How To Calculate Relative Atomic Mass

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How To Calculate Relative Atomic Mass
What is relative formula mass? – The relative formula mass (Mr) is another way to measure the mass of a compound. It’s also sometimes called the relative molecular mass. This value represents the weighted average of the masses of all the atoms in a formula unit compared to the mass of a carbon-12 atom.

A formula unit is the simplest formula of a chemical compound, also known as the empirical formula. When calculating Mr, we use the empirical formula of the compound. The relative formula mass can be used to find the mass of both covalently bonded substances and ionically bonded ones. However, it’s important to note that scientists use the term “relative formula mass” instead of “relative molecular mass” for ionic compounds and ions.

This is because ions are not molecules. An ion is formed when an atom has more or fewer electrons than protons. This causes the ion to have a positive or negative charge. An ion with more electrons than protons has a negative charge, while an ion with fewer electrons than protons has a positive charge. Sodium and chlorine ions ‍ How do we calculate relative formula mass? You can find the relative formula mass of a compound by adding up the relative atomic masses of the atoms it contains. How To Calculate Relative Atomic Mass NaCl (sodium chloride)Sodium and chlorine ‍ Relative Atomic Mass – Key takeaways Relative mass is the mass of an atom or molecule compared to that of 1/12 of a carbon-12 atom. Relative atomic mass (Ar) is the weighted average of the masses of the isotopes of an element compared to 1/12 of the mass of the carbon-12 atom.

The formula for relative atomic mass is ∑ isotope mass x isotope abundance / 100.Relative molecular mass (Mr) is the weighted average of the mass of a molecule compared to 1/12 of the mass of the carbon-12 atom. You use it only when talking about covalent compounds. Relative formula mass (Mr) is the same as relative molecular mass.

You can use it to talk about covalent compounds and ionic ones. What is an atomic number? We call the number of protons in an atom of an element the atomic number. Atomic numbers can help you identify an element. For example, if you find an atom has an atomic number of eight, you can conclude it’s an oxygen atom.

So all the isotopes of an element have the same atomic number! What is the relative mass of an electron? An electron weighs 9.11 x10^-28 grams. That’s 1/2000 of the mass of a proton! What is the relative mass of a neutron? A neutron weighs 1.68 x 10^-24 grams. We can say neutrons have a mass of one unified atomic mass unit (or 1u), approximately.

How do you calculate relative formula mass? You can find the relative formula mass of a compound by adding up the relative atomic masses of the atoms it contains. As an example, the formula mass of NaCl is 58.5. (Mr of sodium = 22.99; Mr of chlorine = 35.45.) ‍ : Relative Atomic Mass

How do you find relative mass on the periodic table?

How to Calculate Relative Atomic Mass – The relative atomic mass of an element is the average mass of an atom, and it takes into account the masses of each isotope and their proportions in the environment. To calculate the relative atomic mass you require the following information:

The abundance of each isotope which tells you the quantity the isotope is present in the environment. This is different for each isotope of an element.

The mass number of each isotope of a particular element. The number of protons in all the isotopes of an atom remains the same, it is only the number of neutrons.

Lets work through an example: Practice Question: A sample of chlorine gas is a mixture of 2 isotopes, chlorine-35 and chlorine-37. These isotopes occur in specific proportions in the sample i.e.75% chlorine-35 and 25% chlorine-37. Calculate the relative atomic mass of chlorine in the sample.1. GCSE Chemistry – Relative Atomic Mass 2. Identify the mass number, For each isotope of the element. GCSE Chemistry – Relative Atomic Mass 3. Write out the relative atomic mass formula to calculate relative atomic mass, GCSE Chemistry – Relative Atomic Mass Equation 4. Substitute identified data. Substitute into the formula and work out the answer. = 35.5 The relative atomic mass of chlorine in this mixture was 35.5. This number is closer to 35 compared to 37 as chlorine-35 is more abundant than chlorine-37.

  1. We can do a sense-check, to make sure that the value seems right.35.5 is in between 35 and 37, as we would expect, and it has a closer mass to Cl-35, which is the more abundant isotope.
  2. This seems fine! You need to be able to rearrange the relative atomic mass equation to work out a missing abundance or mass.

Worked example – relative atomic mass of oxygen. Oxygen has three isotopes. The abundances in percentage are given here. The mass of one of the isotopes is unknown. The average atomic mass of the isotopes is 16.65. Work out the values of x and y. Answer: 1. Set up a table, Put in all the values as shown. GCSE Chemistry – Relative Atomic Mass 2. Work out x, All percentages = 100, so x = 100-(50+35) =15% 3. Form an equation for y. Use the given atomic mass and the equation for calculating it.16.65 = (800 + 595 + 15y) / 100 4. Rearrange to find y, / 15 = y = 18 So the atomic mass of the unknown isotope = 18 and the abundance = 15% →What is the relative atomic mass? Relative atomic mass is a measure of the average mass of an atom of an element compared to a standard unit of mass.

  • It is used to compare the masses of different atoms and elements.
  • How is relative atomic mass calculated? Relative atomic mass is calculated by adding up the masses of all the isotopes of an element and then averaging them based on their relative abundance in nature.
  • This gives us an estimate of the average mass of an atom of that element.

→Why is relative atomic mass important in chemistry? Relative atomic mass is important in chemistry because it helps us understand the properties of different elements and how they behave in chemical reactions. It also helps us predict the masses of molecules and compounds made from different elements.

  1. How does relative atomic mass differ from atomic mass? Atomic mass is the actual mass of an individual atom, whereas relative atomic mass is an average of the masses of all the isotopes of an element.
  2. Atomic mass is a specific value, whereas relative atomic mass is a relative value.
  3. How does relative atomic mass help in chemical calculations? Relative atomic mass is used in chemical calculations to determine the number of moles of a substance, which can then be used to calculate the masses of other substances involved in a reaction.

This information can be used to make predictions about the yields and outcomes of chemical reactions. →What is the standard unit of relative atomic mass? The standard unit of relative atomic mass is the atomic mass unit (amu), which is defined as 1/12 of the mass of a carbon-12 atom.

This unit allows us to compare the masses of different atoms and elements on a relative scale. →How does the periodic table use relative atomic mass? The periodic table is arranged based on the relative atomic masses of the elements. This arrangement allows us to easily compare and predict the properties and behavior of different elements based on their position in the table.

→How does the relative atomic mass affect the reactivity of elements? Elements with lower relative atomic masses tend to be more reactive than elements with higher relative atomic masses. This is because lighter elements have more electrons available to participate in chemical reactions, making them more reactive. How To Calculate Relative Atomic Mass The link to download the notes will be sent to your email

Is relative atomic mass the same as percentage abundance?

What is relative atomic mass? – Relative atomic mass (RAM or ) is the weighted average of the masses of an element’s isotopes compared to of the mass of a carbon-12 atom. All elements have isotopes, but some isotopes are more abundant than others. On a periodic table, the number we see for an atom’s relative atomic mass is an average of the masses of the isotopes of an element.

  • You can calculate the relative atomic mass using this formula:
  • Ar = sum of isotope mass x isotope abundance / 100
  • So the relative atomic mass for chlorine would be:
  • = (35 × 75) + (37 × 25) ÷ 100
  • = (2,625 + 925) ÷ 100 = 35.5

Fig.2: On the periodic table, the relative atomic mass number is the weighted average of all the masses of the isotopes of an element. When we say ‘ weighted average ‘, we mean it takes into account the masses of all the isotopes of that element. The weighted average of the mass of a molecule relative to 112 of the mass of a carbon-12 atom is called the relative molecular mass (Mr or RMM).

  1. An average sample of molecules will have both chlorine-37 and chlorine-35 atoms. This means that the masses of the molecules will vary, like this:
  2. 12 + 1 + (3 x 35) = 118
  3. 12 + 1 + (2 x 35) + 37 = 120
  4. 12 + 1 + 35 + (2 x 37) = 122
  5. 12 + 1 + (3 x 37) = 124
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So a weighted average includes how many of each (or the abundance) of these molecules we find in an average sample of a substance. We calculate the abundance of an isotope as a percentage. Relative molecular mass refers to molecules with a fixed number of atoms joined together by covalent bonding, including noble gases. Fig.3: Hydrogen and Oxygen atomic mass.

  • Hydrogen atomic mass = 1
  • Oxygen atomic mass = 16
  • Mr of H2O
  • = (2×1) + (1×16)
  • = 2 + 16
  • = 18

has no unit because we measure masses by comparing them to the mass of carbon-12. The value we calculate is not the actual mass of an atom but a comparative measure,

What is the formula for relative abundance?

Relative Abundance Formula – The following formula is used to calculate the relative abundance of species in an area.

  • Where RA is the relative abundance of species (%)
  • TS is the total number of species in an area
  • TP is the total sum of the populations of all species in the area

To calculate a relative abundance, divide the total number of species in an area by the total sum of all populations of species in an area, then multiply by 100.

What is meant by relative atomic mass?

Publisher Summary – This chapter discusses the relative atomic masses, molecular masses, and the mole concept. The relative atomic mass of an element is defined as the weight in grams of the number of atoms of the element contained in 12.00 g of carbon-12.

  1. To calculate the relative atomic mass of chlorine, the average mass of one atom of chlorine is found by considering 100 atoms of chlorine.75.53 of these atoms each have a mass of 35 atomic mass units (AMU), and 24.47 atoms each have a mass of 37 AMU.
  2. The word mole has been adopted to represent the Avagadro number of atoms of an element, that is, the relative atomic mass of an element.

Thus, one mole of sodium weighs 23.0 g or one tenth of a mole of sodium weighs 2.3 g. Read full chapter URL: https://www.sciencedirect.com/science/article/pii/B9780750616836500564

Is relative atomic mass the same as atomic mass?

Definition. Relative atomic mass is determined by the average atomic mass, or the weighted mean of the atomic masses of all the atoms of a particular chemical element found in a particular sample, which is then compared to the atomic mass of carbon-12.

What is relative atomic mass to atomic mass?

Relative Molecular Mass – The relative molecular mass is the weighted average of a molecule’s mass relative to one-twelfth of the mass of a carbon-12 atom (Mr or RMM). To calculate the relative mass of a molecule, simply add the relative masses of its constituent elements. This is simple if the relative atomic masses of the elements involved are known.

  • For example – For H 2 SO 4, elements are hydrogen (H), sulphur (S with relative mass =32), and oxygen (O).
  • H 2 SO 4 relative molecular mass = ÷ 100
  • = ÷ 100
  • = (2 + 32 + 64) ÷ 100 = 98

Relative atomic mass is unitless because it is simply the relationship between the mass of any atom and the mass of the C – 12 atom. As a result, this is merely a method of comparing and relating the atomic mass; it lacks a unit. The main difference between relative atomic mass and atomic mass is that relative atomic mass is the ratio of an element’s average atomic mass to one-twelfth of the mass of carbon-12, whereas atomic mass is the total mass of nucleons present in an atom’s nucleus.

  1. The atomic weight in relation to 1/12 the mass of one carbon atom is referred to as relative atomic mass.
  2. Because it is a comparison of atomic weight with 1/12 the mass of a carbon atom, it is referred to as relative atomic mass.
  3. A relative atomic mass (also known as atomic weight; symbol: A r ) indicates how many times an average atom of an element in a given sample is heavier than one-twelfth of an atom of carbon-12.

The formula that can be used to calculate the relative atomic mass: : How To Calculate Relative Atomic Mass – What is Relative Mass? How to Calculate Relative Atomic Mass along with FAQs

How to calculate atomic mass with mass and relative abundance?

Solved Examples – Example 1: Determine the relative abundance of the isotopes if the masses of one isotope of nitrogen, nitrogen-14, are 14.003 amu and another isotope, nitrogen-15, are 15.000 amu. Solution: The average atomic mass of Nitrogen is 14.007 amu.

  • Applying the formula (M1)(x) + (M2)(1-x) = M(E),
  • M1 = 14.003 amu
  • x = unknown relative abundance
  • M2 = 15 amu
  • M (E) = 14.007 amu

14.003(x) + 15 (1 – x) = 14.007 14.003x + 15.000 – 15.000x = 14.007 -0.997x = -0.993 x = 0.993/0.997 x = 0.996 amu Therefore, percent abundance for x = 99.6% and (1 – x) = 0.004 = 0.4%. The abundance of the nitrogen-14 isotope is 99.6%, while the abundance of the nitrogen-15 isotope is 0.4%.

  1. Applying the formula (M1)(x) + (M2)(1-x) = M(E),
  2. M1 = 35 amu
  3. x = unknown relative abundance
  4. M2 = 37 amu
  5. M (E) = 35.45 amu
  6. 35(x) + 37 (1 – x) = 35.45
  7. 35x + 37 – 37x = 35.45
  8. –2 x = – 1.55
  9. x = 1.55/2
  10. x = 0.775

Therefore, percent abundance for x = 77.5% and (1 – x) = 0.225 = 22.5%. The abundance of the Chlorine-35 isotope is 77.5%, while the abundance of the Chlorine- 37 isotope is 22.5%. The percentage of atoms with a specific atomic mass found in a naturally occurring sample of an element is known as its relative abundance.

Calculate the average atomic mass using the atomic masses of each isotope and their percent abundances. Divide each percent abundance by 100 to convert it to decimal form. Multiply this value by the isotope’s atomic mass. Add the atomic masses of each isotope together to get the average atomic mass. The primary distinction between percent abundance and relative abundance is that percent abundance represents isotope abundance, whereas relative abundance represents chemical element abundance.

The percent abundance of a chemical element can be used to calculate its average atomic mass. Isotopes are atoms that have the same atomic number but differ in mass due to a different number of neutrons. The atomic mass of an element is the weighted average of the atomic masses of the element’s naturally occurring isotopes.

Why is carbon-12 used for relative atomic mass?

Why are the atomic masses based on carbon 12 as standard? Answer Verified Hint: Carbon-12 is the more abundant of the two stable isotopes of carbon (the other is carbon-13), accounting for $98.93\%$ of the element. The triple-alpha mechanism, which creates it in stars, is responsible for its abundance.

Complete answer: Note:

Carbon-12 is the more abundant of the two stable isotopes of carbon (the other is carbon-13), accounting for $98.93\%$ of the element. The triple-alpha mechanism, which creates it in stars, is responsible for its abundance. Carbon-12 is particularly important because it serves as the reference point for determining the atomic masses of all nuclides; the atomic mass is, by definition, precisely 12 daltons.

  1. Carbon-12 has six protons, six neutrons, and six electrons.The mass of an atom is its atomic mass.
  2. At rest, 1 dalton equals 12 times the mass of a single carbon-12 atom.
  3. The nucleus’ protons and neutrons account for almost half of an atom’s overall mass, with electrons and nuclear binding energy playing a minor role.

As a result, when calculated in daltons, the numeric value of the atomic mass is almost equal to the mass quantity.Since the chemical atomic weights of carbon 12 are almost equal to those of the natural mix of oxygen, it was selected as the standard.

  1. Since no other nuclide has an identical whole-number mass on this scale except carbon-12.
  2. Six protons, six neutrons, and six electrons make up carbon-12.Carbon-12 is the more abundant of the two stable isotopes of carbon (the other is carbon-13), accounting for $98.93\%$ of the element.
  3. The triple-alpha mechanism, which creates it in stars, is responsible for its abundance.

Carbon-12 is particularly important because it serves as the reference point for determining the atomic masses of all nuclides; the atomic mass is, by definition, precisely 12 daltons. Carbon-12 has six protons, six neutrons, and six electrons. : Why are the atomic masses based on carbon 12 as standard?

What is average atomic mass and how is it calculated?

Calculating Average Atomic Mass – The average atomic mass of an element is the sum of the masses of its isotopes, each multiplied by its natural abundance (the decimal associated with percent of atoms of that element that are of a given isotope). Average atomic mass = f 1 M 1 + f 2 M 2 + + f n M n where f is the fraction representing the natural abundance of the isotope and M is the mass number (weight) of the isotope.

  • For helium, there is approximately one isotope of Helium-3 for every million isotopes of Helium-4; therefore, the average atomic mass is very close to 4 amu (4.002602 amu).
  • Chlorine consists of two major isotopes, one with 18 neutrons (75.77 percent of natural chlorine atoms) and one with 20 neutrons (24.23 percent of natural chlorine atoms). The atomic number of chlorine is 17 (it has 17 protons in its nucleus).
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To calculate the average mass, first convert the percentages into fractions (divide them by 100). Then, calculate the mass numbers. The chlorine isotope with 18 neutrons has an abundance of 0.7577 and a mass number of 35 amu. To calculate the average atomic mass, multiply the fraction by the mass number for each isotope, then add them together.

  • Average atomic mass of chlorine = (0.7577 ⋅⋅ 35 amu) + (0.2423 ⋅⋅ 37 amu) = 35.48 amu Another example is to calculate the atomic mass of boron (B), which has two isotopes: B-10 with 19.9% natural abundance, and B-11 with 80.1% abundance.
  • Therefore, Average atomic mass of boron = (0.199⋅⋅10 amu) + (0.801⋅⋅11 amu) = 10.80 amu Whenever we do mass calculations involving elements or compounds (combinations of elements), we always use average atomic masses.

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Why do we use relative atomic mass?

Relative atomic mass or relative molecular mass was used as the mass of the atoms or molecules were very small. Therefore there was a need to be taken relative to some other elements.

How to find the average atomic mass of an element given abundance and isotope masses?

Calculating Average Atomic Mass – The average atomic mass of an element is the sum of the masses of its isotopes, each multiplied by its natural abundance (the decimal associated with percent of atoms of that element that are of a given isotope). Average atomic mass = f 1 M 1 + f 2 M 2 + + f n M n where f is the fraction representing the natural abundance of the isotope and M is the mass number (weight) of the isotope.

  • For helium, there is approximately one isotope of Helium-3 for every million isotopes of Helium-4; therefore, the average atomic mass is very close to 4 amu (4.002602 amu).
  • Chlorine consists of two major isotopes, one with 18 neutrons (75.77 percent of natural chlorine atoms) and one with 20 neutrons (24.23 percent of natural chlorine atoms). The atomic number of chlorine is 17 (it has 17 protons in its nucleus).

To calculate the average mass, first convert the percentages into fractions (divide them by 100). Then, calculate the mass numbers. The chlorine isotope with 18 neutrons has an abundance of 0.7577 and a mass number of 35 amu. To calculate the average atomic mass, multiply the fraction by the mass number for each isotope, then add them together.

Average atomic mass of chlorine = (0.7577 ⋅⋅ 35 amu) + (0.2423 ⋅⋅ 37 amu) = 35.48 amu Another example is to calculate the atomic mass of boron (B), which has two isotopes: B-10 with 19.9% natural abundance, and B-11 with 80.1% abundance. Therefore, Average atomic mass of boron = (0.199⋅⋅10 amu) + (0.801⋅⋅11 amu) = 10.80 amu Whenever we do mass calculations involving elements or compounds (combinations of elements), we always use average atomic masses.

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  • Mole (unit). Provided by : Wikipedia. Located at : en.Wikipedia.org/wiki/Mole_(unit). License : CC BY-SA: Attribution-ShareAlike
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What is the formula for relative molecular mass?

How to calculate relative formula mass relative molecular mass calculating RFM RMM of compound formula gcse chemistry calculations igcse KS4 science A level GCE AS A2 O Level practice questions exercises How to calculate relative formula mass or relative molecular mass RFM/RMM or just M r

  • How do I calculate relative molecular mass? RMM
  • How to calculate relative formula mass? RFM
  • Is there any difference between RMM and RFM?
  • Does it matter whether the compound is ionic or covalent?
  1. The relative molecular mass/relative formula mass is defined as the sum of all the individual atomic masses of ALL the atoms in the formula (M r ).
  2. If the individual atomic masses of all the atoms in a formula are added together you have calculated the relative formula mass
  3. Atomic masses are listed at the bottom of the page

e.g. for ionic compounds e.g. NaCl = 23 + 35.5 58.5) or molecular mass for covalent elements or compounds,e.g. M r of N 2 = 28 from (2 x 14) or compounds e.g. M r of C 6 H 12 O 6 = 180 from,

  • and more examples of how to calculate relative formula mass are further down the page, you get atomic masses from your periodic table.
  • In a balanced chemical symbol equation, the total of relative formula masses of the reactants is equal to the total relative formula masses of the products (see ).
  • To be honest, the term relative formula mass can be used with any compound whether it be ionic or covalent – it just seems NOT correct to talk about the molecular mass of an ionic compound when it doesn’t consist of molecules, but is that one for the purists!
  • The shorthand M r can be used for the formula of any element or compound and to repeat, ‘it doesn’t matter whether a compound is ionic or covalent ‘.

Numerically M r = Relative formula mass = relative molecular mass = the sum of all the atomic masses for ALL the atoms in a given formula BUT, note, The term relative formula mass is usually applied to ionic compounds. The term relative molecular mass is usually applied to covalent compounds i.e.

  1. Whereas relative atomic mass (section ) only applies to a single atom, anything with at least two atoms in the formula requires the term relative formula mass or relative molecular mass to be used.
  2. WARNING: The most common error is to use atomic/proton numbers instead of atomic masses, unfortunately, except for hydrogen, they are different !
  3. Examples of relative formula mass or relative molecular mass calculations:
  4. How to calculate relative molecular mass = How to calculate relative formula mass
  5. Molecular/formula mass = total of all the atomic masses of ALL the atoms in the molecule/compound.

Watch out for brackets e.g. (OH) 2 means two OH groups to add up!

  • Relative molecular mass calculation Example 2.1
    • The diatomic molecules of the elements hydrogen H 2 and chlorine Cl 2
    • relative atomic masses, Ar : H = 1, Cl = 35.5
    • Formula masses, RMM or M r
      • relative molecular mass for hydrogen H 2 = 2 x 1 = 2
      • relative molecular mass for chlorine Cl 2 = 2 x 35.5 = 71 respectively.
  • Relative molecular mass calculation Example 2.2
    • T he element phosphorus consists of P 4 molecules. (atomic mass of P = 31)
    • relative molecular mass or M r of phosphorus = 4 x its atomic mass = 4 x 31 = 124
  • Relative molecular mass calculation Example 2.3: The compound water H 2 O
    • relative atomic masses are H=1 and O=16
    • relative molecular mass or M r = (1×2) + 16 = 18 (molecular mass of water)
  • Relative molecular mass calculation Example 2.4
    • The compound sulphuric acid H 2 SO 4 when pure, is a covalent compound
    • relative atomic masses are H = 1, S = 32 and O = 16
    • relative molecular mass or M r = (1 x 2) + 32 + (4 x 16) = 98 (molecular mass of sulphuric acid )
  • Relative formula mass calculation Example 2.5
    • The ionic compound magnesium hydroxide Mg(OH) 2 (ionic)
    • relative atomic masses are Mg = 24, H = 1 and O = 16
    • relative formula mass or M r = 24 + 2 x (16+1) = 58
    • Important note on terminology
      • The term relative formula mass is also best applied to giant structure compounds
        • e.g. silicon dioxide SiO 2 (RFM = 28 + 16 + 16 = 60, a 3D giant covalent lattice compound.
  • Relative formula mass calculation Example 2.6
    • The ionic compound aluminium oxide (Al 3+ ) 2 (O 2- ) 3 or just the plain formula Al 2 O 3
    • but it makes no difference to the calculation of relative formula mass or relative molecular mass.
    • relative atomic masses are Al = 27 and O = 16
    • so the relative formula mass RFM or M r = (2 x 27) + (3 x 16) = 102
  • Relative formula mass calculation Example 2.7
    • Calcium phosphate is also ionic but a more tricky formula to work out!
    • (Ca 2+ ) 3 (PO 4 3- ) 2 or Ca 3 (PO 4 ) 2, but it makes no difference to the calculation of relative formula mass or relative molecular mass.
    • atomic masses: Ca = 40, P = 31, O =16
    • relative formula mass or M r = (3 x 40) + 2 x = (120) + (2 x 95) = 310
  • Relative molecular mass calculation Example 2.8
    • Glucose C 6 H 12 O 6 a covalent compound
    • atomic masses: C = 12, O= 16, H = 1
    • relative molecular mass of glucose M r ( C 6 H 12 O 6 ) = (6 x 12) + (12 x 1) + (6 x 16) = 180
  • Relative molecular mass calculation Example 2.9
    • butane C 4 H 10
    • relative atomic masses: C = 12, H=1
    • M r = (4 x 12) + (10 x 1) = 58
  • Relative formula mass calculation Example 2.10
    • copper(II) sulfate (copper sulfate, CuSO 4, ionic)
    • relative atomic masses: Cu = 63.5, S = 32, O = 16
    • M r = 63.5 + 32 + (4 x 16) = 159.5 (its 160 is you use Cu = 64)
  • Relative molecular mass calculation Example 2.11
    • propanol, C 3 H 8 O, CH 3 CH 2 CH 2 OH (the same formula can be expressed in different ways!)
    • relative atomic masses: C = 12, H=1, O = 16
    • M r = (3 x 12) + (8 x 1) + 16 = 60
  • Relative formula mass calculation Example 2.12
    • magnesium nitrate, Mg(NO 3 ) 2
    • relative atomic masses: Mg = 24, N = 14, O = 16
    • M r = 24 + (2 x 14) + (6 x 16) = 24 + 28 + 96 = 148
  • Relative formula mass calculation Example 2.13
    • blue hydrated copper sulfate crystals CuSO 4,5H 2 O, again a bit more tricky!
    • Cu = 63.5, S = 32, H = 1, O =16
    • Just do this carefully in parts eg
    • CuSO 4 = 63.5 + 32 + (4 x 16) = 159.5 (160 if you use Cu = 64)
    • H 2 O = (2 x 1) + 16 = 18, 5 x 18 = 90
    • Formula mass = 159.5 + 90 = 249.5 (250 if Cu = 64)

Self-assessment Quizzes on relative formula mass or relative molecular mass

  • How To Calculate Relative Atomic Mass
  • Above is typical periodic table used in GCSE science-chemistry specifications in calculating relative formula mass or calculating relative molecular mass, and I’ve ‘usually’ used these values in my exemplar calculations to cover most syllabuses

OTHER CALCULATION PAGES

  1. (this page)

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