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Additional Chemistry Calculations. Relative atomic and Formula Masses

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additional chemistry calculations
Additional Chemistry Calculations

Relative atomic and Formula Masses

The mass of an atom is too small to deal with in real terms, so we use ‘relative’ masses – Carbon is given a mass of 12, and everything else is compared with it and given a mass, e.g. Oxygen is ‘heavier’, so its relative mass is 16.

Relative formula mass can be found by adding up the relative atomic masses of each element in a compound.

E.g. Carbon Dioxide (CO2)

Carbon has a relative atomic mass of 12

Oxygen has a relative atomic mass of 16

The relative formula mass of Carbon Dioxide is therefore:

12 + (16 x 2) = 44


Because saying ‘relative formula (or atomic!) mass in grams’ is a bit clumsy, we simply say ‘moles’ instead. This means that 1 mole of Carbon Dioxide is 44 grams, or 44g. Simple!

The relative atomic mass can be found by looking at the periodic table, It is always the larger of the two numbers.

x 2 because it’s “O2”


Percentage of an element in a compound

We can use the relative atomic mass (Ar) of elements and the relative formula mass (Mr) of compounds to find out the percentage composition of different elements.

E.g. What percentage mass of white Magnesium Oxide is actually Magnesium, and how much is Oxygen?

  • Work out the mass of MgO
  • Convert to grams
  • Work out the percentage

24 + 16 = 40



X 100% = 60% is Magnesium, so 40% must be Oxygen!

Work out the formula mass of the compound

Convert this into grams

Work out the percentage by using this equation:


Formula of a compound from its percentage composition

We can also do this backwards! If we know the percentage composition of a compound we can work out the ratio of atoms. This is known as the Empirical Formula. Sometimes this is the same as the molecular formula, but not always (e.g. water has an empirical and molecular formula of H2O. Hydrogen peroxide's empirical formula is HO, but it’s molecular formula is H2O2.

E.g. If 9g of Aluminium react with 35.5g of Chlorine, what is the empirical formula of the compound formed?







  • Mass of element
  • Total mass of compound

x 100%

Divide the mass of each element by its relative atomic mass to find out the number of moles reacted

Create a ratio and simplify if necessary

Write a formula based on the ratio

= 1/3 moles of Aluminium atoms

= 1 mole of Chlorine atoms

Al : Cl

1/3 : 1

1 : 3



Masses of reactants and products

This is an important calculation when we want to know how much of each reactant to react together. For example, sodium hydroxide reacts with chlorine gas to make bleach. If we have too much Chlorine, some will be wasted. Too little and not all of the sodium hydroxide will react.

2NaOH + Cl2 NaOCl + NaCl + H2O

How much Chlorine gas should we bubble through 100g of Sodium Hydroxide to make Bleach?

1. NaOH

23 + 16 + 1 = 40g is one mole of NaOH

2. We have 100g in our reaction so…

100 = 2.5 moles


3. The chemical equation tells us that we need 2 moles of Sodium Hydroxide (2NaOH) for every mole of Chlorine (Cl2).

So we need: 2.5 = 1.25 moles of Chlorine


4. 35.5 x 2 = 71g is one mole of Cl2

So we need 1.25 x 71 = 88.75g of Chlorine to react with 100g of Sodium Hydroxide.







Work out the mass of one mole of Sodium Hydroxide

Calculate how many moles you have in your reaction

Work out how many moles of Chlorine you need

Convert this into a mass for Chlorine


Percentage Yield

Rather than talk about the yield of a chemical reaction in terms of mass (grams, tonnes etc.) we can talk about the percentage yield. This gives us an idea of the amount of product that the reaction really makes, compared to what it could possibly make under perfect conditions. There are many reasons why we don’t make 100% every time, such as:

  • The reaction may be reversible
  • Some product could be left behind in the apparatus
  • The reactants may not be pure
  • It may be difficult to separate the products if more than one are made.

Using this reaction “A + B  C”, it was found that in perfect conditions, scientists could make 2.5g of C. However, when they tried it out, they only made 1.5. What is the percentage yield of this reaction?



The higher the percentage yield and atom economy, the better the reactions are for the Earth’s resources, as there’s less waste!

Amount of product produced

Maximum amount of product possible

x 100%

x 100% = 60% percentage yield