Calculations with Moles

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Relative Molecular Mass

NOTE: The same method is used to find relative molecular mass and relative formula mass

To find the relative molecular mass of a substance, add together the atomic masses of all the atoms in the molecule.


CHcontains 1 carbon atom and 4 hydrogen atoms.

The relative atomic mass of carbon is 12 and of hydrogen is 1

The relative molecular mass is therefore 12 + 1 + 1 + 1 + 1 = 16

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Number of Moles in a Substance

To find the number of moles in a substance:

Number of moles = mass(g) / mass of 1 mole


Find the number of moles in 2g of H2O

1. find the mass of 1 mole: 1 + 1 + 16 = 18g

2. divide the mass you have (2g) by the mass of one mole (18g): 2/18 = 0.1111

so there are 0.1111 moles of H2O in 2g

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Empirical Formula

To find the empirical formula from a molecular formula, just cancel down the numbers of atoms to their simplest form. E.g. H2O2 would have an empirical formula of HO.

To find the empirical formula of a substance when given the mass of the elements:

  • Find the number of moles of each element (mass/mass of 1 mole)
  • Divide the results of this by the smallest one (so if there are 2 elements and one has 1 mole but the other has 2 moles, you divide by 1)
  • These numbers are the numbers of atoms present in the empirical formula.
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Percentage Composition

To find the percentage of a substance made up by one element:

Mass of the element you want to calculate / Molecular mass of entire molecule * 100

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Calculating Masses of Substances from an Equation

E.g. Calculate the mass of oxygen which will react exactly with 0.64g sulfur dioxide according to the equation:

2SO2 + O2 --> 2SO3


2. Work out the number of moles of sulfur dioxide. (0.64 / 32 + 16 + 16 = 0.01 mole)

3. Find the reaction ratio from the equation. (2 moles of sulfur dioxide react with 1 mole of oxygen so the ratio is 2:1)

4. Use the ratio to find the number of moles of oxygen needed. (0.01 / 2 = 0.005 moles of oxygen)

5. Find the mass of 0.005 moles of oxygen. (0.005 * (16 + 16) = 0.16g)

ANSWER = 0.16g oxygen will react with 0.64g sulfur dioxide.

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Percentage Yield

To calculate the percentage yield from an experiment:

% yield = actual yield / theoretical yield * 100

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