Chemistry Calculations

All the chemistry calculations for AQA Additional Chemistry

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Mass number, atomic number, atomic mass, formula m

Mass number = total number of protons + neutrons

Atomic number = number of protons

Isotopes contain the same number of protons and electrons as the atom, but different numbers of neutrons

Relative atomic mass = mass number

Relative formula mass = all mass numbers added together in formula

Relative formula mass = one mole of that substance

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Percentage composition

Percentage composition = how much of an element there is in the compound

1. Work out relative formula mass

2. Divide relative atomic mass of element by formula mass of compound

3. Turn into a percentage (x100)

4. If there are more than one of the atoms in the compound multiply answer by the number of atoms

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Conservation of mass

Mass is never lost or gained in reactions – conservation of mass

This means you can work out the mass of one substance if the others are known


  • Reversible reactions may not go to completion
  • Some product may be lost
  • Some reactants may react in an unexpected way
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Atom economy

Atom economy is the percentage of products that are useful, worked out like percentage composition

High atom economy = good for sustainable development

Mass of desired products ÷ mass of reactants x 100

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Concentration of a solution

Measured in moles/cubic decimetre

The greater the concentration the more dissolved particles there are in the solution

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Volumes of gases

A larger volume contains a larger (proportionally) amount of atoms

One mole of any gas at room temperature occupies approx 24 cubic decimetres

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The Empirical Formula

Used to find formulae of compounds from weights of reactants

1. Find the mass of each element

2. Find the relative atomic mass of each element

3. Divide mass of each element by relative atomic mass

4. Divide results by smallest number to give number of each elements in compound

5. Number of atoms tells you formula

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Percentage yield

In some reactions not all of the products are made

The yield is the actual amount obtained

Percentage yield is actual mass obtained ÷ maximum theoretical mass x 100

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