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Chemical calculations

What you need to know

Reflections and Exam tips


Chemical calculations

The atom is composed of electrons, neutrons and protons.

An atom is the smallest, electrically neutral, particle of an element that can take part in a
chemical change.

The mass of protons and neutrons is the same, so we call their relative masses 1.

mass number - atomic number = neutron number.

Isotopes are elements wiuth the same number of protons but different number of neutrons. Different isotopes of the same element have different physical properties, but the same chemical properties as they have same number of outer shell electrons.

A chemical formula is a useful shorthand method for describing the atoms in a chemical.

The mass of an individual atom is very small and it is much more convenient to measure atomic
masses as relative masses.

Relative atomic mass (Ar) is defined as - the mass of a single atom on a scale on which the mass of an atom of carbon — 12 has a mass of 12 atomic mass units. The relative atomic mass does not have units.

Relative molecular Mr (molar mass) is defined as - the mass of a single molecule on a scale on which the mass of an atom of carbon — 12 has a mass of 12 atomic mass units.

Relative Molecular Mass of a molecule is calculated by adding together the relative atomic
masses of the atoms in the chemical formulae.

Calculate the Molar Mass of sulphuric acid H2SO4 = 2 x 1 + 32 + 4 x 16 = 98.

Chemists measure how much of a particular chemical reacts in grams or measure the volume of a gas.

The mole is the amount of substance, which contains the same number of particles (atoms, ions, molecules, formulae or electrons) as there are carbon atoms in 12 g of carbon -12.

This is known as the Avogadro constant, L, and is equal to 6.02 x 1023 mol–1

The molar mass of a substance is the mass, in grams, of one mole.

The molar volume of a gas is the volume occupied by one mole at room temperature and
atmospheric pressure (r.t.p) and is equivalent to 24 dm3.

Calculate the number of moles of oxygen atoms in 32 g of oxygen atoms:

Number of moles = mass in grams/molar mass of atoms

= 32g/16g/mol = 2 moles

Calculating mass of a given number of moles = the mass of 1 mole x the number of moles of material

Calculate the mass of 3 moles of sulphur dioxide H2O. 1 mole of water has a mass = 16 + (2x1) = 18 g mol–1. Therefore 3 mole of water = 3 x 18 = 54g.

Empirical formula

The simplest whole number ratio of atoms in a compound.

Empirical formulas can be worked out from mass of reactants or percentages of reactants.

Sodium burns in excess oxygen to give a yellow solid oxide that contains 58.97% of sodium.
What is the empirical formula of the oxide?

  ÷ by r.a.m ÷ by smallest Ratio of atoms
  Moles of sodium
58.97/23 = 2.564
  Moles of oxygen
41.03/16 = 2.564 1

Empirical formular = NaO

Molecular Formula Mass = Empirical Formula Mass x a whole number (n).

Calculating percentage mass

The mass of a compound is equivalent to its relative formula mass.

In order to calculate the percentage mass of an element, the formula of compound and relative atomic mass of all atoms needed.

% mass of element = Relative mass of element/Relative formula mass of compound x 100


Atoms are neirther created nor destroyed in a chemical reaction.

Percentage yield can be calculated by comparing the actual yield obtained with the theoretical (expected) yield.

Chemical reactions can never be 100% yield due to:

  1. Reactions being reversible
  2. Not all chemicals reacting
  3. Some products lost during reaction

Atom economy

Atom economy is a measure of the reactants that end up as products.