GCSE Chemistry revision

Relative atomic mass

A simple model of the atom, symbols, relative atomic mass, electronic charge and isotopes

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The mass number of an element is the total number of protons and neutrons in its nucleus, found in the top left of its nuclear symbol.
Relative atomic mass (Ar) is the average mass of all the isotopes of an element, taking into account how common each isotope is.
For example, chlorine has isotopes with mass numbers of 35 and 37, giving it a relative atomic mass of 35.5.

1. Introduction AQA

The mass number of an element is the total number of protons and neutrons in its nucleus, found in the top left of its nuclear symbol.
Relative atomic mass (Ar) is the average mass of all the isotopes of an element, taking into account how common each isotope is.
For example, chlorine has isotopes with mass numbers of 35 and 37, giving it a relative atomic mass of 35.5.

Relative formula mass (Mr) is the average mass of an entire compound, calculated by adding together the Ar values of all atoms in its molecular formula.
Mass is always written as Mr and has no units, as it is a relative measure.
To find Mr, you must account for every atom present in the formula, including any that are multiplied by subscript numbers.

2. What is Relative Formula Mass (Mr )? AQA

Relative formula mass (Mr ) is the average mass of an entire compound, calculated by adding together the Ar values of all atoms in its molecular formula.
Mass is always written as Mr and has no units, as it is a relative measure.
To find Mr, you must account for every atom present in the formula, including any that are multiplied by subscript numbers.

Magnesium chloride has the formula MgCl₂, containing one magnesium atom (Ar = 24) and two chlorine atoms (Ar = 35.5 each).
The calculation is 24 + (35.5 × 2) = 95, so the Mr of magnesium chloride is 95.

Sulfuric acid has the formula H₂SO₄, containing two hydrogen atoms (Ar = 1), one sulfur atom (Ar = 32), and four oxygen atoms (Ar = 16).
The calculation is (2 × 1) + (1 × 32) + (4 × 16) = 98, so the Mr of sulfuric acid is 98.

The percentage mass of an element in a compound is calculated using the formula: (Ar × number of atoms of that element) / Mr × 100 For sulfur in sulfuric acid: 32 × 198 × 100 = 32.7%, meaning sulfur makes up 32.7% of the mass of sulfuric acid.

Calcium hydroxide has the formula Ca(OH)₂, meaning the subscript 2 applies to both the oxygen and hydrogen inside the brackets.
The Ar values are: calcium = 40, oxygen = 16, hydrogen = 1, giving an Mr of 40 + (2 × 16) + (2 × 1) = 74.
The percentage mass of oxygen in calcium hydroxide is 16 × 274 × 100 = 43.2%.

6. Percentage Mass: Calcium Hydroxide Example AQA

Calcium hydroxide has the formula Ca(OH)₂, meaning the subscript 2 applies to both the oxygen and hydrogen inside the brackets.
The Ar values are: calcium = 40, oxygen = 16, hydrogen = 1, giving an Mr of 40 + (2 × 16) + (2 × 1) = 74.
The percentage mass of oxygen in calcium hydroxide is 16 × 274 × 100 = 43.2%.

Always use the periodic table to look up Ar values rather than trying to memorise them.
Pay close attention to brackets in chemical formulae, as the number outside the bracket multiplies all atoms inside it.
Double-check your Mr calculation before using it in a percentage mass question, as an error there will affect your final answer.

7. Key Tips for Exam Success AQA

Always use the periodic table to look up Ar values rather than trying to memorise them.
Pay close attention to brackets in chemical formulae, as the number outside the bracket multiplies all atoms inside it.
Double-check your Mr calculation before using it in a percentage mass question, as an error there will affect your final answer.