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Calculate Relative Formula Mass (AQA GCSE Chemistry: Combined Science)
Revision Note
Relative formula (molecular) mass
How to calculate relative formula mass
- The symbol for the relative atomic mass is Ar
- This is calculated from the mass number and relative abundances of all the isotopes of a particular element
- The symbol for the relative formula mass is Mr and it refers to the total mass of the molecule
- To calculate the Mr of a substance, you have to add up the relative atomic masses of all the atoms present in the formula
Relative formula mass calculations table
Substance | Atoms present | Calculation | Mr |
Hydrogen H2 |
2 x H | (2 x 1) | 2 |
Water H2O |
(2 x H) + (1 x O) | (2 x 1) + (1 x 16) | 18 |
Potassium carbonate K2CO3 |
(2 x K) + (1 x C) + (3 x O) | (2 x 39) + (1 x 12) + (3 x 16) | 138 |
Calcium hydroxide Ca(OH)2 |
(1 x Ca) + (2 x O) + (2 x H) | (1 x 40) + (2 x 16) + (2 x 1) | 74 |
Ammonium sulfate (NH4)2SO4 |
(2 x N) + (8 x H) + (1 x S) + (4 x O) | (2 x 14) + (8 x 1) + (1 x 32) + (4 x 16) | 132 |
- In accordance with the Law of Conservation of Mass, the sum of the relative molecular masses of the reactants will be the same as the sum of the relative molecular masses of the products
Worked example
Calculate the relative formula mass of:
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Sodium chloride, NaCl
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Copper oxide, CuO
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Magnesium nitrate, Mg(NO3)2
Answers:
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Sodium chloride
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NaCl = 23 + 35.5 = 58.5
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Copper oxide
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CuO = 63.5 + 16 = 79.5
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Magnesium nitrate
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Mg(NO3)2 = 24 + (14 x 1 x 2) + (16 x 3 x 2) = 148
-
Examiner Tip
The relative atomic mass of every element is given on the Periodic Table. It is the larger of the two numbers.
Calculating % mass of an element in a compound
- The percentage by mass of an element in a compound can be calculated using the following equation:
- For example, in water:
- Water is a simple molecule with the chemical formula H2O
- So, water is made of two hydrogen (H) atoms and one oxygen (O) atom
- From the Periodic Table, the relative atomic mass of:
- Hydrogen = 1
- Oxygen = 16
- Therefore, the total mass of water is:
- (2 × 1) + 16 = 18
- To find the percentage by mass of hydrogen:
- Percentage of hydrogen = × 100 = 11.1%
- Similarly, the percentage by mass of oxygen is:
- Percentage of oxygen = × 100 = 88.9%
- Note: The total percentage by mass of all the elements should add up to 100%, e.g. 11.1% + 89.9% = 100%
Examiner Tip
- The most common incorrect answer for percentage composition are values above 100%
- This is not possible and usually happens when the numbers in the calculation are "upside down"
- For example, the percentage composition of hydrogen in water:
- CORRECT = x 100 = 11.1%
- INCORRECT = x 100 = 900%
- For example, the percentage composition of hydrogen in water:
Worked example
Calculate the percentage of carbon in the formula of glucose, C6H12O6.
Answer:
- From the Periodic Table, the relative atomic masses are:
- Carbon = 12
- Hydrogen = 1
- Oxygen = 16
- The total mass of glucose is:
- (6 × 12) + (12 × 1) + (6 × 16) = 180
- The equation for percentage composition is:
- Percentage composition = × 100
- So, the percentage composition of carbon in glucose is:
- Percentage of carbon = × 100 = 40%
Worked example
The chemical formula of the fertiliser ammonium nitrate is NH4NO3. Calculate the percentage by mass of nitrogen in ammonium nitrate.
Answer:
- From the Periodic Table, the relative atomic masses are:
- Nitrogen = 14
- Hydrogen = 1
- Oxygen = 16
- The total mass of ammonium nitrate is:
- (1 x 14) + (4 × 1) + (1 x 14) + (3 × 16) = 80
- The equation for percentage composition is:
- Percentage Composition = × 100
- So, the percentage composition of nitrogen in ammonium nitrate is:
- Careful: There are two nitrogen atoms in ammonium nitrate
- Percentage of nitrogen = × 100 = 35%
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