Calculate Relative Formula Mass | AQA GCSE Chemistry: Combined Science Revision Notes 2018

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Relative formula (molecular) mass

The symbol for the relative atomic mass is A_r
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 M_r and it refers to the total mass of the molecule
To calculate the M_r of a substance, you have to add up the relative atomic masses of all the atoms present in the formula

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

Calculate the relative formula mass of:

Answers:

The relative atomic mass of every element is given on the Periodic Table. It is the larger of the two numbers.

The percentage by mass of an element in a compound can be calculated using the following equation:

$% mass of an element = \frac{A_{r} \times number of atoms of the element}{M_{r} of the compound} \times 100$

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 = $(\frac{2 \times 1}{18})$ × 100 = 11.1%
Similarly, the percentage by mass of oxygen is:
- Percentage of oxygen = $(\frac{1 \times 16}{18})$ × 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%

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 = $(\frac{2 \times 1}{18})$ x 100 = 11.1%
  - INCORRECT = $(\frac{18}{2 \times 1})$ x 100 = 900%

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 = $(\frac{total mass of element}{total mass of compound})$ × 100

So, the percentage composition of carbon in glucose is:
- Percentage of carbon = $(\frac{6 \times 12}{180})$ × 100 = 40%

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 = $(\frac{total mass of element}{total mass of compound})$ × 100

So, the percentage composition of nitrogen in ammonium nitrate is:
- Careful: There are two nitrogen atoms in ammonium nitrate
- Percentage of nitrogen = $(\frac{2 \times 14}{80})$ × 100 = 35%