# Understand Relative Atomic Mass and Relative Formula Mass

In this worksheet, students will learn the meanings of relative atomic mass and relative formula mass and how to calculate them.

### QUESTION 1 of 10

The ideas of relative atomic mass and relative formula mass help us link these pictures to real masses of real objects.

The relative atomic mass, or Ar, describes the mass of an element. it's the mass number you can look up in the periodic table.

The mass number is the larger of the two numbers- so the mass number of aluminium is 27.

To work out the relative formula mass (Mr) of a compound or molecule, we just add the relative atomic masses of all the atoms in a chemical formula. Be careful to take account of the number of copies of each atom in the formula. For example:

What is the relative formula mass of carbon dioxide, CO2?

1. Look up Ar for each of the atoms:   C = 12, O = 16.

2. Add up the relative atomic masses, counting numbers of each atom. Mr = 12 + (2 x 16) = 44.

It's a good idea to put brackets round the multiplications for each element; this makes sure that you get the right answer. The reason that relative formula masses are so useful is that they link atomic-scale and real world conservation of mass. One example of this is when we burn methane.

The word equation is methane + oxygen → carbon dioxide + water. In symbols, this is CH4 + 2 O2 → CO2 + 2 H2O.  If we look at the molecules, they look like this:

The relative formula masses of methane and carbon dioxide are (methane) CH4: 12 + (4 x 1) = 16 and (carbon dioxide) CO2 = 12 + (2 x 16) = 44. The relative formula masses work for any units of mass. So 16 g methane burns to produce 44 g carbon dioxide. The extra mass comes from the oxygen  needed to make methane burn.

Relative formula masses are a really powerful tool to predict amounts of chemical produced in reactions. Like any calculations, don't take short cuts, so you don't get confused.

Use a periodic table to find the relative atomic masses of these elements.

What is the relative formula masses an oxygen molecule, O2? The relative atomic mass of oxygen is 16.

16

32

48

What is the relative formula mass of MgCl<sub>2</sub>? The relative atomic masses are 24 for Mg, and 35.5 for Cl.

This is the symbol equation for the thermal decomposition of magnesium carbonate:

MgCO3 (s) → MgO (s) + CO2 (g)

What are the relative formula masses of each of these compounds?

What is the relative formula mass of KF? Look up the relative atomic masses on the periodic table.

What is the relative formula mass of Fe2O3? Look up the relative atomic masses on the periodic table.

This is the symbol equation for the thermal decomposition of magnesium carbonate:

MgCO3 (s) → MgO (s) + CO2 (g)

Earlier on, you worked out that the relative formula masses are

MgCO3 : 24 + 12 + (3 x 16) = 84
MgO: 24 + 16 = 40
CO2: 12 + (2 x 16) = 44

If we have 84 kg magnesium carbonate, how much magnesium oxide can we make?

40 g

40 kg

44 kg

This is the symbol equation for the thermal decomposition of magnesium carbonate:

MgCO3 (s) → MgO (s) + CO2 (g)

Earlier on, you worked out that the relative formula masses are

MgCO3 : 24 + 12 + (3 x 16) = 84
MgO: 24 + 16 = 40
CO2: 12 + (2 x 16) = 44

If we have 42 kg magnesium carbonate, how much magnesium oxide can we make?

20 kg

22 kg

40 kg

This is the symbol equation for the thermal decomposition of magnesium carbonate:

MgCO3 (s) → MgO (s) + CO2 (g)

Earlier on, you worked out that the relative formula masses are

MgCO3 : 24 + 12 + (3 x 16) = 84
MgO: 24 + 16 = 40
CO2: 12 + (2 x 16) = 44

If we need to make 400 kg magnesium oxide, how much magnesium carbonate do we need?

84 kg

840 kg

480 kg

Carbon reacts with oxygen to make carbon dioxide. The reaction is C + O2 → CO2.

How much carbon is needed to make 22 g carbon dioxide? Give your answer in grams.

• Question 1

Use a periodic table to find the relative atomic masses of these elements.

EDDIE SAYS
There are two numbers in each box in the periodic table. The relative atomic mass (or mass number) is the larger of the two.
• Question 2

What is the relative formula masses an oxygen molecule, O2? The relative atomic mass of oxygen is 16.

32
EDDIE SAYS
There are two oxygen atoms in O2, so the formula mass is 2 x 16 = 32.
• Question 3

What is the relative formula mass of MgCl<sub>2</sub>? The relative atomic masses are 24 for Mg, and 35.5 for Cl.

95
EDDIE SAYS
MgCl2 means magnesium and 2 chlorines. The calculation is 24 + (2 x 35.5). The brackets make absolutely sure that you do the multiplication the right way.
• Question 4

This is the symbol equation for the thermal decomposition of magnesium carbonate:

MgCO3 (s) → MgO (s) + CO2 (g)

What are the relative formula masses of each of these compounds?

EDDIE SAYS
MgCO3: 24 + 12 + (3 x 16) = 84 MgO: 24 + 16 = 40 CO2: 12 + (2 x 16) = 44
• Question 5

What is the relative formula mass of KF? Look up the relative atomic masses on the periodic table.

58
EDDIE SAYS
The relative atomic masses are K: 39, F: 19. So the relative formula mass is 39 + 19 = 58.
• Question 6

What is the relative formula mass of Fe2O3? Look up the relative atomic masses on the periodic table.

160
EDDIE SAYS
The relative atomic masses are Fe: 56, O: 16. So the relative formula mass is (2 x 56) + (3 x 16) = 160.
• Question 7

This is the symbol equation for the thermal decomposition of magnesium carbonate:

MgCO3 (s) → MgO (s) + CO2 (g)

Earlier on, you worked out that the relative formula masses are

MgCO3 : 24 + 12 + (3 x 16) = 84
MgO: 24 + 16 = 40
CO2: 12 + (2 x 16) = 44

If we have 84 kg magnesium carbonate, how much magnesium oxide can we make?

40 kg
EDDIE SAYS
The idea here is that we can use the relative formula masses to work out ratios, using any units we need to. So 84 anythings of magnesium carbonate make 40 same anythings of magnesium oxide.
• Question 8

This is the symbol equation for the thermal decomposition of magnesium carbonate:

MgCO3 (s) → MgO (s) + CO2 (g)

Earlier on, you worked out that the relative formula masses are

MgCO3 : 24 + 12 + (3 x 16) = 84
MgO: 24 + 16 = 40
CO2: 12 + (2 x 16) = 44

If we have 42 kg magnesium carbonate, how much magnesium oxide can we make?

20 kg
EDDIE SAYS
We know that 84 kg of magnesium carbonate makes 40 kg of magnesium oxide. If we half both, the ratio still works. So 42 kg magnesium carbonate makes 20 kg magnesium oxide.
• Question 9

This is the symbol equation for the thermal decomposition of magnesium carbonate:

MgCO3 (s) → MgO (s) + CO2 (g)

Earlier on, you worked out that the relative formula masses are

MgCO3 : 24 + 12 + (3 x 16) = 84
MgO: 24 + 16 = 40
CO2: 12 + (2 x 16) = 44

If we need to make 400 kg magnesium oxide, how much magnesium carbonate do we need?

840 kg
EDDIE SAYS
We know that 84 kg of magnesium carbonate makes 40 kg of magnesium oxide. We need ten times as much magnesium oxide (400 ÷ 40 = 10), so we need to start with ten times as much magnesium carbonate.
• Question 10

Carbon reacts with oxygen to make carbon dioxide. The reaction is C + O2 → CO2.

How much carbon is needed to make 22 g carbon dioxide? Give your answer in grams.

6
EDDIE SAYS
This calculation puts all the steps together without any clues, so it's tricky! Start by looking up the atomic masses; C = 12, O = 16. The formula mass of C = 12, and CO2 = 12 + (2 x 16) = 44. So 12 g carbon makes 44 g carbon dioxide. We want to make half of this, so we need to start with half the amount of carbon.
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