# Relative Atomic mass and Relative Formula Mass

In this worksheet, students will learn the meanings of relative atomic mass and relative formula mass, how to calculate them and how to use these ideas to predict the amount of chemical made in a reaction.

### QUESTION 1 of 10

When chemicals react, atoms inside them swap places to make new compounds, which are the products of the reaction. We have the same atoms but in different pattens. 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 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.

Although we often quote mass numbers as whole numbers, most are not exactly integers. The reason for this is that elements have different isotopes (atoms with the same number of protons, but different numbers of neutrons). For example, the relative atomic mass of chlorine is 35.5. This is an average of most chlorine atoms having mass = 35 (called chlorine-35), and some having mass = 37 (called chlorine-37).

Originally, the relative atomic mass scale was based on hydrogen = 1 unit. Later on, it was found that using carbon as the basis of the relative atomic mass scale gave better results. Now 1 unit of relative atomic mass is one-twelfth of the mass of a carbon-12 atom.

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.

Some chemical formulas have brackets- copper nitrate is Cu(NO3)2. That means there are two units of NO3 in the formula. The atom count for this formula is CuN3O6.

The reason that relative formula masses are so useful is that they link atomic-scale and real world conservation of mass.
Let's go back to the burning (or combustion) or methane.

The relative formula masses are CH4: 12 + (4 x 1) = 16.  O2 = (2 x 16) = 32. CO2 = 12 + (2 x 16) = 44. H2O = (2 x 1) + 16 = 18.

The total relative formula mass on the reactants side is 16 + 2 x 32 = 80. (We've doubled the mass of O2 because there are two O2's in the reaction).

The total relative formula mass on the products side is 44 + (2 x 18) = 80.

The clever thing is that the relative formula masses work whatever units we use. So 80 g of reactant make 80 g of product; 80 tonnes of reactant make 80 tonnes of product. They also show balances between individual chemicals. 16 g of methane needs to react with 64 g of oxygen, and they make 44 g of carbon dioxide and 36 g of water.

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.

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

CaCO3 (s) → CaO (s) + CO2 (g)

How many grams of calcium oxide are produced from 100 g of calcium carbonate?

How many grams of calcium oxide are produced from 500 g of calcium carbonate?

Propane burns and gives carbon dioxide and water:

C3H8 (g) + 5O2 (g) → 3CO2 (g) + 4H2O (l)

How much propane (in grams) is needed to produce 36 g water?

Hydrogen and chlorine produce hydrochloric acid:

H2 (g)+ Cl2 (g) → 2HCl (g)

How many grams of HCl acid is made from 35.5 g of chlorine?

propane burns with oxygen to produce carbon dioxide and water:

C3H8 (g) + 5O2 (g) → 3CO2 (g) + 4H2O (l)

How many grams of carbon dioxide are produced when 88 g of propane (C3H8) burns?

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

What is meant by an isotope?

Atoms with the same mass number but different atomic numbers.

Atoms with the same atomic number but different mass numbers.

Atoms with different atomic and mass numbers

What is the basis of the relative atomic mass scale?

Mass of a hydrogen atom

One quarter of the mass of a helium-4 atom

One twelfth of the mass of a carbon-12 atom

The main copper isotopes are copper-63 and copper-65.

70% of copper atoms are copper-63, and 30 % are copper-65.

What is the average atomic mass of a copper atom?

63

63.4

63.6

65

What is the relative formula mass of Ca(OH)2?

What are the relative formula masses of these compounds?

• Question 1

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

CaCO3 (s) → CaO (s) + CO2 (g)

How many grams of calcium oxide are produced from 100 g of calcium carbonate?

How many grams of calcium oxide are produced from 500 g of calcium carbonate?

EDDIE SAYS
relative formula mass of calcium carbonate = 40 + 12 + 16 + 16 + 16 = 100
relative atomic mass of calcium oxide = 40 + 16 = 56
100g of calcium carbonate gives 56g of calcium oxide
500g would give 5 times more, 56 x 5 = 280g calcium oxide
• Question 2

Propane burns and gives carbon dioxide and water:

C3H8 (g) + 5O2 (g) → 3CO2 (g) + 4H2O (l)

How much propane (in grams) is needed to produce 36 g water?

22
EDDIE SAYS
Relative formula mass of propane = 12 + 12 + 12 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 = 44
Relative atomic mass of water = 1 + 1 + 16 = 18, but 4 molecules of water are produced: 18 x 4 = 72
44 g of propane give 72 g of water halving both sides of the reaction, 22 g of propane gives 36 g of water
• Question 3

Hydrogen and chlorine produce hydrochloric acid:

H2 (g)+ Cl2 (g) → 2HCl (g)

How many grams of HCl acid is made from 35.5 g of chlorine?

36.5
EDDIE SAYS

71 g of chlorine give 2 times 36.5 = 73 g of HCl
You have to multiply by 2 because of the 2 in front of HCl in the equation, which shows that 2 molecules of HCl are produced.
35.5 g of chlorine would give half this amount (71/35.5=2), therefore 73/2 = 36.5 g of HCl
• Question 4

propane burns with oxygen to produce carbon dioxide and water:

C3H8 (g) + 5O2 (g) → 3CO2 (g) + 4H2O (l)

How many grams of carbon dioxide are produced when 88 g of propane (C3H8) burns?

264
EDDIE SAYS
Relative formula mass of propane = 12 + 12 + 12 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 = 44
Relative atomic mass of carbon dioxide = 12 + 16 + 16 = 44, but 3 molecules of carbon dioxide are produced: 3 x 44 = 132
44 g of propane give 132 g of carbon dioxide
88 g is double 44 so it gives 132 x 2 = 264 g of carbon dioxide
• Question 5

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 6

What is meant by an isotope?

Atoms with the same atomic number but different mass numbers.
EDDIE SAYS
Isotopes of the same element have the same atomic number (otherwise they're different elements). They have different mass numbers, because they have different numbers of neutrons.
• Question 7

What is the basis of the relative atomic mass scale?

One twelfth of the mass of a carbon-12 atom
EDDIE SAYS
The original basis of the relative atomic mass scale was hydrogen, but carbon turned out to be a better basis. To keep the same scale, the mass of carbon-12 was divided by 12.
• Question 8

The main copper isotopes are copper-63 and copper-65.

70% of copper atoms are copper-63, and 30 % are copper-65.

What is the average atomic mass of a copper atom?

63.6
EDDIE SAYS
To work out the average, do (0.7 x 63) + (0.3 x 65), which gives 63.6. The official answer is a bit lower than this, because the percentage data in the question aren\'t exact.
• Question 9

What is the relative formula mass of Ca(OH)2?

74
EDDIE SAYS
This one is trickier, because of the brackets in the formula. (OH)2 means two lots of OH. We could write it as CaO2H2. That means the formula mass is 40 + (2 x 16) + (2 x 1) = 74.
• Question 10

What are the relative formula masses of these compounds?

EDDIE SAYS
FeO: 56 + 16 = 72 NaCl: 23 + 35.5 = 58.5 SiO2: 28 + (2 x 16) = 60 SrTiO3: 88 + 48 + (3 x 16) = 184
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