# Understand Conservation of Mass

In this worksheet, students consider the mass conserved in chemical reactions. This means that the mass of reactants is equal to the mass of the products and relative formula masses can be used to predict the yield of reactions.

### QUESTION 1 of 10

When chemicals react, atoms inside them swap places to make new compounds which are the products of the reaction. The atoms are the same as before in number, but they have been rearranged. The overall mass stays the same. It is possible to predict the yield of a reaction using the chemical equation.

Example: Calcium carbonate (CaCO3) is thermally decomposed into calcium oxide (CaO) and carbon dioxide (CO2). The symbol equation is as follows:

CaCO3 CaO + CO2

How much carbon dioxide is made when 1000 g of calcium carbonate decomposes?

Answer: In order to calculate this without knowing the amount of calcium oxide made from 1000 g of calcium carbonate, we need to calculate the relative formula masses of the compounds involved. In this case, calcium carbonate and carbon dioxide.

Calcium carbonate has 1 atom of calcium, 1 atom of carbon and 3 atoms of oxygen, and we need to add the relative atomic masses of all those atoms to find the relative formula mass. The relative atomic mass of calcium is 40, of carbon is 12 and of oxygen is 16:

relative formula mass of calcium carbonate: 40 + 12 + 16 + 16 + 16 = 100

Carbon dioxide has 1 atom of carbon and 2 atoms of oxygen, so:

relative formula mass of carbon dioxide: 12 + 16 + 16 = 44

Using the relative formula masses and the equation for the reaction, we know that:

100 g of calcium carbonate gives 44 g of carbon dioxide

1000 g is 10 times more than 100 g, so 1000 g of calcium carbonate gives 10 x 44 = 440 g of carbon dioxide.

Make sure you're confident with how that answer was obtained and then move on to have a go at some questions.

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

CaCO3 CaO + CO

How much calcium oxide is produced from 500 g of calcium carbonate?

Relative atomic mass of calcium = 40, carbon = 12, oxygen = 16

56 g

280 g

310 g

Hydrogen and chlorine produce hydrochloric acid:

H2 + Cl2 2HCl

How much hydrochloric acid (HCL) is made from 6 g of hydrogen?

Relative atomic mass of hydrogen = 1 and chlorine =  35.5

219 g

73 g

111 g

Propane burns and gives carbon dioxide and water:

C3H8 + 5O2 3CO2 + 4H2O

How many grams of water are produced when 22 g of propane (C3H8) burn?

Relative atomic mass of carbon = 12, hydrogen = 1, oxygen = 16

18 g

72 g

36 g

Hydrogen and chlorine produce hydrochloric acid:

H2 + Cl2 2HCl

How much hydrochloric acid (HCL) is made from 35.5 g of chlorine?

Relative atomic mass of hydrogen = 1, chlorine = 35.5

85.5 g

71 g

36.5 g

Propane burns with oxygen to produce carbon dioxide and water:

C3H8 + 5O 3CO2 + 4H2O

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

Relative atomic mass of carbon = 12, hydrogen = 1, oxygen = 16

44 g

264 g

128 g

What is the maximum total mass (in grams) produced from 100 g of reactants?

1 g

50 g

99 g

100 g

This is a general chemical equation for when reactants A and B give products C and D.

A + B C + D

mass of A = 2 g

mass of B = 3 g

mass of C = 1 g

Use the above information to work out the mass of D.

Remember, the mass of the products is equal to the mass of the reactants.

1 g

2 g

3 g

4 g

Calculate how many grams of carbon monoxide are formed from 6 g of carbon.

2C + O2 2CO

Relative atomic mass of C = 12, O = 16

14 g

56 g

72 g

Why is mass conserved in chemical reactions?

The number of atoms changes

The number of molecules is the same

The number of atoms remains the same but they just swap places

In an industrial reaction, 20 g of a wanted substance was expected to be produced, but instead only 10 g was produced.

What was the percentage yield?

45%

50%

55%

• Question 1

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

CaCO3 CaO + CO

How much calcium oxide is produced from 500 g of calcium carbonate?

Relative atomic mass of calcium = 40, carbon = 12, oxygen = 16

280 g
EDDIE SAYS
How did you get on with this first question? It may look tricky, but it is really only a matter of adding up a few numbers. We know the relative atomic masses of calcium = 40, carbon = 12 and oxygen = 16. From the equation, we can see that there is 1 atom of calcium, 1 atom of carbon and 3 atoms of oxygen in calcium carbonate. Therefore the relative formula mass of calcium carbonate = 40 + 12 + 16 + 16 + 16 = 100 In the same way, we can work out the relative formula mass of calcium oxide = 40 + 16 = 56 So, 100 g of calcium carbonate gives 56 g of calcium oxide. This means that 500 g of calcium carbonate would give 5 times more, 56 x 5 = 280 g calcium oxide. Just take it slowly, one step at a time.
• Question 2

Hydrogen and chlorine produce hydrochloric acid:

H2 + Cl2 2HCl

How much hydrochloric acid (HCL) is made from 6 g of hydrogen?

Relative atomic mass of hydrogen = 1 and chlorine =  35.5

219 g
EDDIE SAYS
Here we go: The relative formula mass of hydrogen is 1 + 1 = 2 The relative formula mass of hydrochloric acid is 1 + 35.5 = 36.5 2 g of hydrogen gives 2 x 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.) 6 g of hydrogen would give 3 times more (6 ÷ 2 = 3), therefore 3 x 73 = 219 g of HCl.
• Question 3

Propane burns and gives carbon dioxide and water:

C3H8 + 5O2 3CO2 + 4H2O

How many grams of water are produced when 22 g of propane (C3H8) burn?

Relative atomic mass of carbon = 12, hydrogen = 1, oxygen = 16

36 g
EDDIE SAYS
Are you beginning to get the hang of these yet? Relative formula mass of propane = 12 + 12 + 12 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 = 44
Relative formula 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.
22 g is half of 44 so it gives 72 ÷ 2 = 36 g of water.
• Question 4

Hydrogen and chlorine produce hydrochloric acid:

H2 + Cl2 2HCl

How much hydrochloric acid (HCL) is made from 35.5 g of chlorine?

Relative atomic mass of hydrogen = 1, chlorine = 35.5

36.5 g
EDDIE SAYS
Remember to work through it steadily. The relative formula mass of chlorine is 35.5 + 35.5 = 71 The relative formula mass of hydrochloric acid is 1 + 35.5 = 36.5 You have to multiply this by 2 because of the 2 in front of HCl in the equation, which shows that 2 molecules of HCl are produced. So, 71 g of chlorine give 2 x 36.5 = 73 g of HCl 35.5 g of chlorine would give half this amount: 71 ÷ 35.5 = 2, therefore 73 ÷ 2 = 36.5 g of HCl
• Question 5

Propane burns with oxygen to produce carbon dioxide and water:

C3H8 + 5O 3CO2 + 4H2O

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

Relative atomic mass of carbon = 12, hydrogen = 1, oxygen = 16

264 g
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 6

What is the maximum total mass (in grams) produced from 100 g of reactants?

100 g
EDDIE SAYS
Not a trick question - just the chance to show your understanding of the law of conservation of mass! Can you remember it? The law of conservation of mass states that the mass of reactants is equal to the mass of products, so if there are 100 g of reactants, then 100 g of products would be given.
• Question 7

This is a general chemical equation for when reactants A and B give products C and D.

A + B C + D

mass of A = 2 g

mass of B = 3 g

mass of C = 1 g

Use the above information to work out the mass of D.

Remember, the mass of the products is equal to the mass of the reactants.

4 g
EDDIE SAYS
Despite its appearance, this was just a simple addition and subtraction sum! The mass of the reactants = 2 + 3 = 5 g The mass of reactants = mass of products so the mass of C + mass of D must also equal 5 g. The mass of C = 1 g, so the mass of D = 5 - 1 = 4 g
• Question 8

Calculate how many grams of carbon monoxide are formed from 6 g of carbon.

2C + O2 2CO

Relative atomic mass of C = 12, O = 16

14 g
EDDIE SAYS
Can you remember how to do this? Find the relative formula mass of carbon: 12 x 2 = 24 Find the relative formula mass of carbon monoxide: 12 + 16 = 28 x 2 = 56 So, 24 g of carbon gives 56 g of carbon monoxide. 24 ÷ 6 = 4 so 4 times less carbon monoxide would be produced by 6 g of carbon, so 56 ÷ 4 = 14 g of carbon monoxide are produced.
• Question 9

Why is mass conserved in chemical reactions?

The number of atoms remains the same but they just swap places
EDDIE SAYS
Some similar sounding options here - did they trip you up? The number of atoms is the same as before, they are just rearranged. The overall mass stays the same.
• Question 10

In an industrial reaction, 20 g of a wanted substance was expected to be produced, but instead only 10 g was produced.

What was the percentage yield?

50%
EDDIE SAYS
Sometimes a question can be thrown in which looks really hard. When this happens, try not to panic - the answer may be much more obvious than at first glance! Even if you're not sure about percentage yield, hopefully, you can see that 10 g is half of 20 g, which means that only 50% of what was expected was actually produced. Well done for getting through all these calculations - they were fairly tough so you deserve to feel a bit pleased with yourself!
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