Conservation of mass
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  • Introduction
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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 quality and 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 1000g of calcium carbonate decomposes?

 

Answer: In order to calculate this without knowing at least the amount of calcium oxide made from 1000g of calcium carbonate using conservation of mass, 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, so:

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:

100g of calcium carbonate gives 44g of carbon dioxide

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

  • Question 1
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This is the symbol equation for the thermal decomposition of calcium carbonate:

CaCO3 CaO + CO

 

How much calcium oxide is produced from 500g of calcium carbonate? Type the number of grams (just write the number).

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

  • Question 2
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Hydrogen and chlorine produce hydrochloric acid:

H2 + Cl2 2HCl

 

How much HCl acid is made from 6g of hydrogen? Type your answer in grams (just write the number).

Relative atomic mass of hydrogen = 1 and chlorine = 35.5

  • Question 3
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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? (Just write the number).

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

  • Question 4
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Hydrogen and chlorine produce hydrochloric acid:

H2 + Cl2 2HCl

 

How much HCl acid is made from 35.5 g of chlorine? Type your answer in grams (just type the number).

Relative atomic mass of hydrogen = 1, chlorine = 35.5

  • Question 5
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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? (just write the number).

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

  • Question 6
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What is the maximum total mass (in grams) produced from 100g of reactants?

1g

50g

99g

100g

  • Question 7
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This is a general chemical equation for when reactants A and B give products C and D.

A + B C + D

 

mass of A = 2g

mass of B = 3g

mass of C = 1g

 

Use the above information to work out the mass of D. Remember, the mass of the products is equal to the mass of reactants.

1g

2g

3g

4g

  • Question 8
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Calculate how many grams of carbon monoxide are formed from 6g of carbon. 

2C + O2 2CO

 

Relative atomic mass of C = 12, O = 16

 

(just write the number in the answer box below).

                                    

  • Question 9
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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

  • Question 10
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In an industrial reaction, 20g of a wanted substance was expected to be produced, but instead only 10g was produced. What was the percentage yield?

45%

50%

55%

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