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 (

**CaCO _{3}**

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**.

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- Question 1

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

CaCO_{3} CaO + CO_{2 }

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

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- Question 3

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Propane burns and gives carbon dioxide and water:

C_{3}H_{8} + 5O_{2} 3CO_{2} + 4H_{2}O

How many grams of water are produced when 22 g of propane (C_{3}H_{8}) burn? (*Just write the number*).

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

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- Question 5

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propane burns with oxygen to produce carbon dioxide and water:

C_{3}H_{8} + 5O_{2 } 3CO_{2} + 4H_{2}O

How many grams of carbon dioxide are produced when 88 g of propane (C_{3}H_{8}) burn? (*just write the number*).

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

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- 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

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