Example: When calcium carbonate (CaCO3) is heated calcium oxide is produced. You can use reacting mass calculations to calculate the mass of calcium oxide produced when heating 25 g of calcium carbonate.

     CaCO3     –>         CaO      +      CO2

Step 1: Calculate the amount, in moles, of 25 g of calcium carbonate (CaCO3)

    \[M_r(CaCO_3)=(1 \times 40) + (1\times12) + (3 \times 16) \]

    \[M_r(CaCO_3)= 100\]

    \[Amount = \frac{mass(g)}{M_r} \]

    \[Amount = \frac{25}{100} \]

    \[Amount = 0.25 mol \]

Step 2: Deduce the amount, in moles, of CaO produced from 0.25 mol of CaCO3.

This step involves using the ratio of CaCO3 to CaO from the chemical equation.

     CaCO3     –>         CaO      +      CO2

From the equation you can see that the ratio of CaCO3 to CaO is 1:1.

Therefore if you have 0.25 mol of CaCO3 this will produced 0.25 mol of CaO.

Step 3: Calculate the mass of 0.25 mol of CaO.

    \[M_r(CaO)=(1 \times 40) + (1 \times 16) \]

    \[M_r(CaO)= 56\]

    \[Mass(CaO)=amount \times M_r \]

    \[Mass(CaO)= 0.25 \times 56\]

    \[Mass(CaO)= 14g\]


A simple format for laying out this method can be used.

Example: What mass of ammonia (NH3) is formed when 7 g of nitrogen (N2) is combined with hydrogen (H2).