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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).