Calculations based on chemical equations

When carrying out calculations based on chemical equations, the coefficients of a balanced chemical equation are very important. With these coefficients, we can write a ratio of any two or more substances in the chemical equation. This is known as reaction stoichiometry.

In a chemical reaction, the reaction stoichiometry is the mole ratio of all the reactants and products in the reaction. By knowing the mole ratio of the substances in the reaction, it enables us to calculate the mass of the reactants consumed and the mass of products formed.

The following are the different types of calculations involving chemical equation.

1. Mole-mole calculation
2. Mass-mass calculation
3. Mass-mole calculation
4. Excess reactant and limiting reagent problems

Example

Let's take a look at the balanced chemical equation for the synthesis of water:

2 H₂ (g) + O₂ (g) 2 H₂0 (l)

The coefficient in front of O2 (g) is by default "1". Coefficient of "1" is usually not written in.

Here are some ratios that we can write from the above balanced chemical equation:

1. 2 molecules of H₂ (g) requires 1 molecule of O₂ (g) to produce 2 molecules of H₂0 (l).

The important thing is, the ratio is 2:1:2 for the substances H₂ : O₂ : H₂0.

     In any combination of any two substances, the coefficients in the balanced chemical equation tells us:
       a. the ratio between H2 : O2 is always 2:1.
       b. the ratio between H2 : H20 is always 2:2, or 1:1.
       c. the ratio between O2 : H20 is always 1:2.

2. If we scale up 2 times, then:

      4 molecules of H2(g) requires 2 molecules of O2 (g) to produce 4 molecules of H20 (l).

This ratio of 2:1:2 is always maintained no matter how much we scale up.

3. If we scale up 12 times, then:

    2 dozens molecules of H2 (g) requires 1 dozen molecules of O2 (g) to produce 2 dozens molecules of H20 (l).

This ratio of 2:1:2 is always maintained no matter how much we scale up.

4. If we scale up to a mole, then:

    2 moles of H2 (g) requires 1 mole of O2 (g) to produce 2 moles of H20 (l).

This ratio of 2:1:2 is always maintained no matter how much we scale up.

The important thing is, the ratio is always 2:1:2 for the substances H₂ : O₂ : H₂0.

The coefficients give the mole ratio of the reactants and products.

       a. the ratio between H2 : O2 is always 2:1.
       b. the ratio between H2 : H20 is always 2:2, or 1:1.
       c. the ratio between O2 : H20 is always 1:2.

Content suitability

BCIT courses: CHEM 0011