Using Moles to Balance Equations

Stoichiometry is a concept in chemistry that describes the relationship between the amounts of substances involved in a chemical reaction.

These ratios appear as whole numbers before each substance in the equation, which can be adjusted to balance the equation. We can calculate these numbers by using masses found by experimentation.

Example

To illustrate stoichiometry in action, let’s consider the reaction between sodium (Na) and chlorine (Cl) to form sodium chloride (NaCl). Suppose we have 46 g of sodium reacting with 71 g of chlorine. Using this information, we can balance the chemical equation.

First, we need to note the formulae of the substances involved in the reaction:

  • Sodium: Na
  • Chlorine: Cl2
  • Sodium chloride: NaCl

Calculating the number of moles

To calculate the number of moles of each substance involved in the reaction, use the formula:

Number of moles = Mass (g) ÷ Relative atomic mass.

Sodium:

For sodium, the relative atomic mass is 23. Therefore, the number of moles of Na is:

46 g ÷ 23

Number of moles of Na = 2

Chlorine:

For chlorine, the relative atomic mass is 35.5. Therefore, the number of moles of Cl2 is:

71 g ÷ (35.5 + 35.5)

Number of moles of Cl2 = 1

Sodium chloride:

For sodium chloride, the relative formula mass is (35.5 + 23) = 58.5. Therefore, the number of moles of NaCl is:

117 g ÷ 58.5

Number of moles of NaCl = 2

Balancing the equation

To balance the chemical equation, we need to ensure that the number of moles of each substance is in the lowest possible ratio. We do this by dividing all the numbers by the smallest value, which in this case is 1 (the number of moles of Cl2). Therefore:

  • Number of moles of NaCl = 2 ÷ 1 = 2
  • Number of moles of Cl2 = 1 ÷ 1 = 1
  • Number of moles of Na = 2 ÷ 1 = 2

Now that we have the correct number of moles for each substance, we can write the balanced chemical equation:

Na + Cl2 2NaCl

This equation shows that two moles of sodium react with one mole of chlorine molecules (Cl2) to produce two moles of sodium chloride. The balanced equation ensures that the law of conservation of mass is upheld, which states that the total mass of the reactants must be equal to the total mass of the products in a chemical reaction.

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