By Jack Brubaker – Updated Aug 30, 2022

In every laboratory, chemists perform precise calculations to determine the exact amounts of reactants and products required for a given reaction. This discipline, known as stoichiometry, relies on the concept of the mole—a fundamental unit representing 6.022 × 10²³ formula units of a substance. The mole is defined so that one mole of a compound weighs exactly its formula weight in grams, where the formula weight is the sum of the atomic weights of all atoms in the molecular formula. For example, sodium chloride (NaCl) has a formula weight of 58.44 g mol^‑1, meaning 58.44 g of NaCl contains 6.022 × 10²³ formula units.

Step 1: Write a Balanced Chemical Equation

Start by drafting a balanced equation for the reaction of interest. A balanced equation contains the same number of each type of atom on both sides of the arrow. For instance, the reaction between calcium hydroxide (Ca(OH)₂) and hydrochloric acid (HCl) is written as:

Ca(OH)₂ + HCl → CaCl₂ + H₂O

This version is unbalanced: the left side has two oxygen atoms and one chlorine atom, whereas the right side has two chlorine atoms and one oxygen atom. The correctly balanced equation is:

Ca(OH)₂ + 2 HCl → CaCl₂ + 2 H₂O

Now each element appears in equal numbers on both sides.

Step 2: Determine Formula Weights

Next, calculate the formula weight of every compound in the balanced equation. Formula weight is obtained by summing the atomic weights of all atoms present. For example, calcium chloride (CaCl₂) contains one calcium atom (40.08 g mol^‑1) and two chlorine atoms (35.45 g mol^‑1 each), yielding:

(1 × 40.08) + (2 × 35.45) = 100.98 g mol^‑1

Step 3: Convert Known Mass to Moles

When you know the mass of one reactant or product, convert that mass to moles by dividing by its formula weight. Using the balanced reaction above, suppose you have 10 g of calcium hydroxide. The formula weight of Ca(OH)₂ is 74.10 g mol^‑1, so the moles of Ca(OH)₂ present are:

10 g ÷ 74.10 g mol^‑1 = 0.135 mol (≈ 0.13 mol)

Step 4: Apply the Stoichiometric Ratio

Use the coefficients from the balanced equation to relate the known moles to the unknown moles. In Ca(OH)₂ + 2 HCl → CaCl₂ + 2 H₂O, the stoichiometric ratio of HCl to Ca(OH)₂ is 2:1. Therefore:

0.135 mol Ca(OH)₂ × (2 mol HCl ÷ 1 mol Ca(OH)₂) = 0.270 mol HCl

Thus, 0.270 moles of HCl are required to react completely with 10 g of Ca(OH)₂.

Materials Needed

• Periodic table of elements (for atomic weights)
• Scientific calculator (for division and multiplication)

By following these systematic steps, you can accurately determine the mole quantities necessary for any chemical reaction, ensuring precise stoichiometric calculations in the laboratory.