Reacting masses

• Reacting Masses: Calculations used to determine how much of a reactant is required or how much product is formed in a chemical reaction.

• or M (relative formula mass or molar mass) :The mass of 1 mole of a substance.

Main Formulae:

Moles = Mass (g) /

Steps for Reacting Mass Calculations:

1. Write a balanced equation.

2. Find the moles of the known substance using

3. Use mole ratios from the equation to find the moles of the unknown substance.

4. Convert moles to mass using

5. NB: May also have to use c = n/v and/or c = nv for reactions involving solutions.

Why Mole Ratios Matter:

The coefficients in a balanced chemical equation tell you the ratio of moles of reactants and products. These ratios must be used when converting between substances.

Example 1

Determine the maximum mass of aluminum that can be formed from 1024 g of aluminum oxide.

Example 2:

Question: What is the maximum mass of water that can be produced when 6.00 g of hydrogen reacts with oxygen?

Step 1: Moles of H₂:

of = 2

Step 2: Mole ratio H₂: H₂O = 2:2 → 3.00 mol H₂ makes 3.00 mol H₂O

Step 3: Mass of H₂O:

of H₂O = 18

Example 3:

Question: What is the minimum mass of iron required to react completely with 7.0 moles of oxygen gas.

Moles of O2 = 7.0 mol

• Mole ratio: : Fe = 3:4 → moles of Fe = 7.0 x 4/3 = 9.33

• of Fe = 56

• Mass of Fe = 9.33 x 56 = 522.7g = 520g (2 s.f.)

Give all answers to an appropriate number of significant figures