Moles and reacting masses
Connect the mass of a substance to the amount used in a chemical equation.
Before you begin
- Balanced equations
- Relative atomic masses
Convert mass into amount
The mole measures amount of substance. Molar mass is the mass per mole, commonly expressed in grams per mole. Divide a mass in grams by the molar mass in g/mol to obtain an amount in moles. For a compound, add the atomic contributions in its formula. Keep the formula's subscripts when calculating molar mass.
Use the balanced equation as a mole ratio
Coefficients in a balanced equation describe ratios of particles and amounts in moles, not usually ratios of masses. Convert the known mass to moles, use the coefficient ratio, then convert the required amount back to mass. A prediction assumes the stated reactant is fully used and any other needed reactant is available in sufficient quantity.
Worked example
Using M(Mg) = 24 g/mol, find the amount in 6 g of magnesium.
Show the worked solution
- Write n = m/M with consistent gram units.
- Substitute n = 6/24.
- Check that a quarter of the mass of one mole corresponds to a quarter mole.
0.25 mol.
Try it yourself
In 2H₂ + O₂ → 2H₂O, how many moles of O₂ are required for 4 mol H₂?
Common mistakes
- A balanced equation's coefficient ratio is not a direct mass ratio.
What can you explain now?
Find the mass of 0.5 mol CO₂ using C = 12 and O = 16 g/mol.
Compare with the explanation
22 g.
M(CO₂) = 12 + 2 × 16 = 44 g/mol. Multiply 0.5 mol by 44 g/mol.
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