Chemistry · Higher
Quantitative Chemistry – Moles & Concentration
Relative formula mass, the mole, calculating reacting masses and solution concentration.
Quantitative chemistry lets you calculate how much product you can make and how concentrated solutions are. The mole is the central unit.
Relative formula mass and moles
**Mr** – sum of relative atomic masses (Ar) in a formula.
e.g. H₂O: (2 × 1) + 16 = 18
**Mole** – amount containing Avogadro’s number of particles (6.02 × 10²³).
mass = moles × Mr
moles = mass / Mr
Worked example
How many moles in 36 g of water (Mr = 18)?
Solution
moles = 36 / 18 = **2 mol**
Concentration
concentration (mol/dm³) = moles / volume (dm³)
1 dm³ = 1000 cm³
Convert cm³ to dm³ by dividing by 1000.
Also: concentration (g/dm³) = mass / volume (dm³)
Worked example
2 mol of solute in 0.5 dm³. Concentration?
Solution
2 / 0.5 = **4 mol/dm³**
Reacting masses
1. Write a balanced equation
2. Work out moles of known substance
3. Use the ratio from the equation
4. Convert moles of required substance to mass
Practice questions
Try these without looking at the answers first.
Q1.Mr of CO₂? (C=12, O=16)
Show answer
44
Q2.Moles in 10 g of CaCO₃ (Mr=100)?
Show answer
0.1 mol
Q3.0.25 mol in 500 cm³. Concentration in mol/dm³?
Hint: 500 cm³ = 0.5 dm³
Show answer
0.5 mol/dm³