Concentration of solutions
Concentration measures how much solute is dissolved in a given volume of solution. It is usually given in mol/dm³ (moles per cubic decimetre).
Key idea
Concentration (mol/dm³) = moles of solute ÷ volume (dm³). Remember 1 dm³ = 1000 cm³, so divide a cm³ volume by 1000 first.
Empirical formula
The empirical formula is the simplest whole-number ratio of atoms of each element in a compound. To find it: divide the mass (or percentage) of each element by its Ar to get moles, then divide by the smallest number of moles to get the ratio.
Worked example
A compound is 40% carbon, 6.7% hydrogen, 53.3% oxygen by mass. Find the empirical formula. (C=12, H=1, O=16)
Moles: C = 40 ÷ 12 = 3.33; H = 6.7 ÷ 1 = 6.7; O = 53.3 ÷ 16 = 3.33.
Divide by smallest (3.33): C = 1, H = 2, O = 1.
Empirical formula = CH2O.
Remember
- Moles of solute = concentration × volume (in dm³).
- The molecular formula is a whole-number multiple of the empirical formula (e.g. glucose C6H12O6 is 6 × CH2O).