Solute, solvent and solution
- The solute is the substance that dissolves
- The solvent is the liquid in which the solute dissolves
- The solution is the homogeneous mixture that results
- Most reactions in solution chemistry happen between solutes dissolved in water (an aqueous solution)
What concentration means
- Concentration is the amount of solute dissolved in a fixed volume of the solution
- The greater the amount of solute in a given volume, the more concentrated the solution
- Edexcel uses two units of concentration:
- g/dm³ — grams of solute per dm³ of solution
- mol/dm³ — moles of solute per dm³ of solution
The key formula
- For concentration in mol/dm³:
c=Vn
- where:
- c = concentration in mol/dm³
- n = moles of solute
- V = volume of solution in dm³
- Rearranged:
- moles = concentration × volume (in dm³)
- volume (in dm³) = moles ÷ concentration
Unit conversion:
- cm³ → dm³: divide by 1000
- dm³ → cm³: multiply by 1000
Example. What is the concentration, in mol/dm³, of a solution made by dissolving 8.0 g of NaOH in water to make 250 cm³ of solution?
- Mr(NaOH) = 40, so moles of NaOH = 8.0 / 40 = 0.20 mol
- Volume = 250 / 1000 = 0.25 dm³
- c = 0.20 / 0.25 = 0.80 mol/dm³
Example. 25.0 cm³ of 0.10 mol/dm³ sodium carbonate is exactly neutralised by dilute hydrochloric acid. The equation is:
Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂
If 20.0 cm³ of the acid is required, what is its concentration?
- Moles of Na₂CO₃ = 0.10 × (25.0 / 1000) = 0.00250 mol
- Mole ratio HCl : Na₂CO₃ = 2 : 1, so moles of HCl = 2 × 0.00250 = 0.00500 mol
- Volume of HCl = 20.0 / 1000 = 0.020 dm³
- c(HCl) = 0.00500 / 0.020 = 0.25 mol/dm³