Chemical Formulae, Equations and Calculations

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:

$c = \frac{n}{V}$

• 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?

• M_r(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