Concentration of Solutions Calculations (M = n ÷ V)

Key Concepts

• The concentration of a solution is usually given in moles per litre (mol L^-1 OR mol/L).
      This is also known as molarity.

• Concentration, or Molarity, is given the symbol M.
      A short way to write that the concentration of a solution of hydrochloric acid is 0.01 mol/L is to write [HCl]=0.01M
      The square brackets around the substance indicate concentration.

• M = n ÷ V
      M = concentration of solution in mol/L,
      n = moles of substance,
      V = volume of solution in litres (L)

• This formula can be re-arranged:
      n = M x V
      V = n ÷ M

Examples

1. M = n ÷ V

Calculate the concentration (molarity) of a sodium chloride solution containing 0.125 moles sodium chloride in 0.5 litres of water.

• M = n ÷ V in litres

• n = 0.125mol

• V in litres = 0.5L

• [NaCl(aq)] = M = 0.125 ÷ 0.5 = 0.25M (or 0.25mol/L or 0.25mol L^-1)

2. n = M x V

Calculate the moles of copper sulfate in 250mL of 0.02M copper sulfate solution.

• n = M x V

• M = 0.02M

• V = 250mL = 250 ÷ 1000 = 250 x 10^-3L = 0.250L (since there are 1000mL in 1L)

• n = 0.02 x 250 x 10^-3 = 0.005mol

3. V = n ÷ M

Calculate the volume of a 0.80M potassium bromide solution containing 1.6 moles of potassium bromide.

• V = n ÷ M

• n = 1.6mol

• M = 0.80M

• V = 1.6 ÷ 0.80 = 2.00L

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