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Concentration of Solutions (Molarity) Calculations

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 in mol/L or mol L-1, or Molarity, is given the symbol c (sometimes M).
        For a 0.01 mol L-1 HCl solution we can write :
            [HCl] = 0.01 mol L-1 (concentration implied by square brackets around formula)
            or
            c(HCl) = 0.01 mol L-1 (c stands for concentration, formula given in brackets)

  • c = n ÷ V
        where
          c = concentration of solution in mol L-1 (mol/L or M),
          n = moles of substance being dissolved (moles of solute),
          V = volume of solution in litres (L)

  • This formula can be re-arranged to find:
          moles of solute given molarity and volume of solution: n = c x V
          volume of solution given moles of solute and molarity: V = n ÷ c

Examples

1. Calculating Concentration (c = n ÷ V)

Calculate the concentration in mol L-1 (molarity) of a sodium chloride solution containing 0.125 moles sodium chloride in 0.5 litres of solution.

  1. Extract the data from the question
        n = 0.125 mol
        V = 0.5 L

  2. Write the equation:
        c = n ÷ V

  3. Substitute in the values and solve:
        [NaCl(aq)] = c(NaCl(aq)) = 0.125 ÷ 0.5 = 0.25 mol L-1 (or 0.25 mol/L or 0.25 M)

2. Calculating Moles of Solute (n = c x V)

Calculate the moles of copper sulfate in 250 mL of 0.02 mol L-1 copper sulfate solution.

  1. Extract the data from the question:
        c = 0.02 mol L-1
        V = 250 mL = 250 ÷ 1000 = 250 x 10-3 L = 0.250 L (since there are 1000 mL in 1 L)

  2. Write the equation:
        n = c x V

  3. Substitute in the values and solve:
       n = 0.02 x 250 x 10-3 = 0.005 mol

3. Calculating Volume of Solution (V = n ÷ M)

Calculate the volume of a 0.80 mol L-1 potassium bromide solution containing 1.6 moles of potassium bromide.

  1. Extract the data from the question:
        n = 1.6 mol
        c = 0.80 mol L-1

  2. Write the equation:
        V = n ÷ c

  3. Substitute in the values and solve:
        V = 1.6 ÷ 0.80 = 2.00 L
Practice Questions Homework Checker
For AUS-e-TUTE members:
  1. Click on the Concentration (molarity) drill link:
    Concentration (Molarity) drill
  2. Enter your username and password if prompted.
  3. Click the "New Question" button to begin the drill.
  4. Worked solutions are provided if you need some help!

Not an AUS-e-TUTE Member?

For AUS-e-TUTE members:
1. Enter the moles of solute.
2. Enter volume of solution (L).
3. Click the Calculate button.
4. Concentration of solution in mol/L or M will be shown.

moles of solute: mol
volume of solution: L
Click Calculate:
Calculation :
Concentration = mol L-1
To start again, click

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