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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.

Extract the data from the question
n = 0.125 mol
V = 0.5 L
Write the equation:
c = n ÷ V
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.

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)
Write the equation:
n = c x V
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.

Extract the data from the question:
n = 1.6 mol
c = 0.80 mol L^-1
Write the equation:
V = n ÷ c
Substitute in the values and solve:
V = 1.6 ÷ 0.80 = 2.00 L

Practice Questions

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Concentration (Molarity) drill
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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.

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