Concentration
Learning Objectives
The concept of solution concentration measured with reference to moles (mol L⁻¹) or with reference to mass or volume (g L⁻¹ , mg L⁻¹ , %(m/m), %(m/v), %(v/v), ppm, ppb) in selected domestic, environmental, commercial or industrial applications, including unit conversions
Summary Notes
Units of concentration
Concentration of a substance can be determined by finding the amount of solute per solvent.
The unit of concentration depends on the units for the quantity of solute and quantity of solution.
If the unit of concentration desired is mol/L, it means the quantities required are:
mole of solute
volume of solvent
The common units of concentration are:
mole per liter (mol/L or M)
mass per volume (g/L)
percentages
by mass (%w/w) or (%m/m)
by volume (%v/v)
by mass per volume (%m/v)
Other units of concentration are:
parts per million (ppm)
parts per billion (ppb)
Molar concentration
The most common way of representing concentration.
Units are mol/L or M.
c = concentration (mol L⁻¹ or M) n = amount of substance in moles (mol) v = volume of solvent (L)
Eg. What mass of sodium nitrate is needed to make a 500 mL solution of 0.17 M?
c = n / v n = m/M
n = cv m = nM
n = 0.17 x 0.5 m = 0.085 x 85
n = 0.085 mol m = 7.23 g
Dilution
A concentrated solution may be diluted by the addition of more solvent.
A dilute solution may be made more concentrated by the addition of more solute.
When a solution is diluted, the amount of substance (moles) present in that solution still stays the same despite the volume changes.
This then gives rise to the equation:
c₁ = initial concentration (M) v₁ = initial volume (L) c₂ = final concentration (M) v₂ = final volume (L)
Eg. What volume of 20 M hydrochloric acid is needed to prepare 250 mL of a 2.0 M hydrochloric acid solution?
c₁v₁ = c₂v₂
v₁ = (c₂v₂) / c₁
v₁ = (2 x 0.25) / 20
v₁ = 0.025 L
Videos to help your understanding
How to Visualise One Part per Million
Dilution and Sample Calculations
Practice Material
Teacher's Tip:
Always keep an eye out for the units that you are required to give your concentration in. Do the conversion individually first before putting it into the solute/solvent equation. This reduces the mistakes that you may make during conversion.