You can calculate the amount of a substance in moles in a solution if you know the volume and concentration. You can also work out the concentration of an acid reacting with an alkali, or vice versa.
Concentration in mol/dm3 = amount in mol ÷ volume in dm3
This equation can be rearranged to find the amount of solute or volume of solution.
Divide a volume in cm3 by 1,000 to convert it to a volume in dm3.
0.5 mol of solute is dissolved in 250 cm3 of solution. Calculate its concentration.
250 cm3 = 250 ÷ 1,000 = 0.25 dm3
Concentration = 0.5 ÷ 0.25 = 2.0 mol/dm3
Rearranging the equation: amount = concentration × volume
Calculate the amount of solute dissolved in 2 dm3 of a 0.1 mol/dm3 solution.
Amount = 0.1 × 2 = 0.2 mol
Rearranging the equation: volume = amount ÷ concentration
Calculate the volume of a 2 mol/dm3 solution that contains 0.5 mol of solute.
Volume = 0.5 ÷ 2 = 0.25 dm3
Note that 0.25 dm3 is the same as 250 cm3 (0.25 × 1,000).
Concentrations can also be expressed in grams per decimetre cubed (g/dm3). To convert the two different units of concentration, use the relative formula mass (Mr).