Concentration of solutions
A solutionMixture formed by a solute and a solvent. forms when a soluteThe dissolved substance in a solution. dissolveWhen a substance breaks up and mixes completely with a solvent to produce a solution. in a solventThe liquid in which the solute dissolves to form a solution.. The concentrationA measure of the mass or amount of solute dissolved in a given volume of solvent or solution. of a solution is a measure of how 'crowded' the solute particles are. The more concentrated the solution, the more particles it contains in a given volumeThe volume of a three-dimensional shape is a measure of the amount of space or capacity it occupies, eg an average can of fizzy drink has a volume of 330 ml..
Calculating concentration
The concentration of a solution can be calculated using:
- the massThe amount of matter an object contains. Mass is measured in kilograms (kg) or grams (g). of dissolvedA substance is said to be dissolved when it breaks up and mixes completely with a solvent to produce a solution. solute in grams, g
- the volume of solution (or solvent) in cubic decimetres, dm3
The units for concentration can also be shown as g dm-3, but this means the same as g/dm3.
Worked example
8 g of sodium hydroxide is dissolved in 2 dm3 of water. Calculate the concentration of the sodium hydroxide solution formed.
concentration = \(\frac{\textup{mass~of~solute~in~g}}{\textup{volume~in~dm}^3}\)
concentration = \(\frac{\textup{8~g}}{\textup{2~dm}^3}\)
concentration = 4 g/dm3
Volume units
Apparatus used to measure volumes is usually marked in cm3 or ml. Although these are different units, they describe the same volume. For example, 250 ml = 250 cm3.
Volumes used in concentration calculations must be in dm3, not in cm3 or ml. It is useful to know that 1 dm3 = 1000 cm3. This means:
- divide by 1000 to convert from cm3 to dm3
- multiply by 1000 to convert from dm3 to cm3
For example, 250 cm3 is 0.25 dm3 (250 梅 1000). It is often easiest to convert from cm3 to dm3 before continuing with a concentration calculation.
Question
100 cm3 of dilute hydrochloric acid contains 0.5 g of dissolved hydrogen chloride. Calculate the concentration of the acid in g/dm3.
volume of acid = 100 梅 1000 = 0.1 dm3
concentration of acid = \(\frac{\textup{0.5}}{\textup{0.1}}\)
= 5 g/dm3
Calculating the mass of solute
Rearranging the equation for concentration allows the mass of solute to be calculated:
mass of solute in g = concentration in g/dm3 脳 volume in dm3
Worked example
A solution of sodium chloride has a concentration of 10 g/dm3. What mass of sodium chloride is dissolved in 2 dm3 of the solution?
mass of solute in g = concentration in g/dm3 脳 volume in dm3
= 10 g/dm3 脳 2 dm3
= 20 g
Higher tier
The concentration of a solution can be changed:
- concentration can be increased by dissolving more solute in a given volume of solution - this increases the mass of the solute
- concentration can be increased by allowing some of the solvent to evaporate - this decreases the volume of the solution