# Concentration of solutions

A forms when a in a . The 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 .

## Calculating concentration

The concentration of a solution can be calculated using:

• the of 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 =

concentration =

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 ÷ 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 ÷ 1000 = 0.1 dm3

concentration of acid =

= 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 × 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 × volume in dm3

= 10 g/dm3 × 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
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