Calculating concentrations - Higher - Monitoring chemical reactions - Higher - OCR Gateway - GCSE Chemistry (Single Science) Revision - OCR Gateway

Calculating concentrations - Higher

The concentration of a solution can be calculated using:

\(concentration\ in\ g/dm^{3}\ = \tfrac{mass\ of\ solute\ in\ g}{volume\ in\ dm^{3}}\)

The is the substance that is . The volume is the volume of the solution or the .

Example

5.0 g of sodium hydroxide is dissolved in 250 cm³ of water. Calculate the concentration of the sodium hydroxide solution formed in g/dm³.

250 cm³ = 250 ÷ 1,000 = 0.250 dm³

\(concentration\ = \tfrac{mass}{volume}\)

\(concentration\ = \tfrac{5.0 g}{0.250 dm^{3}}\)

= 20 g/dm³

It is often more useful to know the concentration of a in mol/dm³ so that the of reactant in a given volume can be calculated.

The concentration of a solution can be calculated using:

\(concentration\ in\ mol\ dm^{3}\ = \tfrac{amount\ of\ solute\ in\ mol}{volume\ in\ dm^{3}}\)

Example

0.50 mol of sodium hydroxide is dissolved in 2.0 dm³ of water. Calculate the concentration of the sodium hydroxide solution formed in mol/dm^-3.

\(concentration\ = \tfrac{amount}{volume}\)

\(concentration\ = \tfrac{0.5\ mol}{2.0\ dm^{3}}\)

concentration = 0.25 mol/dm³

100 cm³ of dilute hydrochloric acid contains 0.02 mol of dissolved hydrogen chloride. Calculate the concentration of the acid in mol/dm³.