Tesr learner’s knowledge of acids and bases

The topics covered in this Starter for ten activity are: pH and Kw, pH and acids, pH and bases, acid-base titrations, buffer solutions and more complex buffer solutions.

## Example question

Complete the table below showing some numbers and their common logarithmic values.

Number (n)  log10 n
0.001
0.1
0
3

As water is always slightly ionised, we can write the following equilibrium for water;

H2O(l) ⇌ H+(aq) + OH(aq) ΔH = +ive

As only a very small amount of the water is ionised, we define a new equilibrium constant for this equilibrium called the ionic product of water, Kw;

Kw = [H+(aq)] [OH(aq)]

Like any other equilibrium constant, the value of Kw depends on the temperature of the equilibrium.

(a) Predict what effect increasing the temperature will have on the pH of pure water.

Calculate the pH of pure water (to 2 dp) at each of the temperatures below;

(i) 10 °C, Kw = 0.29 × 10–14 mol2 dm–6

(ii) 25 °C, Kw = 1.01 × 10–14 mol2 dm–6

(iii) 40 °C, Kw = 2.92 × 10–14 mol2 dm–6

Complete the paragraph below;

As the temperature decreases, water becomes (more acidic / less acidic / remains neutral).

## Notes

The full question and answer sheet are available from the ‘Downloads’ section below. An editable version is also available.