The ionic product of water, Kw

At 40 °C the ionic product of water, K_w = 2.92 × 10^–14 mol² dm^–6

Give the expression for K_w

Calculate the pH of pure water at 40 °C Give your answer to 2 decimal places.

35.0 cm³ of 0.150 mol dm^–3 aqueous sodium hydroxide are mixed with 20.0 cm³ of a 0.100 mol dm^–3 solution of hydrochloric acid. The temperature of the solution formed is 40 °C

Calculate the pH of the solution formed. Give your answer to 2 decimal places.

Values of the ionic product of water (K_w) at different temperatures are given.

Which statement is correct?

Calcium hydroxide is almost insoluble in water, but it reacts with dilute hydrochloric acid.

A student adds 100 cm³ of 0.100 mol dm^–3 hydrochloric acid to 0.600 g of solid calcium hydroxide.

Show, by calculation, that the calcium hydroxide is in excess.

The final mixture contains a saturated solution of Ca(OH)₂ at 293 K

At 293 K

• the solubility of Ca(OH)₂ in this solution is 0.400 g dm^–3

• K_w = 6.80 × 10^–15 mol² dm^–6

Calculate the pH of this solution. Give your answer to two decimal places.

Section B

all questions in this section.

Which change causes the pH of 10 cm³ of 1.0 mol dm^–3 NaOH to be halved at 298 K?

This question is about acids and bases.

Calculate the pH of a 0.150 mol dm^–3 solution of ethanoic acid at 25^oC Give your answer to 2 decimal places.

For ethanoic acid, K_a = 1.74 x 10^–5 mol dm^–3 at 25^oC

Strontium is an element in Group 2.

Calculate the pH of a 0.0100 mol dm^–3 solution of strontium hydroxide at 10^oC You may assume that strontium hydroxide is completely dissociated in this solution.

At 10^oC the ionic product of water, K_w = 2.93 x 10^–15 mol² dm^–6

The pH of a barium hydroxide solution is lower at 50 °C than at 10 °C

At 50 °C a 25 cm³ sample of this barium hydroxide solution was neutralised by 22.45 cm³ of hydrochloric acid added from a burette.

Deduce the volume of this hydrochloric acid that should be added from a burette to neutralise another 25 cm³ sample of this barium hydroxide solution at 10 °C

Explain your answer

State how a buffer solution can be made from solutions of potassium hydroxide and ethanoic acid.

Give an equation for the reaction between potassium hydroxide and ethanoic acid.

State how this buffer solution resists changes in pH when a small amount of acid is added.

How buffer solution is made

Equation

How buffer solution resists pH change

A buffer solution is made by adding 2.00 g of sodium hydroxide to 500 cm³ of 1.00 mol dm^–3 ethanoic acid solution.

Calculate the pH of this buffer solution at 25 °C

Give your answer to 2 decimal places.

For ethanoic acid, K_a = 1.74 x 10^–5 mol dm^–3 at 25 °C

This question is about pH.

Pure water dissociates slightly.

The ionic product of water, K_w = [H⁺][OH⁻]

Explain why [H₂O] is not shown in the K_w expression.

Table 5 shows how K_w varies with temperature.

Explain why the value of K_w increases as the temperature increases.

Give the expression for pH.

Calculate the pH of pure water at 50 °C Give your answer to 2 decimal places.

Explain why water is neutral at 50 °C

Calculation

A pH meter is calibrated using a calibration graph. To create the calibration, the pH meter is used to measure the pH of separate solutions, each with a known, accurate pH.

Figure 3 shows the calibration graph.

Use Figure 3 to give the true pH value when the pH meter reading is 5.6

Suggest why the pH probe is washed with distilled water between each of the calibration measurements.

The calibrated pH meter is used to monitor the pH during a titration of hydrochloric acid with sodium hydroxide.

Explain why the volume of sodium hydroxide solution added between each pH measurement is smaller as the end point of the titration is approached.

Table 6 shows data about some indicators.

The student plans to do the titration again using one of the indicators in Table 6 to determine the end point.

State why all three of the indicators in Table 6 are suitable for this titration.

36.25 cm³ of 0.200 mol dm^–3 sodium hydroxide solution are added to 25.00 cm³ of 0.150 mol dm^–3 hydrochloric acid.

Calculate the pH of the final solution at 25 ºC

Which statement about pH is correct?

Which is the concentration of NaOH(aq), in mol dm^–3, that has pH = 14.30?

This question is about different pH values.

For pure water at 40 °C, pH = 6.67 A student thought that the water was acidic.

Explain why the student was incorrect.

Determine the value of K_w at this temperature.

Sodium hydroxide solution was added gradually from a burette to 25 cm³ of 0.080 mol dm^–3 propanoic acid at 25 °C The pH was measured and recorded at regular intervals.

The results are shown in Figure 4.

Use Figure 4 to determine the value of K_a for propanoic acid at 25 °C

Show your working.

Suggest which indicator is the most appropriate for the reaction in Question 09.2 ? Tick (✓) one box.

Indicator	pH range	Tick () one box
methyl orange	3.1 – 4.4
bromothymol blue	6.0 – 7.6
cresolphthalein	8.2 – 9.8
indigo carmine	11.6 – 13.0

A student prepared a buffer solution by adding 0.0136 mol of a salt KX to 100 cm³ of a 0.500 mol dm^–3 solution of a weak acid HX and mixing thoroughly.

The student then added 3.00 × 10^–4 mol of potassium hydroxide to the buffer solution.

Calculate the pH of the buffer solution after adding the potassium hydroxide.

For the weak acid HX at 25 °C the value of the acid dissociation constant,

Give your answer to two decimal places.

A buffer solution has a constant pH even when diluted.

Use a mathematical expression to explain this.

Hydrochloric acid is a strong acid and ethanoic acid is a weak acid.

State the meaning of the term strong acid.

In an experiment, 10.35 cm³ of 0.100 mol dm^–3 hydrochloric acid are added to 25.0 cm³ of 0.150 mol dm^–3 barium hydroxide solution.

Calculate the pH of the solution that forms at 30 °C

Give your answer to 2 decimal places.

The pH of water at 30 °C is 6.92

Give the reason why water is neutral at this temperature.

Identify the oxide that could react with water to form a solution with pH = 2

Tick (✓) one box.

Give the expression for the acid dissociation constant (K_a) for ethanoic acid

A buffer solution contains 0.025 mol of sodium ethanoate dissolved in 500 cm³ of 0.0700 mol dm^–3 ethanoic acid at 25 °C A sample of 5.00 cm³ of 2.00 mol dm^–3 hydrochloric acid is added to this buffer solution.

Calculate the pH of the solution formed.

For ethanoic acid, K_a = 1.76 × 10^–5 mol dm^–3 at 25 °C