pH curves & indicators

This question is about acids and bases.

Nitric acid is a strong acid.

Calculate the pH of 0.150 mol dm^–3 nitric acid.

Give your answer to 2 decimal places.

Propanoic acid is a weak acid.

Calculate the concentration of a solution of propanoic acid with pH = 2.89 at 25 °C

The acid dissociation constant (K_a) for propanoic acid at 25 °C

A student records the pH as an alkali is added to an acid.

Figure 3 shows the pH curve for this reaction.

Which of these combinations gives the pH curve shown in Figure 3 ?

Different indicators change colour across different pH ranges.

Buffer solution B is made by dissolving 0.656 g of sodium ethanoate (M_r = 82.0) in 50 cm³ of 0.120 mol dm^–3 ethanoic acid.

This buffer solution has pH = 4.87

Explain, qualitatively, how this buffer solution resists pH change when a small amount of sodium hydroxide solution is added.

Calculate the pH change that occurs when 1.50 cm³ of 0.200 mol dm^–3 HCl(aq) are added to buffer solution B.

Give your answer to 2 decimal places.

The acid dissociation constant (K_a) for ethanoic acid at 25 °C is 1.78 × 10^–5 mol dm^–3

Figure 4 shows how the pH changes as 0.100 mol dm^–3 sodium hydroxide solution is added to 25.0 cm³ of 0.0800 mol dm^–3 aqueous propanoic acid at 298 K

Propanoic acid is a weak acid.

State the meaning of weak in this context.

Suggest why a student doing an experiment to produce the curve in Figure 4 would add the sodium hydroxide solution dropwise around the equivalence point.

Give an expression for K_a for propanoic acid (CH₃CH₂COOH).

Use this expression to show that pH = pK_a when half of the propanoic acid has reacted with sodium hydroxide.

Use the pH from Figure 4, when half of the propanoic acid has reacted, to calculate

When sodium hydroxide solution is added to aqueous propanoic acid, the solution formed acts as a buffer when between 5 cm³ and 15 cm³ have been added.

Explain why the pH stays approximately constant during this part of the experiment.

Methyl orange and universal indicator are not suitable indicators for the titration of solutions of propanoic acid with sodium hydroxide.

State the reason why each indicator is not suitable.

Universal indicator

This question is about weak acids.

Table 5 shows the pH ranges of some indicators.

Identify the indicator that is most suitable for use in a titration between propanoic acid and sodium hydroxide.

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

Calculate the pH of a 0.100 mol dm^–3 propanoic acid solution. Give your answer to 2 decimal places.

For propanoic acid, pK_a = 4.87

For butanoic acid, K_a = 1.51 × 10^–5 mol dm^–3

20.0 cm³ of 0.100 mol dm^–3 sodium hydroxide solution are added to 25.0 cm³ of 0.100 mol dm^–3 butanoic acid solution.

Calculate the pH of the solution formed.

A student plans to titrate butanoic acid solution with a solution of ethylamine.

Explain why this titration could not be done using an indicator.

Equal volumes of two solutions, each with the same concentration, are mixed together at 298 K

Which two solutions, when mixed, form a solution with a pH >7?

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

A 0.10 mol dm^–3 aqueous solution of an acid is added slowly to 25 cm³ of a 0.10 mol dm^–3 aqueous solution of a base.

Which acid–base pair has the highest pH at the equivalence point?

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.

Some compounds with different molecular formulas have the same relative molecular mass to the nearest whole number.

A dicarboxylic acid has a relative molecular mass of 118, to the nearest whole number.

Deduce the molecular formula of the acid.

Molecular formula

A student dissolved some of the dicarboxylic acid from Question 10.1 in water and made up the solution to 250 cm³ in a volumetric flask. In a titration, a 25.0 cm³ sample of the acid solution needed 21.60 cm³ of 0.109 mol dm³ sodium hydroxide solution for neutralisation.

Calculate the mass, in g, of the dicarboxylic acid used. Give your answer to the appropriate number of significant figures.

Compounds with molecular formula C₆H₁₄O₂ also have a relative molecular mass of 118 to the nearest whole number. These include the diol shown.

Deduce the number of peaks in the¹H NMR spectrum of this diol.

Draw the structure of a different diol also with molecular formula C₆H₁₄O₂ that has a ¹H NMR spectrum that consists of two singlet peaks.

The dicarboxylic acid in question 10.1 and the isomers of C₆H₁₄O₂ in Questions 10.3 and 10.4 all have a relative molecular mass of 118

State why the dicarboxylic acid can be distinguished from the two diols by high resolution mass spectrometry using electrospray ionisation.

Which indicator should be used in a titration to find the concentration of a solution of methylamine using 0.010 mol dm^–3 hydrochloric acid?