Weak acids & Ka

This question is about acids and bases.

Chloroethanoic acid, Cl CH₂COOH, is a weak monobasic acid.

Write the expression for the acid dissociation constant, K_a, of Cl CH₂COOH.

The expression for the acid dissociation constant, K_a, of Cl CH₂COOH can be simplified to:

State one approximation that allows the expression from (a)(i) to be simplified to Expression 19.1.

A student carries out an experiment to determine the pK_a value of a solution of Cl CH₂COOH.

• The concentration of Cl CH₂COOH is 0.090 mol dm^–3.

• The pH of Cl CH₂COOH is 1.95.

Use Expression 19.1 to calculate the pK_a value of Cl CH₂COOH.

Give your answer to 2 decimal places.

Draw a best-fit curve on the graph and calculate the concentration of the CH₃COOH solution.

CH₃COOH concentration =........ mol dm^–3

The end point of the titration can also be found by observing the colour change of an indicator.

The pH ranges of some indicators are shown in the table.

Identify the indicator in the table that would be suitable to observe the end point of the titration between CH₃COOH and Ba(OH)₂.

What is the percentage dissociation of a 0.015 mol dm^–3 solution of methanoic acid, HCOOH (K_a = 1.60 10^–4 mol dm^–3)?

A 0.016%

B 1.1%

C 1.82%

D 10.3%

Your answer

A student carries out an investigation on vitamin C, C₆H₈O₆.

Vitamin C is extremely soluble in water. This means that vitamin C is removed rapidly from the body. ‘Vitamin C ester’ is available in tablet form as a less soluble source of vitamin C which stays in the body for longer.

Suggest why vitamin C is extremely soluble in water.

A ‘vitamin C ester’ tablet contains an ester with the molecular formula C₂₂H₃₈O₇.

This ester can be prepared by reacting vitamin C with a long chain carboxylic acid, C_xH_yCOOH, in the presence of an acid catalyst.

Vitamin C and the long chain carboxylic acid react in a 1:1 molar ratio.

Determine x and y in the formula of this carboxylic acid.

Vitamin C, C₆H₈O₆, is a weak acid (K_a = 7.94 x 10^–5 (mol dm^–3)), which is often referred to as ascorbic acid.

The label on a carton of orange juice lists the mass of vitamin C, in mg, in a typical serving of 150 cm³.

The student carries out an investigation to check the vitamin C content in the orange juice.

Vitamin C can be oxidised by iodine:

The student dilutes 150 cm³ of the orange juice with water to 250.0 cm³ in a volumetric flask.

The student then titrates 25.0 cm³ volume of this solution with 9.60 × 10^–4 mol dm^–3 iodine solution, I₂(aq).

The mean titre of I₂(aq) is 22.50 cm³.

Determine the mass, in mg, of vitamin C in a 150 cm³ serving of the orange juice.

An aqueous solution of ethanoic acid, CH₃COOH, has a concentration of 0.50 mol dm^–3. pK_a for CH₃COOH = 4.76 at 25 °C.

What is the pH of the ethanoic acid solution at 25 °C?

A 2.53

B 2.68

C 4.91

D 5.06

Your answer

This question is about two different types of acid found in organic compounds, carboxylic acids and sulfonic acids, as shown in Fig. 6.1.

Complete Table 6.1 to predict bond angles a and b and name the shapes which makes these bond angles in the functional groups of carboxylic acids and sulfonic acids.

Ethanoic acid, CH₃COOH, and methanesulfonic acid, CH₃SO₂OH, are both monobasic acids. The pK_a values are shown in the table.

A student suggests that 1.0 mol dm⁻³ CH₃SO₂OH should have a lower pH value than 1.0 mol dm⁻³ CH₃COOH.

Write an equation, showing conjugate acid–base pairs, for the equilibrium of CH₃SO₂OH with water and explain, with reasons, whether the student is correct.

Label the conjugate acid–base pairs: A1, B1 and A2, B2.

Carboxylic acids and sulfonic acids both form esters.

Sulfonic acid esters can be hydrolysed by aqueous alkali. The equation shows the alkaline hydrolysis of a sulfonic acid ester.

CH₃SO₂OCH₃ + OH⁻ → CH₃SO₂O⁻ + CH₃OH

In the 3 boxes below, add curly arrows to show the mechanism for this reaction.

In the first box, the hydroxide ion acts as a nucleophile.

This question is about weak acids.

The K_a values of three weak acids are shown in Table 20.1.

Calculate the pH of 0.0800 mol dm⁻³ C₂H₅COOH(aq).

Give your answer to 2 decimal places.

Show by calculation that 20.0 cm³ of NaOH(aq) is required to reach the end point.

Calculate the pH of the final solution.

Give your answer to 2 decimal places.

On the axes below, sketch a pH curve for the pH changes during the addition of 45.0 cm³ of 0.100 mol dm⁻³ NaOH(aq) to 25.0 cm³ of 0.0800 mol dm⁻³ C₂H₅COOH(aq).

The student considers using the four indicators in Table 20.2 for the titration.

Explain which indicator would be most suitable for the titration.

The student repeats the experiment starting with 25.0 cm³ of 0.0800 mol dm⁻³ HCN(aq) and adding a total of 45.0 cm³ of 0.100 mol dm⁻³ NaOH(aq).

Predict one similarity and one difference between the pH curve with C₂H₅COOH(aq) and the pH curve with HCN(aq). Use the information in Table 20.1, and your answer to (b)(iii).

Similarity........

Difference........

The student calculates the pH of 0.0800 mol dm⁻³ HIO₃(aq). The student assumes that the equilibrium concentration of HIO₃(aq) is the same as the initial concentration of HIO₃(aq).

The student measures the pH, and finds that the measured pH value is different from the calculated pH value.

Explain why the measured pH is different from the calculated pH.

Dettol ® is a disinfectant containing the antiseptic chloroxylenol, shown below.

Chloroxylenol is a weak Brønsted–Lowry acid.

What is the systematic name of chloroxylenol?

Predict the number of peaks in a¹³C NMR spectrum of chloroxylenol.

Name the functional group responsible for the acidity of chloroxylenol and describe a simple test which would confirm the presence of this group.

Functional group........

Test........

A student measures the pH of the contents in a bottle of Dettol ® as 5.14.

The label on the bottle shows the percentage of chloroxylenol in Dettol ® as 4.80% i.e. 100 cm³ of Dettol^® contains 4.80 g of chloroxylenol.

Assume the following:

• Chloroxylenol is the only acidic component in Dettol ®.

• Chloroxylenol is a weak monobasic acid.

• The density of Dettol ® is 1.00 g cm −3.

Write the equation, using molecular formulae, for the acid dissociation of chloroxylenol.

Calculate the acid dissociation constant, K_a, for chloroxylenol.

K_a =........ mol dm⁻³

Dettol^® contains other chemicals including -terpineol, shown below.

-Terpineol is a chiral compound.

Show with an asterisk, (*), the chiral centre(s) in the structure of -terpineol.

-Terpineol meets the requirements for E / Z isomerism. However, only one E / Z isomer of -terpineol exists.

Explain

• why -terpineol meets the requirements for E / Z isomerism

• whether -terpineol is an E- or Z- isomer

• why only one E / Z isomer of -terpineol exists.

-Terpineol contains two functional groups.

For each functional group, choose a reagent that reacts with that group only. Draw the structures for the organic products of the reactions.

Show structures for organic compounds.

Reagent(s)........

Name of functional group that reacts........

Structure of organic product

Reagent(s)........

Name of functional group that reacts........

Structure of organic product

This question is about acids and bases found in the home.

Ethanoic acid, CH₃COOH, is the acid present in vinegar.

A student carries out an experiment to determine the pK_a value of CH₃COOH.

• The concentration of CH₃COOH in the vinegar is 0.870 mol dm^–3.

• The pH of the vinegar is 2.41.

Write the expression for the acid dissociation constant, K_a, of CH₃COOH.

Calculate the pK_a value of CH₃COOH.

Give your answer to two decimal places.

Determine the percentage dissociation of ethanoic acid in the vinegar.

Give your answer to three significant figures.

Many solid drain cleaners are based on sodium hydroxide, NaOH.

• A student dissolves 1.26 g of a drain cleaner in water and makes up the solution to 100.0 cm³.

• The student measures the pH of this solution as 13.48.

Determine the percentage, by mass, of NaOH in the drain cleaner.

Give your answer to three significant figures.

Sodium carbonate, Na₂CO₃, is a base used in washing soda.

Na₂CO₃ contains the carbonate ion, CO₃^2–, shown below.

Draw the ‘ dot-and-cross ’ diagram for the carbonate ion.

Show outer electrons only and use different symbols for electrons from C and O, and any ‘extra’ electrons.