Titrations

What volume, in cm³, of 0.250 mol dm^–3 barium hydroxide solution is required to exactly neutralise 25.0 cm³ of 0.115 mol dm^–3 hydrochloric acid?

A 5.75

B 11.50

C 23.00

D 54.35

The burette readings from an experiment are shown:

The burette has an uncertainty of ±0.05 cm³.

What is the percentage uncertainty of the titre?

A 0.17%

B 0.19%

C 0.34%

D 0.37%

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.

Aspirin tablets are used for pain relief.

The structure of aspirin is shown below.

A student uses the reaction of aspirin with cold NaOH(aq) to determine the mass of aspirin in one tablet.

In this reaction, 1 mol of aspirin reacts with 1 mol of cold NaOH(aq).

The student’s method is outlined below.

Step 1 The student reacts three aspirin tablets with 100 cm³ of 0.500 mol dm^–3 NaOH(aq). The NaOH is in excess. A colourless solution forms.

Step 2 The colourless solution from Step 1 is made up to 250.0 cm³ with distilled water.

Step 3 A 25.00 cm³ sample of the diluted solution from Step 2 is titrated with 0.200 mol dm^–3 HCl(aq) in the burette.

The HC l (aq) reacts with excess NaOH(aq) that remains in Step 1:

The student repeats the titration to obtain concordant (consistent) titres.

Titration results The trial titre has been omitted.

The burette readings have been read to the nearest 0.05 cm³.

Analysis of results From the results, the student can determine the following.

1. The amount, in mol, of excess NaOH(aq) that remains after the reaction of aspirin with NaOH(aq).

2. The amount, in mol, of NaOH(aq) that reacted with the aspirin.

Use the results to determine the mass, in mg, of aspirin in one aspirin tablet.

Aspirin reacts with hot NaOH(aq), under reflux.

Draw a labelled diagram of suitable apparatus for reflux.

In this reaction, 1 mol of aspirin reacts with 3 mol of hot NaOH(aq).

Complete the equation for the reaction of aspirin with an excess of hot NaOH(aq).

Show structures for organic compounds.

This question is about acids and buffer solutions.

Succinic acid, HOOC(CH₂)₂COOH, is a weak dibasic acid that is used in tablet form in health supplements.

A student plans to determine the mass of succinic acid in one tablet of a succinic acid health supplement.

The student carries out a titration with potassium hydroxide.

The end point occurs when both acidic protons in succinic acid have been replaced as shown in Equation 19.1.

The student uses the following method.

Stage 1 The student crushes four tablets of the health supplement and dissolves the powdered tablets in distilled water.

Stage 2 The student makes up the solution from Stage 1 to 250.0 cm³ in a volumetric flask.

Stage 3 The student titrates 10.0 cm³ portions of the solution obtained in Stage 2 with 0.0600 mol dm^–3 potassium hydroxide, using phenolphthalein as the indicator.

The student carries out a trial titration, followed by three further titrations. The results are shown below.

Complete the table and calculate the mean titre that the student should use for analysing the results.

Use the student’s results and Equation 19.1 to calculate the mass, in mg, of succinic acid in one tablet of the health supplement.

Give you answer to 3 significant figures.

Glycolic acid, HOCH₂COOH, (pK_a = 3.83) is a weak monobasic acid used in some skincare products.

A buffer solution is prepared by adding 60.0 cm³ of 0.750 mol dm^–3 glycolic acid to 40.0 cm³ of 0.625 mol dm^–3 potassium hydroxide, KOH.

Explain why a buffer solution is formed.

Calculate the pH of the buffer solution that has been prepared.

Give your answer to 2 decimal places.

A small amount of aqueous ammonia, NH₃(aq), is added to the buffer solution.

Explain, in terms of equilibrium, how the buffer solution would respond to the added NH₃(aq).

A student carries out an investigation to identify two metals, M and X, by two different methods.

In Stage 1, a redox reaction takes place between M and HC l (aq), forming hydrogen and a solution containing M in the +2 oxidation state.

Write an overall equation, with state symbols, for this reaction. Write half-equations for the oxidation and reduction processes.

Overall equation........

Oxidation half-equation........

Reduction half-equation........

In Stage 1, suggest two observations that would confirm that all of metal M has reacted.

In Stage 3, write the ionic equation for the reaction taking place in the titration.

Metal M can be identified following the steps below.

1. The amount, in mol, of excess HC l (aq) that remains after the reaction of M with HC l (aq).

2. The amount, in mol, of HC l (aq) that reacted with M.

3. The identity of metal M.

Analyse the results to identify metal M.

The student is provided with the carbonate of an unknown metal, X₂CO₃.

The student measures the mass loss when the X₂CO₃ is reacted with an excess hydrochloric acid. The equation is shown below.

X₂CO₃(s) + 2HCl (aq) → 2XCl (aq) + CO₂(g) + H₂O(l)

The reaction is carried out using this method:

Step 1 Add 100 cm³ HCl (aq) to a conical flask and weigh.

Step 2 Add X₂CO₃ to the conical flask and immediately reweigh.

Step 3 After 5 minutes, reweigh the conical flask and contents.

Calculate the amount, in mol, of CO₂ released in the reaction.

After analysing the results, the student was told that their molar mass of X₂CO₃ was incorrect.

The student evaluated the experiment for possible reasons for the incorrect result.

The student wondered whether the reaction was complete when the mass was recorded after 5 minutes (Step 3).

How could the student modify the experimental procedure to be confident that the reaction was complete?

The student finds out that carbon dioxide is slightly soluble in water.

State and explain how the solubility of CO₂ would affect the calculated molar mass of X₂CO₃.

This question is about -amino acids, RCH(NH₂)COOH.

Table 17.1 shows the R groups in four amino acids.

In the boxes, draw the organic products for the reactions of serine shown below.

What is the R_f value of serine (ser) in solvent W?

Analyse the chromatograms to identify the unknown amino acid.

Explain your reasoning.

Name of unknown amino acid........

Explanation........

A student titrates a standard solution of barium hydroxide, Ba(OH)₂, with nitric acid, HNO₃.

25.00 cm³ of 0.0450 mol dm^–3 Ba(OH)₂ are needed to neutralise 23.35 cm³ of HNO₃(aq).

What is the concentration, in mol dm^–3, of the nitric acid?

A 0.0241

B 0.0482

C 0.0900

D 0.0964

Your answer

A student carries out two experiments in the laboratory based on succinic acid (butanedioic acid), (CH₂COOH)₂.

Succinic acid is esterified by ethanol, C₂H₅OH, in the presence of an acid catalyst to form an equilibrium mixture.

The equilibrium constant, K_c, for this equilibrium can be calculated using the amounts, in moles, of the components in the equilibrium mixture, using expression 5.1.

A student carries out an experiment to determine the value of K_c for this equilibrium.

• The student mixes together 0.0500 mol of succinic acid and 0.150 mol of ethanol, with a small amount of an acid catalyst.

• The mixture is allowed to reach equilibrium.

• The student determines that 0.0200 mol of succinic acid are present in the equilibrium mixture.

Which technique could be used to determine the equilibrium amount of succinic acid?

Write the equation for the equilibrium reaction that takes place.

Draw the skeletal formula of the ester present in the equilibrium mixture.

K_c is the equilibrium constant in terms of equilibrium concentrations.

Why can expression 5.1 be used to calculate K_c for this equilibrium?

Calculate the value of K_c for this reaction.

Show your working.