How Much? The Amount of Chemical Change (DP IB Chemistry: SL): Exam Questions

An analysis of a 2.54 g antacid tablet containing Mg(OH)₂ was carried out by titration using 40.00 cm³ of 1.25 moldm^-3 sulfuric acid. The acid was in excess.

i) Write an equation for the reaction.

[1]

ii) Determine the amount, in mol, of sulfuric acid.

[1]

The excess sulfuric acid reacted with 21.45 cm³ of 1.51 moldm^-3 NaOH.

Determine the amount of excess acid present.

Calculate the amount of sulfuric acid that reacted with the Mg(OH)₂

Determine the mass of Mg(OH)₂ that was present in the tablet.

Determine the percentage mass of Mg(OH)₂ that was present in the tablet.

The chlorine level in a swimming pool is regulated in parts per million (ppm).

i) Explain the meaning of ppm in the context of an aqueous solution.

[1]

ii) A recommended range for chlorine is 1.0 to 3.0 ppm. Express this concentration range in mol dm^-3.

[3]

The concentration of a sample of chlorine water was determined by a two-step titration. First, the chlorine reacts with excess iodide ions:

Cl₂ (aq) + 2I^- (aq) → 2Cl^- (aq) + I₂ (aq)

The iodine produced was then titrated with a standard solution of sodium thiosulfate, Na₂S₂O₃:

I₂ (aq) + 2S₂O₃^2- (aq) → 2I^- (aq) + S₄O₆^2- (aq)

Titration results are shown in Table 1.

Table 1

i) Using the data from Table 1, determine the mean titre.

[2]

ii) The concentration of the sodium thiosulfate solution was 0.120 mol dm^-3. Calculate the amount, in moles, of sodium thiosulfate used in the titration.

[1]

Determine the amount, in moles, of chlorine present in the original 25.0 cm³ sample of chlorine water.

Calculate the concentration of the chlorine water in:

i) mol dm^-3

[1]

ii) g dm^-3

[1]

A student investigated the reaction between aluminium and aqueous copper(II) sulfate:

2Al (s) + 3CuSO₄ (aq) →  3Cu (s) + Al₂(SO₄)₃ (aq)

The student dissolved 2.00 g of copper(II) sulfate pentahydrate (CuSO₄.5H₂O) in water and added 0.25 g of aluminium foil. After the reaction was complete, the solid copper was collected, dried, and weighed.

The students results are shown in Table 1.

Table 1

 Show by calculation that copper(II) sulfate is the limiting reagent.

Calculate the percentage yield of copper in the experiment.

Determine the percentage uncertainty in the mass of copper produced, and the overall percentage error for the experiment.

Discuss the impact on the percentage yield of copper from the following errors:

i) The copper collected is not fully dried out before the beaker is weighed.

[1]

ii) The student misread the instructions and used 1.0 mL of hydrochloric acid. 

[1]

Citric acid, C₆H₈O₇ , is present in lemon juice and is classed as a weak acid. 10.00 cm³ of citric acid is reacted with sodium hydroxide, NaOH (aq) , with a concentration of 12.0 g dm^-3 to form sodium citrate, Na₃C₆H₅O₇ , and water. 32.10 cm³ of sodium hydroxide was required to react with the lemon juice. 

State the balanced equation for this reaction. 

Calculate the mass, in grams, of sodium hydroxide that reacted with the lemon juice. 

Determine the concentration, in mol dm^-3, of citric acid in the sample of lemon juice. 

A group of students investigated the rate of reaction between sodium thiosulfate and hydrochloric acid by measuring the amount of time taken for a cross marked on a piece of paper to become obscured by a yellow precipitate. 

Na₂S₂O₃ (aq) + 2HCl (aq) → 2NaCl (aq) + SO₂ (g) + H₂O (l) + S (s) 

Initially they measured out 15.00 cm³ of 0.900 mol dm^-3 hydrochloric acid and then added 40.00 cm³ of 0.0150 mol dm^-3 aqueous sodium thiosulfate.

The mark on the paper was obscured 38 seconds after the solutions were mixed.

Their teacher made up 3.00 dm³ of sodium thiosulfate solution using sodium thiosulfate pentahydrate crystals, Na₂S₂O₃•5H₂O. 

Calculate the required mass, in grams, of these crystals. Give your answer to 2 decimal places. 

Using sections 2 and 4 of the Data booklet, calculate the volume of gas produced, in dm³, in this reaction if it were collected at a temperature of 300 K and 1.00 x 10⁵ Pa. 

A different group of students decided to measure the rate of reaction by collecting the volume of sulfur dioxide produced over a period of time.

The students attempted to collect the gas in a measuring cylinder over water, but were unsuccessful. Suggest why they were unsuccessful.  

Determine the pH of the acid used and suggest how pH could be used to measure the rate of reaction. 

Determine the reagent in excess in this reaction and state the amount, in moles, that will be in excess. 

An experiment was performed to determine the percentage purity of an impure sample, A, of hydrated iron(II) sulfate, FeSO₄.7H₂O.

A known mass of sample A was dissolved in dilute acid and made up to a 500.0 cm³ standard solution. In a titration, a 25.00 cm³ aliquot of this solution required 22.10 cm³ of 0.0200 mol dm^-3 potassium dichromate(VI), K₂Cr₂O₇, for complete reaction.

The reaction involves the oxidation of Fe²⁺ to Fe³⁺ and the reduction of Cr₂O₇^2- to Cr³⁺.

Deduce the balanced net ionic equation for this reaction.

Using data from section 7 of the data booklet, calculate the mass of FeSO₄.7H₂O in the original 500.0 cm³ solution.

A student performed a titration to identify an unknown dicarboxylic acid, X, which only contains carbon, hydrogen and oxygen. 

A 1.513 g sample of X was dissolved and made up to a 250.0 cm³ standard solution. This solution was placed in a burette. A 25.00 cm³ aliquot of 0.112 mol dm^-3 NaOH(aq) was titrated, and the titration results were recorded.

The equation for the reaction is:

X (aq) + 2NaOH (aq) → Na₂X (aq) + 2H₂O (l)

i) Calculate the mean titre. 

[1]

ii) Determine the amount, in moles, of X present in the original 250.0 cm³ sample.

   [3]

i) Determine the molar mass of X.

[1]

ii) Suggest a possible structure for X.

[2]

A student prepared some phenyl benzoate by reacting phenol with benzoyl chloride in alkaline conditions. The equation for the reaction is:

The table shows the data recorded by the student:

State the names of two functional groups found in the product

Determine the following quantities from the data in part a):

i) The amount, in mol, of phenol used

[2]

ii) The theoretical yield, in g, of phenyl benzoate 

[2]

iii) The percentage yield of phenyl benzoate

[1]

State the number of significant figures associated with the mass of phenyl benzoate obtained and calculate the percentage uncertainty associated with this mass.

Another student repeated the experiment and obtained an experimental yield of 145%. 

The teacher checked the student's calculations and found no errors. Suggest an explanation for this result.