How Much? The Amount of Chemical Change (SL IB Chemistry)

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.

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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 should lie between 1.0 and 3.0 ppm. Explain the meaning of ppm and express this concentration range in moldm^-3.

The amount of dissolved chlorine can be analysed by reacting with excess iodide ions under acidic conditions, and titrating the liberated iodine against standard sodium thiosulfate solution in a two-step process:

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

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

A 25.0 mL sample of chlorine water was analysed and the volume of 0.120 moldm^-3 sodium thiosulfate solution, Na₂S₂O₃, needed to react with the iodine was recorded in Table 1.

Table 1

Calculate the mean titre and determine the number of moles of sodium thiosulfate that reacted.

Determine the amount of chlorine, in mol, present in the sample of chlorine water.

Calculate the concentration of the chlorine water in moldm^-3 and in gdm^-3.

Aluminium will react with copper(II) sulfate solution according to the following equation:

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

The reaction is quite slow at room temperature, but when chloride ions in the form of hydrochloric acid are added, the rate increases significantly. The chloride ions catalyse the reaction.

An experiment was carried out to determine the yield of the reaction. A student made a solution of aqueous copper(II) sulfate by dissolving 2.00 g of copper(II) sulfate pentahydrate, CuSO₄.5H₂O (M_r  249.72 g mol^-1) in 10.0 mL of distilled water in a small beaker.

To this solution she added 0.25 g of aluminium foil followed by 2.0 mL of 6.0 mol dm^-3 hydrochloric acid.

After the reaction was complete, she collected, dried, and weighed the copper that was produced.

She recorded the measurements in Table 1 below.

Table 1

Use the data to show that the copper sulfate is the limiting reagent in the experiment and calculate the mass of aluminium in excess.

Calculate the actual yield and 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 systematic errors:

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. 

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. 

A student carried out an experiment involving a solution of potassium dichromate(VI), K₂Cr₂O₇, with iron(II) sulfate, to find the mass of FeSO₄.7H₂O in an impure sample, A. 

The student recorded the mass of A, dissolved the sample in water and then made the solution up to 500 cm³. After an excess was added, the student found that 25.00 cm³ of this solution reacted with 22.10 cm³ of a 0.020 mol dm^–3 solution of K₂Cr₂O₇.

Deduce the full equation for the reaction between acidic Cr₂O₇^2- (aq) and Fe²⁺ (aq) to form Cr³⁺ (aq) and Fe³⁺ (aq).

Use section 7 of the Data booklet to determine the mass, in grams, of FeSO₄.7H₂O in sample, A. Give your answer to three significant figures.

A student performs a titration to determine the molar mass and structure of a dicarboxylic acid, X, which only contains carbon, hydrogen and oxygen. 

The student prepares a 250.0 cm³ solution from 1.513 g of X.

The solution of X is added to the burette and titrated with 25.00 cm³ aliquot of 0.112 mol dm^-3 NaOH (aq). 

The student recorded their results in the table below:

Using section 7 in the Data booklet, suggest a structure for X.

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):

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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.