1.2 Reacting Masses & Volumes

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. 

Phosphine, PH₃, is a gas formed by heating phosphorous acid, H₃PO₃, in the absence of air, as shown in the equation below.

4H₃PO₃ (s) → PH₃ (g) + 3H₃PO₄ (s)

3.45 × 10⁻² mol of H₃PO₃ is completely decomposed by this reaction.

State the expected molecular shape and expected bond angle in PH₃ (g).

Calculate the volume of phosphine gas formed, in cm³, at 100 kPa pressure and 210 °C.

1.85 g of white phosphorus was used to make phosphine according to the equation.

P₄ (s) + 3OH⁻ (aq) + 3H₂O (l) → PH₃ (g) + 3H₂PO₂⁻ (aq)

This phosphorus was reacted with 75.00 cm³ of 4.50 mol dm^-3 sodium hydroxide solution. Deduce, showing your working, which was the limiting reagent. 

Using section 2 of the Data booklet determine the volume of phosphine, measured in cm³ at standard temperature and pressure, that was produced. Give your answer to 3 significant figures. 

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 6 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 6 in the Data booklet, suggest a structure for X.

An empty 1.5 dm³ Tupperware container has been kept in the fridge without a lid at 5 °C. The container is removed from the fridge and allowed to reach a temperature of 21 °C. Using your knowledge of Charles's Law, determine the volume of gas, in cm³, that escaped from the container. 

A balloon contains 2500 mL of helium gas at a temperature of 75 °C. Determine the new volume in mL of the gas when the temperature changes to 55 °C assuming the pressure is constant. Give your answer to three significant figures. 

A 10.0 L container of helium gas with a pressure of 33 000 Pa at 0 °C is heated until the new pressure is 200 000 Pa. Determine the new temperature of the gas assuming the volume remains constant. 

Gas cylinders of helium, like the one shown below, are sometimes used to inflate party balloons.

Figure 1

A typical 11-inch party balloon has a fully inflated volume of 14.1 dm³. The pressure in the gas cylinders is 20,000 kPa. If the gas cylinder can fill 160 balloons at 298 K and 108 kPa, what is the total volume of helium inside the gas cylinder in dm³?

Sketch a graph to show the relationship between the volume and temperature of an ideal gas at constant pressure. Describe the relationship between the two variables.

Deep sea divers sometimes breathe mixtures of helium and oxygen in their scuba diving tanks when the conditions are very deep, so they can avoid nitrogen narcosis.

If a bubble of gas escapes from a scuba tank at 60 m depth where the pressure is 588 kPa and the temperature is 8 ^oC, determine the increase in the size of the bubble by the time it reaches the surface where the pressure is 100 kPa and the temperature is 20 ^oC.

Commercial aircraft are fitted with oxygen cannisters that provide a supply of oxygen in case of the loss of cabin pressure. The cannisters contain sodium chlorate(V) which produces oxygen in the following decomposition reaction.

2NaClO₃ (s) " 2NaCl (s) + 3O₂ (g)

Determine the mass of sodium chlorate(V) needed to produce 10.0 dm³ of oxygen at 298 K and 90 kPa.

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.

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

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.

Ammonia and nitrogen monoxide react together according to the following equation:

                        4NH₃ (g) + 6NO (g) → 5N₂ (g) + 6H₂O (g)

50.0 dm³ of ammonia is reacted with 50.0 dm³ of nitrogen monoxide at 150.0 ^oC and 100 kPa. Identify the excess reagent and determine the volume of nitrogen produced.

Determine the volume of excess gas and hence total volume of gas after the reaction has finished.

What mass of nitrogen is produced in the reaction? Express your answer to an appropriate number of significant figures.

Explain why calculating the gas volumes in the reaction is likely to be more accurate at 150 ^oC than at room temperature.

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.