Experimental techniques: notes and practice questions

A liquid mixture is prepared containing propanone and butanone. The mole fraction of propanone in the mixture is 0.450. The vapour pressure of pure propanone at 25 °C is 30.8 kPa.

(a) Assuming the mixture behaves as an ideal solution, calculate the partial vapour pressure of propanone above the mixture at 25 °C.

(b) Propanone has a boiling point of 56 °C and butanone has a boiling point of 80 °C. Outline how this mixture can be separated by fractional distillation.

A student investigates the decomposition of hydrogen peroxide, H₂O₂, in a commercial antiseptic solution when exposed to light over a period of 7 days. The reaction is:

$2\text{H}_2\text{O}_2(\text{aq}) \rightarrow 2\text{H}_2\text{O}(\text{l}) + \text{O}_2(\text{g})$

The concentration of H₂O₂ is determined by titration with a standard solution of acidified potassium manganate(VII), KMnO₄. Several identical flasks are prepared. One flask is kept in darkness, while others are exposed to a light source for different durations each day.

(a) Identify two variables, other than the volume of the solution, that should be controlled in this experiment.

(b) (i) The antiseptic solution is diluted with deionized water before titration. Suggest why this is necessary.

(b) (ii) Identify a possible systematic error associated with the control sample kept in darkness.

(b) (iii) Suggest how the experimental setup could be improved to check if the systematic error identified in (b)(ii) is significant.

(c) The following data are collected for one of the titrations:

Final burette reading = $24.50 \pm 0.05~\text{cm}^3$

Initial burette reading = $2.30 \pm 0.05~\text{cm}^3$

Calculate the percentage uncertainty of the titre.

(d) The initial concentration of H₂O₂ in all flasks was $0.88~\text{mol}~\text{dm}^{-3}$. The concentration after 7 days was measured for each flask.

(i) Calculate the average rate of decrease in hydrogen peroxide concentration, in $\text{mol}~\text{dm}^{-3}~\text{day}^{-1}$, for the sample exposed to light for 4 hours daily over the 7-day period.

(d) (ii) The student's hypothesis is: "A greater decrease in hydrogen peroxide concentration will be observed in solutions exposed to light for longer durations, due to photochemical decomposition."

Discuss, with reference to the data, the extent to which the results support this hypothesis.

(d) (iii) State one implication of the results for the storage of hydrogen peroxide solutions.

(e) Suggest a relevant extension to this investigation that would provide further information on the stability of hydrogen peroxide solutions.

Solid rocket boosters are used to provide large amounts of thrust for spacecraft launches. A common oxidizer used in these boosters is ammonium nitrate, $NH_4NO_3$.

(a) When ignited, ammonium nitrate decomposes to produce nitrogen gas, oxygen gas, and water vapour. Deduce the balanced chemical equation for this decomposition, including state symbols.

(b) Calculate the total number of moles of gas produced from the complete decomposition of 100.0 g of ammonium nitrate.

(c) The gaseous products are ejected at a temperature of 800 °C. Calculate the total volume, in $dm^3$, that these gases would occupy at a pressure of 1.01 × 10⁵ Pa.

(d) Explain why water vapour deviates more from ideal gas behaviour than oxygen gas does, especially at lower temperatures and higher pressures.

(e) Some advanced propellants use ammonium perchlorate, $NH_4ClO_4$, which produces toxic chlorine gas, $Cl_2$, upon decomposition. Suggest, including a relevant equation, one reason why the release of chlorine gas into the atmosphere is an environmental concern.

Caffeine is a stimulant found in tea leaves. A student carried out an experiment to extract caffeine from tea. The procedure involved adding solid sodium carbonate ($Na_2CO_3$) to boiling water with tea leaves, followed by a liquid-liquid extraction using dichloromethane ($CH_2Cl_2$) as the organic solvent. The crude caffeine obtained after evaporating the dichloromethane was then purified by recrystallization.

(a) State one key property of a solvent required for a liquid-liquid extraction and suggest why dichloromethane is a suitable choice for extracting caffeine from the aqueous solution.

(b) The aqueous solution from the tea leaves is alkaline due to the presence of sodium carbonate. Suggest how the addition of sodium carbonate improves the separation of caffeine from other acidic compounds, such as tannins, also present in tea.

(c) During the final recrystallization step, the student cooled the hot solution of crude caffeine rapidly in an ice bath. Outline how this would affect the purity of the caffeine obtained.

(d) Dichloromethane is a chlorinated hydrocarbon. State one reason, other than its toxicity, why the use of such solvents is being phased out.

A student investigates the rate of reaction between magnesium and sulfuric acid. The reaction produces hydrogen gas.

(a) Write the balanced chemical equation, including state symbols, for the reaction between magnesium and sulfuric acid.

(b) Describe two different experimental methods to measure the rate of this reaction. For each method, state the variable that would be measured.

The student carries out the reaction using 0.50 g of magnesium and excess 1.0 mol dm⁻³ sulfuric acid. The experiment is performed twice: once with a single ribbon of magnesium and once with the same mass of magnesium powder. The results are plotted on the graph below.

(c) (i) Identify which curve, A or B, represents the reaction with magnesium powder. Explain your answer using collision theory.

(ii) Explain why both curves reach the same final volume of hydrogen gas.

A student is presented with several samples of matter and asked to classify them.

(a) Define the term homogeneous mixture.

(b) Classify each of the following substances as an element, a compound, a homogeneous mixture, or a heterogeneous mixture.

• Air

• Graphite

• Sodium chloride solution

• A mixture of sand and iron filings

(c) Describe a method to separate the iron filings from the sand.

A food chemist is analyzing the components of a new artificial sweetener mixture using paper chromatography. A small spot of the mixture is applied to a chromatography paper, and the paper is placed in a sealed chamber with a solvent system consisting of $70\%$ ethanol and $30\%$ water. After some time, the solvent front moves up the paper, separating the components.

(a) Define the stationary phase and the mobile phase in this paper chromatography experiment.

(b) Explain why it is important that the initial spot of the sweetener mixture is applied above the solvent level in the chamber, and why the solvent front should not be allowed to reach the very top of the chromatography paper.

(c) After the chromatography run, the solvent front travelled $9.0\text{ cm}$ from the start line. Two distinct components, Sweetener A (yellow) and Sweetener B (blue), were observed to have travelled $7.2\text{ cm}$ and $4.5\text{ cm}$ respectively from the start line.

Calculate the retardation factor, $R_F$, for Sweetener A and Sweetener B. Show your working.

(d) The stationary phase (paper) is polar due to cellulose, and the mobile phase is a moderately polar mixture of $70\%$ ethanol and $30\%$ water.

Deduce, with a reason, which sweetener (A or B) is more polar.

(e) If Sweetener A and another component had very similar $R_F$ values and were not well separated, suggest one modification to the experiment that could improve their separation.

A student plans to determine the concentration of copper(II) ions in a sample of industrial wastewater using colorimetry. The blue colour of the solution is due to the hydrated copper(II) ion, $[Cu(H_2O)_6]^{2+}$. To do this, they must first prepare a standard solution of copper(II) sulfate and construct a calibration curve.

(a) Describe the procedure for preparing a standard aqueous solution of known concentration from solid hydrated copper(II) sulfate, $CuSO_4 \cdot 5H_2O(s)$.

(b) Describe how this standard solution can be used to construct a calibration curve.

(c) Explain how this curve can be used to determine the concentration of copper(II) ions in the wastewater sample.

A student is tasked with determining the concentration of copper(II) ions, $Cu^{2+}(aq)$, in a sample of industrial wastewater using a colorimeter. The blue colour of the solution is due to the hydrated copper(II) ion, $[Cu(H_2O)_6]^{2+}$. To do this, a calibration curve must first be prepared using a standard solution of hydrated copper(II) sulfate, $CuSO_4 \cdot 5H_2O(s)$.

(a) Describe the essential steps to prepare a standard aqueous solution of copper(II) sulfate of a known concentration.

(b) Describe how this standard solution can be used to construct a calibration curve.

(c) Explain how the calibration curve can be used to determine the concentration of $Cu^{2+}(aq)$ in the wastewater sample.

A voltaic cell is constructed using a copper half-cell, $Cu^{2+}(aq) + 2e^- \rightleftharpoons Cu(s)$.

(a) A solution of copper(II) ions is required for the half-cell. State the chemical formula of a suitable soluble copper(II) compound.

(b) The copper half-cell is connected to another half-cell to form a complete voltaic cell. A salt bridge is used to connect the two half-cells. Describe the function of the salt bridge.

(c) A high-resistance voltmeter is used to measure the potential difference of the cell. State the units of cell potential.