Experimental techniques & chemical analysis Banker

1. (a) A solution of an unknown salt is tested. Aqueous sodium hydroxide gives a white precipitate that dissolves in excess to give a colourless solution. Aqueous ammonia gives a white precipitate that does not dissolve in excess. Which cation is present?A Al³⁺B Ca²⁺C Zn²⁺D Cr³⁺ [1]

Four gases, A to D, are collected in test-tubes during practical work.

1. (a) Gas A relights a glowing splint. Identify gas A. [1]
2. (b) Gas B turns damp red litmus paper blue. Identify gas B. [1]
3. (c) Gas C bleaches damp litmus paper. Identify gas C. [1]
4. (d) Gas D turns acidified aqueous potassium manganate(VII) from purple to colourless. Identify gas D. [1]
5. (e) Describe the test for carbon dioxide and its positive result. [2]

A student in a Lahore school laboratory tests a blue-green solid X dissolved in water.Test 1: flame test — blue-green flame.Test 2: aqueous sodium hydroxide added dropwise then in excess — light blue precipitate, insoluble in excess.Test 3: aqueous ammonia added dropwise then in excess — light blue precipitate, which dissolves in excess to give a dark blue solution.Test 4: dilute nitric acid added, then aqueous silver nitrate — white precipitate.

1. (a) Name the cation in X. Give one observation from the table that supports your answer. [2]
2. (b) Name the anion in X. [1]
3. (c) Explain why dilute nitric acid, and not dilute hydrochloric acid, is used to acidify the solution in test 4. [2]
4. (d) Suggest the formula of X. [1]

A student collects a colourless liquid from a well near Mithi in the Thar desert and wants to know whether it is water and whether it is pure.

1. (a) Describe a chemical test that shows the liquid contains water. Give the result. [2]
2. (b) The chemical test does not show that the water is pure. Describe a physical test that would show whether the water is pure. [2]
3. (c) The well water boils at 101.5 °C. What does this tell you about it? [1]

Solution Y contains one cation and one anion.Test 1: aqueous sodium hydroxide added — green precipitate, insoluble in excess.Test 2: the mixture from test 1 is warmed — no gas that affects damp red litmus paper.Test 3: aluminium foil is added to the warm mixture — a gas is produced that turns damp red litmus paper blue.

1. (a) Identify the cation in Y. Explain your answer. [2]
2. (b) Identify the anion in Y. [1]
3. (c) Explain why test 2 was needed before the result of test 3 could be used. [2]
4. (d) Suggest what the student would see if the green precipitate in test 1 was left to stand in air for some time. [1]

1. (a) Which reagent can be used to tell aqueous sodium sulfite apart from aqueous sodium sulfate?A dilute nitric acid followed by aqueous barium nitrateB acidified aqueous potassium manganate(VII)C aqueous sodium hydroxideD a flame test [1]

A technician in Peshawar has four unlabelled bottles of colourless solution. She knows they are zinc chloride, aluminium chloride, potassium bromide and potassium iodide.

1. (a) Using aqueous ammonia only, describe how the technician could tell the zinc chloride solution from the aluminium chloride solution. Give the result for each. [3]
2. (b) Describe how the technician could tell the potassium bromide solution from the potassium iodide solution. Give the result for each. [3]
3. (c) Suggest a test that would show that the potassium solutions both contain potassium ions. Give the result. [1]

A student investigates how the concentration of hydrochloric acid affects the rate of its reaction with marble chips. She collects the carbon dioxide over water in an upturned measuring cylinder and records the volume every 30 seconds.

1. (a) Suggest one reason why the measuring cylinder is not a good way to measure the volume of gas. [1]
2. (b) Name a piece of apparatus that would give more accurate measurements of the gas volume. [1]
3. (c) Explain why the student should use the same mass of marble chips each time. [1]
4. (d) Suggest why the student should repeat each experiment. [1]

A student investigates how temperature affects the time taken for a cross drawn on paper under a flask to disappear when dilute hydrochloric acid is added to aqueous sodium thiosulfate.

1. (a) State the independent variable. [1]
2. (b) State the dependent variable. [1]
3. (c) State two variables that should be controlled. [2]
4. (d) Suggest how the student could keep the solution at a steady temperature. [1]

Rock salt from a mine in the Salt Range contains sodium chloride mixed with insoluble rock.

1. (a) Plan an experiment to find the percentage by mass of insoluble impurities in a sample of crushed rock salt. You may use common laboratory apparatus, including an electronic balance and an oven. [6]

A student measures the temperature change when 25 cm³ of dilute hydrochloric acid is mixed with 25 cm³ of aqueous sodium hydroxide in a glass beaker.

1. (a) Explain why a polystyrene cup is better than a glass beaker for this experiment. [2]
2. (b) Suggest one further improvement to reduce heat loss. Explain why it is an improvement. [2]
3. (c) The student uses a measuring cylinder to measure 25 cm³ of acid. Suggest a more accurate piece of apparatus and explain why it is an improvement. [2]

A student in a Karachi college compares the energy released by burning two liquid fuels, ethanol and propan-1-ol. Each fuel is burned in a spirit burner under a copper can containing water, and the temperature rise of the water is measured.

1. (a) Give two variables that must be kept the same to make this a fair comparison. [2]
2. (b) The temperature rise measured is much smaller than expected from data books. Suggest two reasons. [2]
3. (c) Describe how the student finds the mass of fuel burned. [1]

Antacid tablets are sold in many pharmacies in Pakistan. Three brands, P, Q and R, each claim to relieve indigestion by neutralising stomach acid.

1. (a) Plan an experiment to find which brand of antacid tablet neutralises the most hydrochloric acid per tablet. You are provided with dilute hydrochloric acid of known concentration, methyl orange indicator and common laboratory apparatus. Each tablet is an insoluble base. [6]

A metal higher in the reactivity series than copper displaces copper from aqueous copper(II) sulfate, and the reaction releases heat.

1. (a) Plan an experiment to put three metals, magnesium, zinc and iron, in order of reactivity using the temperature change when each metal powder is added to aqueous copper(II) sulfate. Say how you will use your results. [6]

A student adds a piece of magnesium ribbon to excess dilute sulfuric acid and collects the hydrogen in a gas syringe.Time / s: 0, 30, 60, 90, 120, 150, 180Volume of gas / cm³: 0, 46, 66, 75, 79, 80, 80

1. (a) Describe how the rate of reaction changes during the experiment. [2]
2. (b) Calculate the average rate of reaction in the first 30 s, in cm³/s. Give your answer to 2 significant figures. [2]
3. (c) State the total volume of hydrogen produced. [1]
4. (d) Suggest why the reaction stops. [1]

1. (a) Different masses of magnesium are reacted with excess dilute hydrochloric acid at room temperature and pressure. 1 mol of gas occupies 24 dm³.Mass of Mg / g: 0.024, 0.048, 0.072, 0.096Volume of H₂ / cm³: 24, 48, 60, 96Which mass gave an anomalous result?A 0.024 gB 0.048 gC 0.072 gD 0.096 g [1]

25.0 cm³ of dilute hydrochloric acid is placed in a polystyrene cup. Aqueous sodium hydroxide of concentration 1.00 mol/dm³ is added from a burette in portions, and the highest temperature is recorded after each addition.Volume of NaOH / cm³: 0.0, 4.0, 8.0, 12.0, 16.0, 24.0, 28.0, 32.0, 36.0Temperature / °C: 21.5, 24.0, 26.5, 27.6, 31.4, 33.0, 32.4, 31.8, 31.2

1. (a) Which temperature reading is anomalous? [1]
2. (b) When the two straight lines of best fit are drawn, the first line has the equation T = 21.5 + 0.62V and the second line has the equation T = 36.6 − 0.15V, where V is the volume of NaOH added in cm³. Calculate the volume of sodium hydroxide at which the lines meet. Give your answer to 1 decimal place. [2]
3. (c) Calculate the concentration of the hydrochloric acid in mol/dm³. Give your answer to 3 significant figures. [3]
4. (d) Explain why the temperature falls after the point where the two lines meet. [1]

A student titrates 25.0 cm³ of aqueous sodium carbonate with dilute hydrochloric acid using methyl orange.Titration 1: initial burette reading 3.40 cm³, final 28.35 cm³Titration 2: initial 1.90 cm³, final 26.30 cm³Titration 3: initial 0.10 cm³, final 24.55 cm³

1. (a) Calculate the volume of acid used in each titration. [2]
2. (b) State which titrations are concordant and calculate the mean titre. [2]
3. (c) Suggest why titration 1 gave a larger volume. [1]

The solubility of a salt Z in water was measured at different temperatures.Temperature / °C: 10, 20, 30, 40, 50, 60, 70Solubility / g per 100 g water: 21, 32, 46, 64, 85, 110, 138

1. (a) Use the data to estimate the solubility of Z at 45 °C. [1]
2. (b) A saturated solution of Z in 50 g of water is cooled from 70 °C to 20 °C. Calculate the mass of Z that crystallises. [2]
3. (c) Describe the trend shown by the data. [1]

A student electrolyses aqueous copper(II) sulfate with a constant current and weighs the cathode at intervals.Time / min: 5, 10, 15, 20, 30Mass of copper deposited / g: 0.06, 0.12, 0.17, 0.24, 0.36

1. (a) Identify the anomalous result and suggest what its value should have been. [2]
2. (b) State the relationship between mass of copper deposited and time. [1]
3. (c) Predict the mass of copper deposited after 25 minutes. [1]
4. (d) Suggest one reason why the 15-minute result was lower than expected. [1]

1. (a) A student needs exactly 25.0 cm³ of aqueous sodium hydroxide to put into a conical flask for a titration. Which apparatus should be used?A beakerB measuring cylinderC volumetric pipetteD gas syringe [1]

1. (a) The burette reading is 12.60 cm³ before the titration and 36.15 cm³ after. Calculate the volume added. [1]
2. (b) A stop-watch shows 1:23 (minutes:seconds). Give this time in seconds. [1]
3. (c) Name the units used for the volume of a gas collected in a gas syringe. [1]
4. (d) Name the piece of apparatus used to measure the mass of a solid. [1]

A thermometer has 1 °C divisions. At the start the liquid level is halfway between the 19 °C and 20 °C marks. At the end it is level with the 27 °C mark.

1. (a) Record the starting and final temperatures to the nearest 0.5 °C. [2]
2. (b) Calculate the temperature rise. [1]
3. (c) Explain why recording the final temperature as '27' is not acceptable. [1]

1. (a) Which apparatus is best for measuring the volume of carbon dioxide given off every 20 seconds during a reaction?A a buretteB a gas syringeC a volumetric pipetteD a balance [1]

Separating mixtures is an important skill in a chemistry laboratory.

1. (a) Describe how pure, dry crystals of salt can be obtained from a mixture of sand and salt. [4]
2. (b) Name the method used to separate ethanol (boiling point 78 °C) from water. [1]
3. (c) Explain why this method works. [1]

1. (a) A pure substance melts at 136 °C. A student's sample of the same substance melts over the range 128 °C to 133 °C. Which conclusion is correct?A The sample is pure, because it melts below 136 °C.B The sample is impure, because it melts over a range below 136 °C.C The sample is impure, because impurities raise the melting point.D The sample is a different substance, because pure substances have no melting point. [1]

A student in Islamabad titrates 25.0 cm³ of aqueous potassium hydroxide with dilute nitric acid from a burette.

1. (a) Explain why the burette is rinsed with the acid, not water, before it is filled. [1]
2. (b) Explain why the conical flask is placed on a white tile. [1]
3. (c) Thymolphthalein is used as the indicator. State the colour change at the end-point when acid is added to the alkali. [1]
4. (d) Explain why the acid is added drop by drop near the end-point and the flask swirled. [2]

Three students carry out the same titration of 25.0 cm³ of aqueous sodium hydroxide with dilute hydrochloric acid, but each makes a different change to the method.

1. (a) Student 1 left an air bubble in the burette tip, which came out during the titration. State and explain the effect on the measured titre. [2]
2. (b) Student 2 rinsed the pipette with water and did not dry it before measuring the alkali. State and explain the effect on the measured titre. [2]
3. (c) Student 3 added water to the conical flask during the titration to wash the sides. State the effect on the titre. [1]

A food colouring used in mithai (sweets) is tested by paper chromatography. The solvent front moves 8.0 cm from the starting line. The colouring gives two spots, at 2.4 cm and 5.6 cm from the starting line.

1. (a) Calculate the Rf values of the two spots. [2]
2. (b) Explain why the starting line is drawn in pencil, not ink. [1]
3. (c) Explain why the starting line must be above the level of the solvent. [1]
4. (d) How many dyes are in the food colouring? [1]

A sample containing one amino acid is analysed by chromatography. The solvent front travels 12.5 cm and the spot travels 4.0 cm.Rf values of amino acids in this solvent: glycine 0.26; alanine 0.32; valine 0.51; leucine 0.70

1. (a) Amino acids are colourless. State what must be done to the chromatogram so that the spots can be seen. [1]
2. (b) Calculate the Rf value of the spot. Give your answer to 2 decimal places. [2]
3. (c) Use the table to identify the amino acid. [1]
4. (d) Explain why the Rf values in the table are only useful if the same solvent is used. [1]