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DSEE 🇹🇿 NECTA ✓ Official NECTA answers

Chemistry · Paper 2B · 2016

5 questions

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Questions (5)

Section A

1. 10 marks

A technician prepared a solution coded S1 by dissolving an unknown mass of sodium hydroxide pellets in water. Another solution, coded S2, was made by dissolving 3.65 g of hydrochloric acid in 1 dm³ of water. You are required to determine the concentration of solution S1 by titrating it with S2 using methyl orange as the indicator. Instructions: • Fill the burette with solution S2 • Pipette 25.0 cm³ of S1 into a conical flask • Add 2–3 drops of methyl orange and titrate to the end point • Repeat the titration and record the average volume of acid used

(a) State the colour change observed during titration. 1 mk
(b) What is the balanced chemical equation between S1 and S2? 2 mk
(c) Calculate the concentration of S2 in mol/dm³. 2 mk
(d) Using an average titre of 25.0 cm³, calculate the number of moles of HCl used. 1 mk
(e) Use the mole ratio to determine the number of moles of NaOH in 25.0 cm³. 1 mk
(f) Calculate the concentration of NaOH in mol/dm³ and g/dm³ (Molar mass of NaOH = 40 g/mol). 2 mk
(g) Suggest two reasons why the technician used pellets of NaOH instead of prepared solution. 1 mk

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2. 10 marks

You are given: • 0.2 M potassium iodide labeled W1 • 0.2 M hydrogen peroxide labeled W2 • 0.1 M sulfuric acid labeled W3 • Starch indicator • Stopwatch Equal volumes of W1, W2, and W3 are mixed in a clean beaker. A few drops of starch solution are added, and the time for the blue-black colour to appear is recorded. The procedure is repeated at 25°C, 35°C, 45°C, 55°C, and 65°C.

(a) What causes the blue-black colour during the reaction? 1 mk
(b) Complete the following table by filling in the temperature in Kelvin and suggest reasonable times. Temperature (°C) Temperature (K) Time (s) 25 72 35 52 45 36 55 24 65 14 3 mk
(c) Write the net ionic equation for the reaction between hydrogen peroxide and iodide. 2 mk
(d) Plot a graph of temperature (K) against time (s). 1 mk
(e) Explain how temperature affects the reaction rate based on the data above. 2 mk
(f) If the reaction was done at 15°C, what would be the expected trend in time? Explain. 1 mk

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3. 10 marks

A salt labeled Y is suspected to be a sulphate of a transition metal. Perform the following tests to identify the ions present: (a) Observe and record the appearance of salt Y. (b) Heat a small amount in a test tube and record observations. (c) Dissolve a portion in water and test with aqueous sodium hydroxide. (d) Test a second portion with aqueous ammonia solution. (e) Test a third portion with barium chloride followed by dilute hydrochloric acid. (f) Test a fresh solution with silver nitrate followed by dilute nitric acid.

(i) Construct a table to show observations and inferences. 3 mk
(ii) What is the identity of the cation and anion present in salt Y? 2 mk
(iii) Write a balanced chemical equation for the reaction with barium chloride. 2 mk
(iv) Give one distinguishing property of the salt that confirms it is a transition metal compound. 3 mk

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