Chemistry · Paper 2A · 2022

5 questions 🇹🇿 NECTA ✓ Official NECTA answers

CSEE 🇹🇿 NECTA ✓ Official NECTA answers

Chemistry · Paper 2A · 2022

5 questions

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Questions (5)

Section A

1. 10 marks

Table: Experimental data

Experimental data
Experiment No.	Volume of NN (cm³)	Volume of LL( cm³)	Time (sec)	Volume of distilled water (cm³)
1	10	20	0	10
2	10	15	10	15
3	10	10	15	10

(a) Describe the procedure you would follow to determine the molar mass of the unknown solid using solution B. 7 mk

(a) Complete filling the experimental table. 2 mk

(a(i)) What was the volume of the pipette used? 0 mk

(b) What does represent in the experimental table 1 mk

(b) Perform the titration calculations using the following results obtained from the experiment. Table 1: Titration of 25.0 cm³ of solution A against solution B. | Titre No. | Initial burette reading (cm³) | Final burette reading (cm³) | Volume of solution B used (cm³) | |-----------|-----------------------------------|----------------------------------|--------------------------------------| | 1 | 0.0 | 20.5 | 20.5 | | 2 | 20.5 | 40.8 | 20.3 | | 3 | 40.8 | 61.1 | 20.3 | Average volume of solution B used = 20.3 cm³ Assume that the unknown solid is a monoprotic acid. 18 mk

(a(ii)) What was the colour change at the end point? 0 mk

(a(iii) Calculate the average volume of the acid used to neutralize the base. 0 mk

(d) How was the factor of concentration varied in this experiment? 2 mk

((a)(i) Add solution B from the burette to the conical flask, swirling constantly until the pink colour disappears at the equivalence point. 0 mk

(e) Plot a graph of volume of solution LL against time. 1 mk

((a)(v) Repeat the titration two more times (titrations 2 and 3) to obtain concordant results. 0 mk

(f) Use the graph you have drawn in (e) above to explain how the variation of concentration affects the rate of chemical reaction. 1 mk

((b)(i) Hence, determine the concentration of solution A in mol/dm$^3$ and g/dm$^3$. 10 mk

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2. 0 marks

You are provided with the following: LL: A solution of 0.13 M $Na_2S_2O_3$ (sodium thiosulphate); NN: A solution of 2 M HCl; Distilled water, Stopwatch or stop clock; A sheet of white paper marked X. Procedure (i) Measure 20 cm³ of solution LL and put it into a 100 cm³ beaker. (ii) Place the beaker containing solution LL on the top of a letter X drawn on a sheet of paper. (iii) Measure 10 cm³ of NN; pour it into a beaker containing solution LL and immediately start the stopwatch. Swirl the beaker with contents twice. (iv) Look down vertically through the mouth of the beaker so as to see the cross at the bottom of the beaker. Stop the clock when the letter X is invisible. (v) Record the time taken for the letter X to disappear completely. (vi) Repeat the experiment using the data shown in the following table.

(a) Describe how you would prepare a standard solution of approximately 0.05 M copper (II) sulphate from solution C. 5 mk

(b) You are asked to investigate the reaction between copper (II) sulphate and sodium sulphite. Describe the procedure you would follow and the observations you expect to make. 15 mk

((a)(i) Record all observations, including any change in temperature, gas evolution, or colour change. 0 mk

((b)(i) Suggest another experiment you could perform to confirm the reactivity of substance C and substance D. 5 mk

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