Chemistry · Paper 2C · 2020

3 questions 🇹🇿 NECTA ✓ Official NECTA answers

CSEE 🇹🇿 NECTA ✓ Official NECTA answers

Chemistry · Paper 2C · 2020

3 questions

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Questions (3)

Section A

1. 0 marks

The experiment involves observing the disappearance of letter X when a mixture of solutions A1 and A2 is heated at different temperatures. The procedure is as follows: Measure specific volumes of A1 and A2, mix them in a flask, add a solution N, swirl the mixture, observe from above, and record the time taken for letter X to disappear completely. Repeat the procedure by varying the temperature as shown in the table.

Experimental Data
Volume of A (cm3)	Volume of A2 (cm3)	Temperature of A1 (oC)	Time (s)
30	30	40	1
30	30	50
30	30	60
30	30	70
30	30	80

(a) Complete filling the table. 0 mk

(a) Calculate the average titre volume. 1 mk

(b) (i) Plot a graph of volume N (vertical axis) against time (horizontal axis) taken for the letter X to disappear completely. (ii) State what does the shape of the graph indicate. 0 mk

(b) Find the volume of the acid which was required to complete neutralization. 1 mk

(d) Calculate the molarities of the acid and base. 1 mk

(d) Write the electronic configuration of the product which causes the solution to be cloudy (milky). 0 mk

(e) Write the ionic equation for the reaction between N and N 1 2. 0 mk

(e) Determine the molar mass of the univalent element carbonate and name of the univalent element. 1 mk

(f) Plot a graph of volume of A against the rate of the reaction. 1 0 mk

(f) Write the chemical formula of the univalent element carbonate. 1 mk

(g) State why N was added to N . 3 1 0 mk

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Section B

2. 25 marks

Study the reaction between sodium thiosulphate and hydrochloric acid at different temperatures. The chemicals provided are labelled as A$_1$ for 0.05 M sodium thiosulphate and A$_2$ for 1.0 M hydrochloric. Also, you are provided with a piece of white paper marked X on which a 100 cm$^3$ beaker containing the reaction mixture to be placed during the experiment. Measure 30cm$^3$ of A$_2$ and put it into the 100 cm$^3$ beaker place on top of the X marked on the piece of paper. Then, measure 30 cm$^3$ of A$_1$, put it into boiling tube, insert a thermometer into it and heat on water bath until it reaches a temperature of 40$^\circ$C. The next step, transfer the heated solution A$_1$ into the beaker containing solution A$_2$ and immediately starting the stop watch.

(a) Complete filling the table. 5 mk

(b) (i) Plot a graph of volume N (vertical axis) against time (horizontal axis) taken for the letter X to disappear completely. (ii) State what does the shape of the graph indicate. 4 mk

(d) Write the electronic configuration of the product which causes the solution to be cloudy (milky). 2 mk

(e) Write the ionic equation for the reaction between N and N. 3 mk

(f) Plot a graph of volume of A₁ against the rate of the reaction. 2 mk

(g) State why N was added to N . 2 mk

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