← Back to papers
CSEE 🇹🇿 NECTA ✓ Official NECTA answers

Chemistry · Paper 2B · 2021

2 questions

📖 View PDF ⬇
🎯 Quiz from this
💰 Sit full exam 200 TZS

3-hour timed simulation · auto-graded · counts toward ranking

Pay & start exam →
Questions (2)

Section A

1. 25 marks

You are given two unlabeled beakers containing solution of an acid and an alkali. Identify the solutions (acid and alkali) by pouring about 2 cm3 of each solution into separate test tubes and adding few drops of phenolphthalein indicator (POP) in both test tubes. Label the identified alkali as A and the acid as C. Given the information that, solution A contained 5.6 g of pure potassium hydroxide in 1 dm3 of solution and solution C had 6.0 g of an impure sulphuric acid in 1 dm3 . Answer the following questions:

(a) What was the colour of phenolphthalein indicator (POP) indicator in solution A and C, respectively? 2 mk
(b) Titrate the acid (in a burette) against the alkali (in a conical flask) using two drops of phenolphthalein indicator (POP) as an indicator. Repeat the process and obtain three titre values. Record the results in a tabular form. 7 mk
(c) What was the volume of the pipette used? 1 mk
(d) What was the colour change at the end point? 2 mk
(e) Calculate the average volume of the acid used. 3 mk
(f) Showing your procedures clearly, determine the percentage purity of the sulphuric acid. 10 mk

Upgrade to see model answers.

2. 25 marks

Determine the effect of concentration on the rate of chemical reaction. Study the reaction between sodium thiosulphate and hydrochloric acid. The chemicals provided are labelled as QQ containing 3.16 g/dm3 sodium thiosulphate solution, TT containing 7.3 g/dm3 hydrochloric acid solution, and distilled water. You are also provided with a stop watch and a piece of white paper marked X on which a 100 cm3 beaker containing the reaction mixture. Proceed as follows: (i) Place 100 cm3 beaker on top of the sheet labelled by letter X in such a way that the letter X is visible when viewed from above. (ii) Using measuring cylinder, measure 50 cm3 of solution QQ and pour it

(a) Complete the table by filling the empty column. 0 mk
(b) Write a balanced chemical equation for the reaction between sodium thiosulphate and hydrochloric acid. 0 mk
(c) Which substance from the chemical reaction in part (b) produced obscured the mark X? 0 mk
(d) Use the data in the experiment table to draw a concentration-time graph, by allocating time on x-axis and concentration on the y-axis. 0 mk
(e) What conclusion can you draw from the concentration-time graph and regarding the disappearance of mark X. 0 mk

Upgrade to see model answers.

💡 Unlock all answers & explanations
Premium · 3000 TZS/mo