Chemistry · Karatasi 2B · 2015

5 maswali 🇹🇿 NECTA ✓ Majibu rasmi ya NECTA

DSEE 🇹🇿 NECTA ✓ Majibu rasmi ya NECTA

Chemistry · Karatasi 2B · 2015

5 maswali

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Maswali (5)

Sehemu A

1. 0 alamas

You are provided with two solutions: R1: A solution of potassium hydroxide of unknown concentration R2: A standard solution of sulphuric acid containing 0.05 mol/dm³ You are required to determine the concentration of R1 using titration and phenolphthalein as the indicator. Instructions: Pipette 25.0 cm³ of R1 into a clean conical flask. Add 2–3 drops of phenolphthalein. Titrate against R2 from the burette. Repeat the process and record all readings including a rough titration and three accurate ones.

(a) What was the colour change at the end point of the titration? 2 alama

(a) What was the colour change at the end point of the titration? 1 alama

(b) From your burette readings, determine the average volume of R2 used. (Use 25.0 cm³ if not given) 2 alama

(b) From your burette readings, determine the average volume of R2 used. (Use 25.0 cm³ if not given) 5 alama

(d) Determine the number of moles of R2 used in the average titre. 5 alama

(d) Determine the number of moles of R2 used in the average titre. 2 alama

(e) Use the mole ratio to find the moles of R1 in 25.0 cm³. 2 alama

(e) Use the mole ratio to find the moles of R1 in 25.0 cm³. 5 alama

(f) Hence calculate the concentration of R1 in mol/dm³. 2 alama

(f) Hence calculate the concentration of R1 in mol/dm³. 5 alama

(g) Calculate the concentration of R1 in g/dm³ (Molar mass of KOH = 56 g/mol). 5 alama

(g) Calculate the concentration of R1 in g/dm³ (Molar mass of KOH = 56 g/mol). 2 alama

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2. 0 alamas

In a reaction involving the effect of concentration on rate, a student was given: • Solution T1: 0.5 M sodium thiosulphate • Solution T2: 2 M hydrochloric acid • A white paper marked “+” • Stopwatch and other apparatus Instructions: The student added 10 cm³ of T1 and 10 cm³ of T2 in a beaker placed over the paper with “+” mark. The time taken for the mark to disappear was recorded. This was repeated using more dilute T1 each time (diluted with distilled water), keeping the total volume constant at 20 cm³.

(a) Why does the mark “+” disappear during the reaction? 5 alama

(a) Why does the mark “+” disappear during the reaction? 1 alama

(b) Complete the following table with sample times: 25 alama

(b) Complete the following table with sample times: Experiment T1 (cm³) Water (cm³) T2 (cm³) Time (s) 1 10 0 10 22 2 8 2 10 28 3 6 4 10 35 4 4 6 10 48 5 2 8 10 70 5 alama

(d) What conclusion can be drawn about the effect of concentration on rate of reaction? 2 alama

(e) State two precautions that must be observed during this experiment. 2 alama

Boresha ili kuona majibu ya mfano.

3. 0 alamas

You are given a salt labeled Z, which contains a single cation and a single anion. Perform tests to identify them. Tests to perform: (a) Record the appearance of the salt. (b) Heat the dry salt in a test tube and observe any changes. (c) Dissolve a portion in distilled water and divide into three test tubes. (d) Add aqueous sodium hydroxide dropwise, then in excess, to the first portion. (e) Add aqueous ammonia dropwise, then in excess, to the second portion. (f) Add barium chloride followed by dilute hydrochloric acid to the third portion. (g) Add silver nitrate followed by dilute nitric acid to a fresh portion.

(i) Create a table showing your observations and inferences. 10 alama

(ii) Identify the cation and anion present in the salt. 2 alama

(iii) Write two balanced equations for any two positive tests observed. 4 alama

(iv) State one confirmatory test that distinguishes this salt from a chloride. 1 alama

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