WAEC SSCE - Chemistry - 2011 (Practical)

Question 1 Report

(a) A colourless gas P was given off when dilute tetraoxosulphate (VI) acid was added to zinc salt Q. On bubbling the gas through lime water, a white precipitate R was formed. Identify P, Q and R.

(b) Name a suitable apparatus that could be used to perform each of the following activities in the laboratory

(i) storage of dilute silver trioxonitrate (V):

(ii) heating copper metal;

(iii) separation of a mixture of water and Kerosene. Give one reason for each of your answers in (b)

Answer

Answer Details

(a) The colourless gas P turns lime water milky (white precipitate R), so P is carbon(IV) oxide, CO₂. A zinc salt that gives CO₂ with dilute acid is a trioxocarbonate(IV), so Q is zinc trioxocarbonate(IV), ZnCO₃, and R is calcium trioxocarbonate(IV), CaCO₃.
\(ZnCO_3 + H_2SO_4 \rightarrow ZnSO_4 + H_2O + CO_2\); \(CO_2 + Ca(OH)_2 \rightarrow CaCO_3 + H_2O\)

(b) Suitable apparatus, with a reason for each:

Storage of dilute silver trioxonitrate(V): a dark (amber/brown) reagent bottle, because silver trioxonitrate(V) is light-sensitive and decomposes in bright light.
Heating copper metal: a crucible (on a pipe-clay triangle), because it can withstand strong, direct heating.
Separating water and kerosene: a separating funnel, because the two liquids are immiscible and settle into layers that can be run off separately.

Read lesson note on Acids, Bases And Salts (JAMB)

Read lesson note on Separation Of Mixtures And Purification Of Chemical Substances (JAMB)

Acids, Bases And Salts

Separation Of Mixtures And Purification Of Chemical Substances

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Question 2 Report

Credit will be given for strict adherence to the instructions, for observations precisely record and for accurate inferences. All tests, observations and inferences must be clearly entered in your answer book in ink, at the time they are made.

C is a mixture of two salts. Carry out the following exercises on C. Record your observations and identify any gas(es) evolved. State the conclusion drawn from the result of each fest

(a) Put all of C into a boiling tube and add about 5cm\(^3\) of distilled water. Stir thoroughly and filter. Keep both the residue and the filtrate.

(b) To about 2 cm\(^3\) of the filtrate, add few drops of Pb(NO\(_3\))\(_{(aq)}\). Boil the mixture and then allow to cool.

(c)(i) Put the residue in a test tube and add dilute HNO\(_{3}\). Shake the mixture and divide the solution into two portions

(ii) to the first portion from (c)(i), add NaOH\(_{(aq)}\) in drops acid then in excess.

(iii) To the second portion from (c)(ii), add aqueous ammonia in drops and then in excess

Answer

Answer Details

C is a mixture of two salts. The tests, observations and inferences are recorded below.

Test	Observation	Inference
(a) All of C + about 5 cm³ of distilled water, stirred thoroughly and filtered	C partly dissolved; a colourless filtrate was obtained with a white residue left on the filter paper	C contains a soluble salt (in the filtrate) and an insoluble salt (the residue)
(b) About 2 cm³ of the filtrate + a few drops of Pb(NO₃)₂, boiled then cooled	A white precipitate formed; it dissolved on boiling and reappeared as needle-like crystals on cooling	Cl^- confirmed (PbCl₂ is soluble in hot water and recrystallises on cooling)
(c)(i) Residue + dilute HNO₃	Effervescence occurred; a colourless, odourless gas that turned lime water milky was evolved	CO₂ evolved from CO₃^2-; the residue is a trioxocarbonate(IV)
(c)(ii) Solution from (c)(i) + NaOH(aq) in drops then in excess	A white gelatinous precipitate formed which dissolved in excess NaOH	Zn²⁺ or Al³⁺ present
(c)(iii) Solution from (c)(i) + aqueous ammonia in drops then in excess	A white gelatinous precipitate formed which dissolved in excess ammonia	Zn²⁺ confirmed

Ionic equations: \(Pb^{2+} + 2Cl^- \rightarrow PbCl_2\) and \(CO_3^{2-} + 2H^+ \rightarrow H_2O + CO_2\).

Read lesson note on Separation Of Mixtures And Purification Of Chemical Substances (JAMB)

Separation Of Mixtures And Purification Of Chemical Substances

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Question 3 Report

Burette readings (initial and final) must be given to two decimal places. Volume of pipefte used must also be recored but no account of expeririental procedure is required. All calculations must be done in your answer book.

A is 0.100 mol dm\(^{-3}\) solution of an acid. B is a solution of KOH containing 2.8 g per 500 cm'\(^3\)

(a) Put A into the burette and titrate it against 20.0 cm\(^3\) or 25.0 cm\(^3\) portions of B using methyl orange as an indicator. Repeat the titration to obtain consistent titres. Tabulate your readings and calculate the average volume of A used.

(b) From your results and the information provided above, calculate the:

(i) number of moles of acid in the average titre;

(i) number of moles of KOH in the volume of B pipetted;

(ii) mole ratio of acid to base in the reaction. [H = 1.00, O = 16.0, K = 39.0]

Answer

Answer Details

(a) Burette readings and average titre

Burette reading (cm³)	Rough	1st titration	2nd titration
Final reading	24.10	47.00	23.10
Initial reading	01.10	24.10	00.00
Volume of A used	23.00	22.90	23.10

Average titre \(= \dfrac{23.00 + 22.90 + 23.10}{3} = 23.0\ \text{cm}^3\) of A. (A 25.0 cm³ portion of B is pipetted.)

(b)(i) Number of moles of acid A in the average titre
\(= \dfrac{0.100 \times 23.00}{1000} = 2.30\times10^{-3}\ \text{mol} = 0.00230\ \text{mol}\).

(ii) Number of moles of KOH in the volume of B pipetted
Molar mass of KOH \(= 39 + 16 + 1 = 56\ \text{g mol}^{-1}\).
500 cm³ of B contains 2.8 g, so 1000 cm³ contains \(\dfrac{2.8 \times 1000}{500} = 5.60\ \text{g}\).
Concentration of B \(= \dfrac{5.60}{56} = 0.100\ \text{mol dm}^{-3}\).
Moles of KOH in 25.0 cm³ of B \(= \dfrac{0.100 \times 25.0}{1000} = 2.50\times10^{-3}\ \text{mol} = 0.00250\ \text{mol}\).

(iii) Mole ratio of acid to base
\(= 0.00230 : 0.00250 = 1 : 1\).

Read lesson note on Stoichiometry And Chemical Reactions (WAEC)

Stoichiometry And Chemical Reactions

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