### Chemistry Past Questions

4001

(a) Draw a labelled sketch of the laboratory set-up for dissolving hydrogen chloride acid.

(ii) Give one chemical test to distinguish between hydrochloric acid and ethanoic acid

(b) Outline a suitable laboratory procedure for obtaining a fairly pure sample of sodium chloride crystals from a solution of it that is contaminated with some methyl orange:

(c) Name one substance used in the laboratory for:

(i) drying ammonia gas,

(ii) testing for the presence of water;

(iii) converting copper (I) oxide to copper.

WAEC 1994
4002

C and D are samples of two different simple salts. Carry out the following exercises on them. Record your observations and identify any gases evolved. State the conclusion you draw from the result of each test.

a) Heat about one-half of C in a dry test tube until no further change is observed. Allow to cool.

(b)(i) To the cooled residue from (a) above, add about 5cm$$^3$$ of dilute hydrochloric acid and warm.

(ii) To about 2cm$$^3$$ of the clear solution from (b)(i) above add aqueous ammonia in drops until it is in excess.

(c)  Put all of D in a boiling tube and add about 10cm$$^3$$ of distilled water. Shake thoroughly and divide into two portions.

(d)(i) To the first portion from (c) above, add about 2cm$$^3$$ of barium chloride solution, followed by dilute hydrochloric acid in excess. Warm the mixture.

(ii) To the second portion from (c) above, add 2 or 3 of acidified potassium tetraoxomanganate (VII) solution and shake.

WAEC 1994
4003

A is a solution containing 0.050 mol. dm of tetraoxosulphate (VI) acid. B is a solution of anhydrous trioxocarbonate (IV).

(a) Put A into the burette and titrate with 20cm$$^3$$ or 25cm$$^3$$ portions of B using methyl orange as an indicator. Record the volume of your pipette. Tabulate your burette readings and calculate the average volume of A used

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

(i) Concentration of solution B in mol. dm$$^{-3}$$

(ii) mass of sodium tetraoxosulphate (VI) that would be formed in solution of 1dm$$^3$$ of solution B were neutralized by solution A

(iii) volume of carbon (IV) oxide at s.t.p. that would be liberated in (b)(ii) above. The equation for the reaction is: N$$_2$$CO$$_{3(aq)}$$ + H$$_2$$SO$$_{4(aq)}$$ $$\to$$  Na$$_2$$SO$$_{4(aq)}$$ + H$$_2$$O$$_{(l)}$$ + CO$$_{2(g)}$$ [O = 16; Na = 23, S = 32; molar volume of gases of s.t.p. = 22.4dm$$^3$$

WAEC 1994
4004

(a) State;

(ii) One disadvantage of evaporation of salt solutions to dryness over crystallization.

(b)(i) List two normal salts which when dissolved in pure water are acidic to litmus

(ii) Mention the phenomenon that accounts for the behaviour of the salts in (b)(i) above.

(c)(i) Outline a suitable laboratory method for separating a mixture of glucose and starch.

(i) Give two chemical tests that would enable you to identify three solids suspected to be glucose, sucrose and starch.

WAEC 1993
4005

You are provided with two aqueous solutions labelled C and D. Carry out the following exercises on them. Record your observations and identify any gases evolved. State the condition you draw from the result of each test.

(a)(i) Mix C and D in a beaker thoroughly.

(ii) Filter the mixture. Keep both the residue and the filtrate.

(b)(i) To about 5cm$$^3$$ of the filtrate, add barium chloride solution, followed by excess dilute hydrochloric acid in a boiling tube. Divide the resulting solution into two portions.

(c)(i) To the first portion of the solution from (b)(ii) above, add sodium hydroxide solution in excess.

(ii) To the second portion of the solution from (b)(ii) above, add aqueous ammonia in drops until it is in excess.

WAEC 1993