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1. A is 0.100 mol dm-3 solution of an acid. B is a solution NAOH containing 2.8 g per 500 cm3.
(a) Put A into the burette and titrate it against 25.0 cm3 portions B using methyl orange as 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;
(ii) number of moles of NAOH in the volume of B pipetted;
(iii) mole ratio of acid to base in the reaction
[Na= 23.00 H=1.00, O = 16.0]
ANS: (a) titration soon.
(b) (i) number of moles of acid = 0.100 x VA
1000
= X mole(s) [3sig. Fig to score]
1000cm3 contains 0.100 mole(s)
VA will contain 0.100 x VA
1000
= X moles [3 Sig. Fig. to score]
(ii) Number of moles of NAOH in B
500cm3 of B contains 2.8g of KOH
1000cm3 of B will contain 2.8 x 1000 = 5.6 KOH
500
Molar mass of NAOH = 23 + 16 + 1 or 40 gmol-1
Conc of B = 5.6 = 0.100 mol dm-3
56
(iii) Mole ratio of acid to base = X: Y to nearest whole number ratio.
- 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 test.
(a) Put all of C into a boiling tube and add about 5cm3 of distilled water. Stir thoroughly and filter. Keep both the residue and the filtrate.
(b) To about 2 cm3 of the filtrate, add few drops of Pb(NO3)2(aq)‑Boil the mixture and then allow to cool.
(c) (i) Put the residue in a test tube and add dilute HNO3. Shake the mixture and divide the solution into two portions.
(ii) To the first portion from (c)(i), add NaOH(aq) in drops and then in excess.
(iii) To the second portion from (c)(i), add aqueous ammonia in drops and thein in excess.
2020 WAEC Chemistry 3 Practical (Alternative A) Answer
Paper 2
2. 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 test.
(a) Put all of C into a boiling tube and add about 5cm3 of distilled water. Stir thoroughly and filter. Keep both the residue and the filtrate.
(b) To about 2 cm3 of the filtrate, add few drops of Pb(NO3)2(aq)‑Boil the mixture and then allow to cool.
(c) (i) Put the residue in a test tube and add dilute HNO3. Shake the mixture and divide the solution into two portions.
(ii) To the first portion from (c)(i), add NaOH(aq) in drops and then in excess.
(iii) To the second portion from (c)(i), add aqueous ammonia in drops and thein in excess.
ANS:
| TEST | OBSERVATION | INFERENCE |
| a) C+water, mixture stirred and filtered | Partly dissolves/ soluble Colourless filtrate White residue (do not accept ppt /solid) | C contains soluble and insoluble salts |
| b) Filtrate + Pb (NO3)2(aq). Boiled then cooled | White precipitate formed Precipitate dissolves Precipitate re-appears | Cl-, SO42- or CO32. Cl- present Cl- confirmed |
| c) (i) Residue + dil HNO3 | Effervescence /bubbles /gas evolved colourless, odourless gas. Gas turns lime water milky. | CO2 evolved from CO32- |
| (ii) Solution from (c)(i) + NaOH(aq) in drops then in excess | White gelatinous ppt formed. Precipitate dissolves | Zn2+ or A13+ Zn2+ or A13+ |
| (iii) Solution from (c)(i) + HN3(aq) in drops then in excess | White gelatinous. Precipitate forms. Precipitate dissolves | Zn2+ or A13+ Zn2+ confirmed |
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