WAEC 2020 CHEMISTRY Practical Question And Answer

Further maths

💯*2020 CHEMISTRY PRACTICAL ANSWERS

1) ANS: (a) titration answer 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 KOH 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 KOH = 39 + 16 + 1 or 56 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.
.
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.

3725: 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

3aii
Red litmus contains a weak diprotic acid. When it is exposed to a basic compound, the hydrogen ions react with the added base. The conjugate base formed from the litmus acid has a blue color, so the wet red litmus paper turns blue in alkaline solution.

3c(i)
When exposed to light, silver chloride decomposes into gray metallic silver and chlorine. The light sensitivity of the silver chloride and other silver halides, such as silver bromide and silver iodide, forms the basis of the photographic process.

3cii)
The ammonia combines with silver ions to produce a complex ion called the diamminesilver(I) ion, [Ag(NH3)2]+. This is a reversible reaction, but the complex is very stable, and the position of equilibrium lies well to the right.

3d)
Pure copper(II) sulfate is white. It is also known as anhydrous copper(II) sulfate because it has no water in it. When water is present in a sample of copper(II) sulfate it turns blue. … This colour change can be used to detect the presence of water (or water vapour).

*Stay tuned Typing in Progress*
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