2020 WAEC Chemistry Practical Answers Released – See Waec Chemistry Specimen Here. The WAEC chemistry practical 2020 alternative A paper for SSCE will now be written on Thursday, 3rd September 2020 from 08:30am to 10:30am (1st Set) and 11:00am to 13:00pm (2nd Set).
We made this post to highlight samples of the apparatus, Waec chemistry practical specimen, graphs and calculations that was used in the Chemistry Practical Examination questions and syllabus.
Here, we will be providing you with the WAEC Chemistry practical 2020 past questions and materials that will be used for Examination preparation. Read answers below.
WAEC Chemistry Practical 2020 Questions & Answers:
1. A is 0.100 mol dm-3 solution of an acid. B is a solution KOH 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 KOH in the volume of B pipetted;
(iii) mole ratio of acid to base in the reaction
[H = 1.00, O = 16.0, K = 39.0]
ANS: (a) titration soon.
(b) (i) number of moles of acid = 0.100 x VA
= X mole(s) [3sig. Fig to score]
1000cm3 contains 0.100 mole(s)
VA will contain 0.100 x VA
= 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
Molar mass of KOH = 39 + 16 + 1 or 56 gmol-1
Conc of B = 5.6 = 0.100 mol dm-3
(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.
|a) C+water, mixture stirred|
|Partly dissoves/ 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|
|Cl-, SO42- or CO32.|
|c) (i) Residue + dil HNO3||Effervescence /bubles /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|
3. (a) Explain briefly the observations in each of the following processes:
(i) when carbon(IV) oxide is bubbled through lime water, it turns milky but the milkiness disappears when the gas is bubbled for a long time;
(ii) a precipitate of calcium hydroxide is insoluble in excess sodium hydroxide solution whereas that of lead (II) hydroxide is soluble.
(b) (i) What is a primary standard solution?
(ii) Calculate the mass of sodium trioxocarbonate(IV) required to prepare 250 cm3 of 0.15 moldm-3 solution.
[Na = 23.0; O = 16.0; C = 12.0]
(c) Name one gas that can be collected by:
(i) upward displacement of air;
(ii) downward displacement of air.
ANS: (a) (i): Insoluble CaCO3 formation is responsible for the milkness produce when CO2 is bubbled through lime water while the disappearance of milkness is due to the formation of soluble Ca(HCO3)2.
Lime water turns milky with CO2 because CaCO3/ CaCO3(s) is formed. Milkiness disappear when excess CO2 reacts with CaCO3 in water medium forming the soluble Ca(HCO3)2/ Ca(HCO3)2 (aq).
(ii) Calcium hydroxide is not amphoteric. Does not react with an alkali NaOH whereas lead (II) hydroxide is amphoteric so reacts with excess NaOH.
(b) (i) Primary standard solution is one whose concentration is known and can be used to standardize another solution.
(ii) M (Na2CO3) = 106 gmol-1
m(Na2CO3) = C x M x V
= 0. 15 x 106 x 0. 25
= 3. 98 g
(c) (i) Carbon(IV) oxide, sulphur (IV) oxide, hydrogen chloride, oxygen, nitrogen (IV) oxide, chlorine, hydrogen sulphide.
(ii) Ammonia, oxygen, hydrogen, methane.
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