Waec Chemistry 2020 Past Questions and Answers

Question 56:

• WAEC 2020

All your burette readings (initial and final), as well as the size of your pipette, must be recorded but no account of the experimental procedure is required. All calculations must be done in your answer booklet.
A is a solution containing 5.00 g of HNO${}_3$ in 500 cm${}^3)ofsolution.BisasolutionofNaOHofunknownconcentration.(a)PutAintotheburetteandtitrateitwith20.0cm{\text{(}}^3$ or 25.0 cm${}^3$ portions of B using methyl orange as an indicator. Repeat the titration to obtain concordant titre values. Tabulate your results and calculate the average volume of acid used. Equation of the reaction is HNO${}_{3(aq)}$ + NaOH${}_{(aq)}$ $\to$ NaNO${}_{3(aq)}$ + H${}_2$O${}_{(l)}$
(b) From your results and the information provided. calculate the: (i) concentration ot A In mol dm${}^{-3}$
(ii) concentration of B in mol dm${}^{-3}$.
(iii) concentration of B in gdm${}^{-3}$
(iv) mass of NaNO${}_3$ formed. If 250 cm${}^3$ of NaOH were neutralised. [Molar mass of NaOH = 40g mol${}^{-1}$, NaNO${}_3$ = 85 gmol${}^{-1}$. Credit will be given for strict adherence to the instructions. for observations precisely recorded and for accurate inferences. All tests, observations and inferences must be clearly entered in this booklet, in ink, at the time they are made.

Question 57:

• WAEC 2020

C is an organic compound. Carry out the following exercises on C. Record your observations and identity any gas(es) evolved. State the conclusions you draw from the results of each test.
(a) Put about 10 drops of C on a watch glass and ignite it using a burning splint.
(b)(i) Put about 1 cm${}^3$ of C in a test tube and add about 1 cm${}^3$ of distilled water. Shake the test tube.
(ii) Put about 1 cm${}^3$ of C in a test tube and add about 2 cm${}^3$ of acidified K${}_2$Cr${}_2$O${}_7$ solution. Warm the mixture gently and leave to stand for 5 minutes.
(c) Put few crystals of specimen D in a test tube and add about 2cm${}^3$ of C followed by about 2 cm${}^3$ of 10 % NaOH${}_{(aq)}$ Shake the test tube vigorously.
(d) State the class of compounds to which C belongs.

Question 58:

• WAEC 2020

(a) A zinc salt, E when heated strongly, produced a brown gas with pungent smell, a colourless gas that rekindled a glowing splint, and a residue that was allowed to cool.
(i) identify the salt E.
(ii) Write an equation for the decomposition of E.
(iii) State what would be observed when the residue was allowed to cool.
(b) Describe how 250cm${}^3$ of 0.2 mol dm${}^3$ H${}_2$SO${}_4$ could be prepared from 150 cm${}^3$ of a 1.0 mol dm${}^3$ stock solution of the acid.
(c) State the effect of aqueous solution of Al${}_2(S{O}_4{)}_3$ on litmus paper.