WAEC - CHEMISTRY PRACTICAL ANSWER– May/June Expo

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CHEMISTRY 2019 PAPER 3



*Q1*
A is a solution containing 6gdm3 of NaS2O3.5H2O.

B is a solution containing iodine in potassium iodine.

Put A into the burette and titrate it against 20cm3 or 25cm portion of B. Add about 2cm3 of starch solution as indicator just before the end point. Repeat the exercise to obtain a consistent titre values. Tabulate your burette readings and calculate the average volume of A used.


¶ From your result and the information provided, calculate

i) concentration of A in moldm3.

ii)Amount of I2 present the end of the titration

iii)Write a balance equation for the reaction involved in the titration.

iv) Suggest the reason why the starch solution was added just before the end point of the titration.




[I =127; Na =23, S=32;O=16]

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*Q2*

A is a solution containing 0.0242moldm3 of S2O3²-

B is a solution containing 5gdm-³ impure I2 in KI solution

Put A into the burette and titrate it against 20cm³ or 25 cm³ portion of B. Add about 25cm³ of starch solution just before the end point as indicator. Repeat the exercise to obtain a consistent titre values. Record the volume of your pipette. Tabulate your burette readings and calculate the average volume of A used.




¶ From your result and the information provided, calculate the


i) Concentration of B in moldm-³


ii) Concentrate of B in moldm-³

iii) Mass concentration of iodine in solution B.

The equation for the reaction is as follows;

2Na2S2O3.5H2O + I2-----------> 2NaI + NaS4O6 + 10H2O


[H =1, O= 16;Na =32;I =127]

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*Q3*

A is a solution containing 0.0242moldm³ Na3S2O3

B is a solution containing 2.5gdm³ of I2 in KI solution.


Put A into the burette and titrate it against 25cm³ portion of solution B. Add about 5cm³ of starch solution as indicator just before the end point. Repeat the exercise to obtain a constant titrate values. Tabulate your burette readings and calculate the average volume of A used.



¶ From the your result and the information provided, calculate.

i) Amount of Na2S2O3 present in solution A at the end of the titration.


ii)Concentration of B in gdm ³


iii) percentage purity of I2 in solution B


iv) concentration of A in gdm³



The equation 2S2O3³- + I2 --------> S4O6² + 2. Practise problems okay
at work with your paper

(2cii)
Test; solution in 2(c)(i) + NH3 solution in drops and then in excess

Observation: light pale blue precipitate is formed precipitate dissolved in excess solution of NH3 to give a deep blue solution

Inference; Cu^2+ confirmed.


(3ai)
On adding BaCl2 solutions to a portion of saturated Na2CO3 precipitate is formed,precipitate dissolves on adding excess dilute HCl
(3aii)
Q is reducing agent like SO2, H2S, CO
(3bi)
calcium oxide(Cao)
(3bii)
concentrated H2SO4
(3c)
NaOH pellet is delinquescent because it absorb moisture from the atmosphere to form solution

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