WAEC 2019 Chemistry Practical Answer Question



Burette reading|ROUGH|1st |2nd |3rd
Final |13.60|15.50|14.80|12.50
Initial |1.00 |3.10 |2.40 |0.00
Vol. acid used |12.60|12.60|12.40|12.50
Volume of S2O3²- =12.60+12.40+12.50/3
Conc. in g/dm³ = molar mass * conc in mol/dm³
But molar mass of Na2S2O3
=46 + 64 + 48
Therefore 15.8g/dm³ = 158g/mol * conc of A in
Conc of A in mol/dm³ = 15.8/158
Using CaVa/CbVb = na/nb
0.1*12.50/Cb * 25.00 = 2/1
50Cb = 1.25
Cb = 1.25/50
Cb = 0.025mol/dm³
The conc of I2 in B = 0.025mol/dm³
Gram conc of I2 = molar mass * molar conc
= (127*2)*0.025
Percentage mass of I2 in sample = 6.35/9.0
= 0.7056 * 100%
There have to be a change in the colour of the
mixture before it is added. The end point is
known when the blue colour formed as starch is
added, changes to colourless


Test: C + Water and filtered
Observation: part of C dissolves, colour less
filtrate is formed
Inference: C is a mixture of a soluble salt
Test: 2cm^2 of filterate of salt C + AgNO3(aq)
then dilute HNO3
Observation: white precipitate is formed
Inference: Cl^- present
Test: Solution im 2(b)(i) + excess NH3 solution
Observation: precipitate disorder in excess
solution of NH3
Inference; Cl^- confirmed
Test: Resident + dilute HCl + shake
Observation; residue dissolved to form a blue
Inference; Cu^2+ present
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.


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


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