Waec 2019 Chemistry Practical Answers – May/June Expo

Use your school end point in number 1. ours is 12.50cm3.


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 mol/dm³

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 *100%
= 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 solution
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,precipitate 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

Posted on May 6, 2019


  1. Guest says:

    thank you

  2. Anonymous says:


  3. Guest says:

    Nice work

  4. Anonymous says:


  5. Anonymous says:


  6. Mathew says:

    Please gidifanc. Help us expo the answer

  7. Guest says:

    please I need help

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