WAEC 2019 - PRACTICAL CHEMISTRY ANSWER

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VERY IMPORTANT MESSAGE:
Please make sure you use your school end point in that number 1. In my answer, you will see that i used 12.50cm3. Make sure you use what your school used. So try edit my 12.50cm3 and put yours, then recalculate. Meaning that you should replace my 12.50cm3 with your school's end point anywhere you see it on my work. Some school got 12.90cm3, 13.00cm3, some even got 18.50cm3, etc

ANSWERS:



(1a)
DRAW A TABLE WITH THE FOLLOWING

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
=37.50/3
=12.50cm³

(1bi)
Conc. in g/dm³ = molar mass * conc in mol/dm³
But molar mass of Na2S2O3
=(23*2)+(32*2)+(16*3)
=46 + 64 + 48
=158g/mol
Therefore 15.8g/dm³ = 158g/mol * conc of A in mol/dm³

Conc of A in mol/dm³ = 15.8/158
=0.1mol/dm³

(1bii)
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

(1biii)
Gram conc of I2 = molar mass * molar conc
= (127*2)*0.025
=6.35g/dm³

Percentage mass of I2 in sample = 6.35/9.0 *100%
= 0.7056 * 100%
=70.56%

(1c)
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





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(2)
PLS DRAW A TABLE FOR THESE:
TEST|OBSERVATION | INFERENCE

(2a)
TEST: Salt C+10cm^3 of distilled water and shake with filter
OBSERVATION: Salt C dissolves partly in water to give colourless filerate and residue
INFERENCE: Salt C contains both soluble and insoluble salts

(2bi)
TEST: 2cm^3 of filterate of salt C +AgNO3(aq) then dil HNO3
OBSERVATION: Formation of white ppt then the white ppt remains
INFERENCE: Cl- or CO3^2- present

(2bii)
TEST: Solution from 2bi + NH3(aq) in excess
OBSERVATION: The white ppt formed in 2bi dissolves in NH3(aq)
INFERENCE: Cl- confirmed

(2ci)
TEST: Residue of C + 2cm^3 of dil HCl then shake
OBSERVATION: Effervescence occurs with the liberation of a gas that is odourless and colourless but turns lime water milky and turns moist blue litmus paper to red
INFERENCE: Acidic gas is evolved ie CO2 is evolved from CO3^2- or HCO3-

(2cii)
TEST: Mixture from 2ci +NH3(aq) in drops then in excess
OBSERVATION: Blue gelatinous ppt is formed in drops which turns deep blue colouration in excess
INFERENCE: Cu^2+present, Cu^2+ confirmed


(2&3)



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(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 deliquescent because it absorb moisture from the atmosphere to form solution






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ANSWER COMPLETED
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