2019 Waec Practical Chemistry Expo

2019 Waec Practical Chemistry Expo

(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
=(232)+(322)+(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
= (1272)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

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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 delinquescent because it absorb moisture from the atmosphere to form solution



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