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Waec 2016 chemistry practical alt. a Answers and Questions – expo

Waec 2016 chemistry practical alt. a Answers and Questions – expo

Wednesday 13th April 2016
CHEMISTRY PRACTICAL ALT. A 9.30AM – 11.30AM
VERIFIED WAEC 2016 CHEMISTRY PRACTICAL ALTERNATIVE. A QUESTIONS AND ANSWERS

(1)
TABULATE:
Volume of pipette
Burette reading(cm3)|Rough titre(cm3)|1st titre(cm3)|2nd titre(cm3)
Final reading(cm3) |20.00|19.60|19.60
Initial reading(cm3)|0.00 |0.00 |0.00
Volume of acid used |20.00|19.60|19.60
Average titre=(19.60 19.60)/2 cm3
=39.20/2 cm3
=19.60cm3
Equation=>2MnO4^- 5C2O4^2- 16H^ ->2NA^2 8H2O 10CO2
(1bi)
Conc of B in mol/dm3
CaVa/CbVb=Na/Nb
(0.05*19.6)/(Cb*25.0)=5/2
5*Cb*25=2*0.05*19.6
Cb=(2*0.05*19.6)/(5*25)=1.96
Cb=0.0157mol/dm3
Cb=0.016mol/dm3
(1bii)
Conc in B in g/dm3
=0.0157*158
=2.4806g/dm3
=2.481g/dm3
(2a)
Observation: C dissolved partially in solution
Inference:Salt C in a mixture of soluble and insoluble
(2b)Observation:White precipitate is formed.White precipitate dissolved in excess of NH3(aq), Inference:Cl^- is present.Cl^- is confirmed
(2ci)
Observation: Effervescence occur with liberation colourless odourless gas which turn lime water milky
Inference: CO2(g) from HCO3^- or CO3^2-
(2cii)
Observation:White precipitate is formed which is soluble in excess,
Inference: Pb^2 ,Zn^2 ,Al^3 may be present
(2ciii)
Observation:white precipitate is formed which is soluble in excess,
Inference:Zn^2
(3a)
(i)Simple distillation-wide boiling point difference between constituent liquid
(ii)Chromatography-Relative migration of constituents of mixture over an absorbent or rf-value difference of constituents of mixture
(iii)Fractional distillation-Close boiling point between constituent liquid
(3b)
(i)For easy identification of the end point ie the point of complete titration where the colour change is observed
(ii)For accurate titre value ie to minimize the error margin
(3c)
C1=2.5mol/dm3, V1=?,C2=0.20mol/dm3,V2=500cm3
C1V1=C2V2
V1=C2V2/C1
V1=(0.20*500)/2.5
V1=100/2.5
V1=40cm3
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Posted by on April 12, 2016.

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