Waec Gce Chemistry Practical Answers – Jan/Feb 2019 Expo

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(1a)

H2SO4(aq) + 2NaOH(aq) —-> Na2SO4(aq) + 2H2O(l)

 

(1bi)

Volume of Acid Va = 21.60cm³

Volume of Alkali Vb = 25.0cm³

Mass of NaoH

2.0g = 500cm³

Xg = 1000cm³

X = 1000×2.0/500

X = 2×2 = 4.0g/dm³

 

Then mass conc = molar conc × molar mass

.:. molar conc of B = mass conc/molar mass

 

Where the molar mass of NaOH

=23+16+1 = 40gmol^-1

.:. Conc of NaOH in moldm^-³ = 4.0g/dm³/40g/mol

= 0.100mol/dm^-³

 

(1bii)

CaVa/CbVb = na/nb

Ca×21.60/0.1×25 = 1/2

Ca = 0.1×25/2×21.6

CA = 0.058mol/dm^-³

Concentration of H2SO4 in mol/dm^-³ = 0.058mol/dm-³

 

(1biii)

Recall that mass conc of H2SO4

= Molar conc × Molar mass

Where molar mass of H2SO4

=2(1)+32+4(16)

= 2 + 32 + 64

= 98g/mol

Mass conc of H2SO4 = 0.058× 98

= 5.684g/dm^-³

= 5.68g/dm^-³

 

(1c)

(i) I would ensure the air bubble present is removed before taking the initial reading.

(ii) I would ensure I keep my eye in level with the liquid surface while taking the burrette reading.

(iii) I would ensure there is no leakage from the burette during titration.

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(2ai)

INFERENCE: NO3- suspected, Gas is acidic.

 

(2aii)

OBSERVATION: Color changes from yellow to red

 

(2bi & 2bii)

INFERENCE: NO3- confirmed

 

(2ci)

OBSERVATION: White ppt, solube in excess

INFERENCE: PB2+, Al3+, Ca2+ suspected

 

(2cii)

OBSERVATION: Precipitate dissolves and reappears on cooling

INFERENCE: Pb2+ present

 

(2d)

OBSERVATION: A white ppt of AgCl. Dissolves in NH3

INFERENCE: Cl- suspected

Cl- Present

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(3a)

(i) Boiling tube

(ii) Evaporating dish

(iii) Desiccator.

 

(3b)

Fill a container with warm water and pour the mixture NaCl and activated charcoal. Stir properly to dissolve it. The activated charcoal will float instantly and it is filtered off using a filter paper, the dissolved salt(filtrate) is then heated till the H2O is evaporated leaving the NaCl as residue.

 

(3c)

(i) It can be use for teeth whitening and health.

(ii) It can be use for skin care.

 

(3d)

(i) NaCl is Not decomposed by heat

(ii) Na2CO3.10H2O effloresces, losing all or most of its water of crystallization

 

(3e)

The acid used in this titration is dilute sulphuric acid. Nitric acid is not used as it is itself an oxidising agent and hydrochloric acid is usually avoided because it reacts withKMnO4 according to the equation given below to produce chlorine and chlorine which is also an oxidising agent in the aqueous solution.

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