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NABTEB 2018 CHEMISTRY PRACTICAL ANSWER EXPO

Real and Confirmed NABTEB 2018 Chemistry practical Questions and Expo Answer – May/June Expo

NABTEB CHEMISTRY PRACTICAL

(1a)
Volume of Pipette = 25.0cm
TABULATE
Burette reading |rough |1st |2nd|3rd|
Final burette reading (cm³) |24.90| 39.50|34.80|24.50
Initial Burette reading (cm³) |0.00|15.00|10.00|0.­00
Volume of E used (cm³) |24.90|24.50|24.80|2­4.50

Average volume of acid used (VE)
= (24.50+24.50)cm/2
= 24.50cm³
VE = 24.50cm³

(1b)
Concentration of E (CE) in moldm
= mole x 1000/volume
4.90g ==> 500cm³
xg ==> 1000cm³
x = 4.90x1000gdm³/500
=9.80gdm³
Molar mass (H2S04) = 1×2 + 32 + 16×4
= 2+ 32 + 64
= 98gmol
Concentration (mole dm³) = 9.80gdm³/98
= 0.100moldm³

(ii) CEVE/CFVF = Ne/Nf
= 0.100×24.50/Cf x 25.0 =1/1
Cf = 0.100×24.50/25×1
Cf = 2.45/25 moldm³
= 0.0980moldm³

(iii) Concentration of F in g/dm
Conc (gdm³) = molar mass x conc
Molar mass (Na3CO3) = 23×2 + 12 + 16 x3
=106gmol³
Conc (gdm³) = 106gmol x 0.0980
=10.39gdm³




(2)
TABULATE
Test | Observation| Inference
UNDER TEST
- G + 5cm of distilled water

- 1st portion + HNO3(aq) + AgNO3(aq)
- 2nd portion + NaOH + heat

- Residue + H2SO4(aq)
- 1st portion + NaOH(aq) in drops than excess
- 2nd portion + NH3(aq) in drops than excess


UNDER OBSERVATION
- Sample G dissolves partially in water colourless filtrate black residue - A white precipitate is formed
- A purgent irritating, choking gas is liberated which turns moist red litmus paper blue - Residue dissolve completely on warming
- A pale blue precipitate is formed which is insoluble in excess
- A pale blue precipitate is formed which dissolved in excess to give deep blue solution


UNDER INFERENCE
- G is a mixture of soluble and insoluble salt
- Cl present
- Gas is NH3 from NH4
- Residue is soluble in acid
- Cu2+ is present
- Cu2+ present





(3a)
Effervescence of brownish gas which turns a moist litmus paper red and also it gives a black residue of lead sulphide. Pb (NO3)(aq) + H2S(g) ==> Pbs(s)black + H2O(i) + NO(g)

(3bi)
- H2SO4 ==> strong acid
- CH3COOH ==> weak acid


(3bii)
- H2SO4 is strong because it ionise completely in water
- While CH3COOH is weak acid because it ionise partially in water

(3c)
Heat the mixture, ammonium chloride subline and it then condenses to solid NHCL

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