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Nabteb 2018 Chemistry Practical Questions And Answers Expo/Runs

NABTEB CHEMISTRY PRACTICAL

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(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

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(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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