NABTEB 2018 - CHEMISTRY PRACTICAL ANSWERS

Friday 11/05/2018 Chemistry Paper II(Practical) 9.00am - 11.00am +++++++++++++++++++++++++++++++++++++ ============================= COMPLETED ============================= Click here for image versions ============================= (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 = (24.50+24.80+24.50)cm/3 = 24.50cm³ = 24.50cm³ (1bi) Concentration of E (CE) in moldm = mole x 1000/volume 4.90g ==> 500cm³ xg ==> 1000cm³ x = 4.90x1000gdm³/500 =9.80gdm³ Molar mass (H2S04) = 1x2 + 32 + 16x4 = 2+ 32 + 64 = 98gmol Concentration (mole dm³) = 9.80gdm³/98 = 0.100moldm³ (1bii) CEVE/CFVF = Ne/Nf = 0.100x24.50/Cf x 25.0 =1/1 Cf = 0.100x24.50/25x1 Cf = 2.45/25 moldm³ = 0.0980moldm³ (1biii) Concentration of F in g/dm Conc (gdm³) = molar mass x conc Molar mass (Na3CO3) = 23x2 + 12 + 16 x3 =106gmol³ Conc (gdm³) = 106gmol x 0.0980 =10.39gdm³ ============================================= (2) TABULATE: Test | Observation| Inference (2a) TEST: G + 8cm³ of distilled water OBSERVATION: Sample G dissolves partially in water colourless INFERENCE: G is a mixture of soluble and insoluble salt (2bi) TEST: 1st portion + HNO3(aq) + AgNO3(aq) OBSERVATION: A white precipitate is formed INFERENCE: Cl- present (2bii) TEST: 2nd portion + NaOH + heat OBSERVATION: A purgent irritating, choking gas is liberated which turns moist red litmus paper blue INFERENCE: Gas is NH3 from NH4+ (2c) TEST: Residue + H2SO4(aq) OBSERVATION: Residue dissolve completely on warming INFERENCE: Residue is soluble in acid (2ci) TEST: 1st portion + NaOH(aq) in drops than excess OBSERVATION: A pale blue precipitate is formed which is insoluble in excess INFERENCE: Cu2+ present (2cii) TEST: 2nd portion + NH3(aq) in drops, then excess OBSERVATION: A pale blue precipitate is formed which dissolved in excess to give deep blue solution INFERENCE: Cu2+ present =============================================== (3a) Effervescence of brownish gas which turns a moist litmus paper red and a black precipitate or residue of lead sulphide is formed. 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 ionises completely in water while CH3COOH is weak acid because it ionise partially in water (3c) Heat the mixture, ammonium chloride sublime and it then condenses to solid NH4Cl =============================== COMPLETED ===============================