WAEC 2018/19 MAY/JUNE CHEMISTRY PRACTICAL QUESTIONS and ANSWERS WAEC – CHEMISTRY PRACTICAL =========================== KEEP REFRESHING THIS PAGE =========================== PLS NOTE THAT YOU ARE REQUIRED TO USE UR SCHOOL’S TITRE VALUE IN NUMBER 1. OUR SCHOOL GOT 15.30CM3. REPLACE WITH UR SCHOOLS OWN AND RECALCULATE. (1a) Volume of pipette used=25.0cm^3 TABULATE Reading: Rough|1st|2nd|3rd Final reading: 16.00|15.40|17.20|15.30 Initial reading: 0.00|0.00 |2.00 |0.00 Vol of acid used: 16.00|15.40|15.20| 15.30 Average volume of A used = (VA1 + VA2 + VA3) cm³ VA = (15.40 + 15.20 + 15.30)/3cm³ VA = 45.90/3 = 15.30cm³. B contains 4.80g/250cm³ Hence x8 = 1000cm³ X = 4.80 × 1000/250 = 4.80 × 4 = 19.2gdm³ (1bi) Hence molar concentration of B in moldm3 Mass Conc(8dm³) = molar Conc × molar mass Molar Conc of B = mass conc/molar mass = 19.2gdm³/127gmol Cb = 0.151moldm3 (1bii) Concentration of A in moldm-³, CA is given by CAVA/CBVB = na/nb Where CA = ? VA = 15.30cm³, na = 1 CB = 1.51moldm-³, VB = 25.0cm³, nb = 5 CA × 15.30/0.151 × 250= 1/5 Therefore CA = 0.151×25×1/15.30×5 = 3.775/76.50 = 0.049moldm-³ Hence Conc. Of A in moldm-³ = 0.049moldm-³ (1biii) No of moles of Fe²+ in the volume of B pipetted n= cv/1000 = 0.151×25.0/1000 = 3.775/1000 = 0.003775moles of Fe²+ ================================ (2a) TEST: C + about 5cm3 of distilled water OBSERVATION: C dissolves to form a clear solution; Turns blue litmus paper to pink INFERENCE: C is a soluble salt; C is fairly acidic (2ai) TEST: 1st portion of C + NaOH(aq) in drops; then excess OBSERVATION: pale-blue precipitate forms in drops of NaOH(aq) which persists in excess of NaoH(aq) INFERENCE: cu²+ present. (2aii) TEST: 2nd portion of C + NH3(aq) in drops and then in excess OBSERVATION: pale blue precipitate forms in drops of NH3(aq). Ppt dissolves to form a deep blue coloration. INFERENCE: Cu²+ present. Cu²+ confirmed (2aiii) TEST: 3rd portion of c + AgNO3(aq) + HCL(aq) OBSERVATION: a creamy white ppt forms. INFERENCE: Halogen/Halides present. (2bi) TEST: D + about 5cm³ of distilled H2O OBSERVATION: D dissolves to form a clear solution. The test tube feels warmth INFERENCE: D is a soluble salt. (2bii) TEST: 2cm³ of D + HCL(aq) OBSERVATION: effervescence occurs, an odourless and colourless gas evolved. Precipitate dissolves in dil HCL(aq) INFERENCE: CO32-, so32-present CO32- present ================================ (3a) A white precipitate of barium sulphate will be formed by the instant reaction between sulphuric acid and barium chloride. H2SO4(aq) + BaCl2(aq) -> BASO4 + 2HCl (3b) Iron II sulphide reacts with hydrochloric acid, releasing the malodorous and very toxic gas, hydrogen sulphide FeS(aq) + 2HCl -> Fecl2(aq) + H2S(g) (3c) When solid iron fillings are added to dilute aqueous hydrochloric acid, Iron (II)Chloride OR ferrous chloride is formed, with the liberation of hydrogen gas. Fe(s) + 2HCL(aq) -> Fecl2 + H2(g)
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Created at 2018-04-22 08:26:19
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