Waec 2015/16 chemistry 3(practical) (alt.A) ouestions and answers available.

PLS IN NUMBER 1, TAKE NOTE OF UR SCHOOL END POINT OR TITRE VALUE. EACH SCHOOL HAS A DIFFERENT TITRE VALUE FROM UR CHEMISTRY LAB. OUR END POINT HERE IS 21.70cm3. ANYWHR U SEE IT IN OUR WORK, EDIT IT WITH UR SCHOOL'S AVERAGE TITRE VALUE. THEN RE-CALCULATE ONCE U EDIT IT IN ANY SOLUTION. (1) TABULATE br /> Burrette Readings(cm3),1st Reading | 2nd Reading | 3rd reading| Final burette Reading|23.70|45.00|26.00| Initial burette Reading|2.00|23.50|4.30| Volume of acid used|21.50|20.70|20.90| Average titre value = (21.70 21.50 21.70)cm^3 /3 =(64.90)/3 = 21.70cm3 (1b) A IS 0.100 mol 2.50g of Na2CO3 and Na2SO4 in 250cm^3 Na2CO3 2HNO3->2NaNO3 H2O CO2 Conc of B in mol/dm^3 Conc of Na2CO3 in the mixture Na2CO3 =106 VA=21.7cm^3 CA=0.100 VB=25cm^3 CB=? NA=2 NB=1 CAVA/CBVB=NA/NB 0.1*21.7/CB*25=2/1 CB=(0.1*21.7*1)/(25*2) =0.0434M (ii) molar concentration =mass conc/molar mass mass conc=molar conc*molar mass =0.0434*106 =4.69g/dm^3 (iii) 2.50g of Na2CO3 Na2SO4->250cm^3 Xg of Na2CO3 Na2SO4->1000cm^3 Xg=2.50*1000/250 Xg=10g % of Na2CO3=4.6/10*100 =46% (2) TABULATE Test | Observation | Inference (i) TEST br /> Sample C + water 5cm^3 of C + NaoH in drop and in excess | OBSERVATION br /> soluble white geletenous precipitate in drop. White soluble or dissolve (in excess). INFERENCE br /> Soluble salt. Zn^2+ or Al^3+ present. (ii) TEST br /> 5cm^3 of C + NH4OH in drop, then in excess. OBSERVATION br /> white geletenous precipitate in drop. White geletenous ppt dissolve in excess. INFERENCE br /> Zn^2+ or Al^3+ present. Zn^2+ confirmed (in excess). (iii) TEST br /> Solution of sample C + dilute HN3 + BaCl2(aq) + HCl(aq) OBSERVATION br /> No visible reaction. White ppt form. White ppt insoluble in dilute Hcl. INFERENCE br /> SO3^2- or SO4^2- may be present. SO4^2- confirmed. (iv) TEST br /> Sample D + dilute Hcl(aq) OBSERVATION br /> A colourless and odourless gas evolve with a effervescence. The gas turn like water milky. The gas turn blue litmus to red. INFERENCE br /> The gas in CO2 from CO3^2- or HCO3-(3ai) fountain experiment (3aii) to show that HCL is extremely soluble in water (3aiii) Ammonia gas (NH3) (3aiv) The water turn red immediately (3av) No (3avi) because of its solubility in water (3b) u=>(heated) v + CO2 CaCo3(s) =>(heated) Cao(s) + Co2(g) v + H20 => w Cao(s) + H20(l) => Ca (OH)2(aq) U = CaCa(3), V = Cao, W = Ca (OH)2