1. The equivalence point of a titration of a weak base with strong acid is 6.24,
ID: 1056650 • Letter: 1
Question
1. The equivalence point of a titration of a weak base with strong acid is 6.24, and the titration curve drops steeply from pH 7 to 5. Which of the following indicators will NOT be suitable for this titration? (A) methyl red, pH transition range 4.8-6.0 (B) bromocresol purple, pH transition range 5.2-6.8 (C) cresol purple, pH transition range 7.6-9.2 (D) p-nitrophenol, pH transition range 5.6-7.6 (E) chlorophenol red, pH transition range 4.8-6.4 2. The than those formed by similar monodentate ligands. is the ability of multidentate ligands to form more stable metal complexes (A) ligand effect (B) multidentate effect (C) isomer effect (D) chelate effect (E) Lewis effect 3. A 1.000 g organic sample containing nitrogen is subjected to Kjeldahl analysis. The ammonia gas generated when ammonium is neutralized is distilled into 50.00 mL of 0.1000 M HCI to reform ammonium. If 17.29 mL of 0.1000 NaOH is required to titrate the unreacted HCI, calculate the percent by mass nitrogen in the original sample.Explanation / Answer
2) Ans = D- chelate effect
Chelate effect of multidentate ligands forms more stable complexes than monodentate ligands.
5)
[NH3] = molarity x volume in Litres = 0.1 M x 0.025 L = 0.0025 mol
[HCl] = molarity x volume in Litres = 0.09 M x 0.021 L = 0.00189 mol
Kb = 1.76 x 10-5
NH3 + HCl ----------------> NH4Cl
0.0025 mol 0.00189 mol 0
-----------------------------------------------------------------------------
0.0025 - 0.00189 0 0.00189 mol
= 0.00061 mol
Hence,
[NH3] = 0.00061 mol
[NH4Cl] = 0.00189 mol
Then,
pOH = - log Kb + log [NH4Cl]/[NH3]
= - log (1.76 x 10-5) + log (0.00189 mol /0.00061 mol)
= 5.24
pOH = 5.24
Then,
pH = 14 - pOH = 14 - 5.24 = 8.76
Therefore,
pH = 8.76
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