Question

Section B ANALYTICAL CHEMISTRY 3. Answer ALL parts. (a) Answer each of the following i) ii) Define buffer capacity. (b) A solution was prepared by dissolving 1.572 grams of NaB (Relative Molecular Mass 169), the sodium salt of a weak base, in 100 0 cm of water. pK, for the weak acid HB is 6.3. Calculate the pi of the solution, K.-1 x 1014 (c) 20.0 cm' of the basic solution is titrated with 0.134 mol dm HCL. Calculate: i) the equivalence volume, ii) the pH at the equivalence point, ili) the pH at the point of half neutralization, iv) the pH 5.0 cm after the equivalence point. (d) Using your answers to parts (b) and (c) above as a guide, sketch the variation of pH with titrant volume and select a suitable indicator for the titration from the table, below, of indicators and their pK, values. Give a reason for your choice of indicator. indicator pk, indicatorpK, 4.2 5.0 bromothymol blue 7.1 methyl orange methyl yellow bromocresol green chlorophenol red6.0b phenol red thymol blue thymolphthalein methyl red cresol purple alizarin yellow R 8.3 8.9 phenolpbthalein 9.7 11.0

Answer 1

a)

ii) Buffer capacity- Its measures the efficiency of buffers in resiststing changes of PH.

buffer capacity calculated as:

∆B= gram equivalent of strong acid/base to change pH of 1 liter of buffer solution
∆PH = the pH change caused by the addition of strong acid/base

Buffer:- Buffers compounds that resist the change in PH by addition of small amounts of acids or bases

a) i ) Difference between Gravimetric nd volumetric analysis.

Volumetric analysis method-

1) volumetric analysis deals with the volumes and concentrations of substances

eg. If you react it with a solution of HCl of known concentration, then you are performing a volumetric analysis.

analysis is the determination of concentration of Fe2+2+ by reacting it with KMnO44.

2) Gravimetric analysis method_

i)gravimetric analysis deals with measurement of mass/ density etc.

eg.If you dry it out and weigh it, then you are performing a gravimetric analysis.

A common example of a gravimetric analysis is determination of empirical formula of MgO by burning Mg in air