This question was attempted by most candidates and the performance was good.
In part(a), candidates were able to list three errors in the table as follows:
- absence of unit/no unit
- values used for averaging were not concordant
- arithmetic errors in the second/third titration.
In(b)(i), candidates correctly mentioned methyl orange/red or phenolphthalein as suitable indicator for the experiment.
In (b)(ii), some of the candidates could not give the colour of the indicator before the titration experiment. However, a good number of them were able to give the colour at the end point. The expected response from candidates was any of 1 or 2 as stated below.
SuSuitable indicator Colour before titration Colour at the end of titration
1. Methyl orange/red yellow orange
2. Phenolphthalein red or pink colourless
In part (c), most candidates did not know that the end point will not be affected by addition of 5.00cm3 of distilled water because the specific amount of the alkali in 25.0cm3 of K2CO3((aq) remains the same.
In (d)(i) and (ii), candidates correctly calculated the number of moles of acid in the average titre recorded and determined the percentage purity of K2CO3 if 18.8g of impure K2CO3 was used by the student as follows.
- 1000 cm3 contain 0.10 moles
∴ 23.4 cm3 will contain 0.10 x 23.4 = 0.00234 mol
(ii) CA VA = 1 mole ratio
CB = CA VA making CB the subject of the formula
= 0.10 x 23.4 correct substitution
0.0936 moldm-3 correct evaluation to 3 sig. figures and unit
mole ratio of K2CO3 : H2 SO4 = 1 : 1
25cm3 of K2CO3 contains 0.0234 moles
∴ 1000cm3 of K2CO3 will contain 1000 x 0.0234
= 0.0936 moldm-3
Molar mass of K2CO3 = 78 + 12 + 48/138gmol-1
Mass of pure K2CO3 = 0.09326 x 138
% Purity = 12 . 9 x 100