Question 1B
D is a solution of a dibasic acid, H2Y containing 1.4 g in 500 cm3 solution.
E is 0.105 moldm-3 KOH.
(a) Put D into the burette and titrate it against 20.0 cm3 or 25.0 cm3 portions of E using methyl orange as indicator. Repeat the titration to obtain concordant titre values.
Tabulate your results and calculate the average volume of D used.
The equation for the reaction is:
H2Y(aq) + 2KOH(aq) --> K2Y(aq) + 2H2O(l)
(b) From your results and the information provided, calculate the:
- concentration of D in moldm-3;
- molar mass of H2Y;
- relative atomic mass of Y;
- number of hydrogen ions in 1 dm3 of D.
[NA = 6.02 x 1023]
Observation
In part (a), majority of the candidates were able to carry out the titration and record their results in tabular form. They were able to calculate the average titre value. It was reported that majority of them did not use concordant values.
In part (b), majority of the candidates calculated concentration of D in moldm-3. However, some of them could not calculate the relative atomic mass of Y.
The expected answers include:
(a) Two concordant titres Averaging
(iv) Number of hydrogen ions
H2Y(aq) → 2H+ + Y2-
I mole of H2Y contains 2 moles of H+ ions
a moles of H2Y will contain 2x a moles of H+ ions
Number of H+ ions is 6.02 x 1023 x2xa
= say d ions