Question 1
- A was prepared by dissolving 6.50 g of an impure solid Y(OH)2.XH2O in 250 cm3 of solution.
B is a solution of HCl containing 0.025 mol in 250 cm3.
Titration of 25 cm3 portions of A with B using methyl orange as indicator gave the following results:
Burette readings / cm3 |
1 |
2 |
3 |
Final reading |
33.20 |
30.00 |
|
Initial reading |
16.80 |
7.50 |
1.50 |
Volume of B used/cm3 |
|
|
16.65 |
(a) (i) Complete the table.
(ii) Calculate the average volume of B used.
Equation of the reaction is:
(b) From the information provided, calculate the:
concentration of
I. Y(OH)2. XH2O in moldm-3;
II. Y2+ ions in moldm-3;
III. OH- ions in moldm-3.
(ii) mass of pure Y(OH)2. XH2O in 250 cm3 of A.
(iii) percentage purity of Y(OH)2. XH2O.
[Y(OH)2. XH2O = 150 g mol-I]
Observation
In part (a), majority of the candidates could not complete the table and calculate the average volume of B used.
In part (b), majority of the candidates could not calculate the concentration of the base in moldm-3. Also, only few of them could calculate the concentration of Y2+ ions in moldm-3. In addition to this, majority of them could not calculate the mass of pure Y(OH)2. XH2O in 250 cm3 of A.
The expected answers include:
(a) (i)
Burette reading / cm3 |
1 |
2 |
3 |
Final reading |
33.20 |
49.60 |
18.15 |
Initial reading |
16.80 |
33.20 |
1.50 |
Volume of B used / cm3 |
16.40 |
16.40 |
16.65 |
(ii) Average volume of B used
VB = 16.40 + 16.40
2
= 16.40 cm3
(b) (i)
I. Concentration of Y(OH)2.XH2O in moldm-3
From the equation of reaction
nY(OH)2.XH2O = 1 n(HCl) 2
CHCl = 0.025 mol
0.25 dm3
=0.100moldm3
CY.XH2O = CHCl xVHCl
2 x VY(OH)2.XH2O
= 0.100 x 16.40
2 x 25.0
= 0.0328 moldm-3
OR
(b) (i)
Concentration of Y(OH)2.XH2O
CHCI in moldm-3
250 cm3≡ 0.025 mol
1000 cm3 = 0.025 x 1000= 0.100 mol
250
CHCI=0.100moldm3=CB
CAVA = 1 mole ratio
CBVB 2
= 0.0328 moldm-3
OR
II. Y2+ ions in moldm-3
Y(OH)2.XH2O → Y2+ + 2OH+XH2O
n(Y2+) = 1
n(Y.XH2O) 1
C(Y2+) = C(Y(OH)2.XH2O)
= 0.0328 moldm-3
III. OH- ions in moldm-3
n(OH-) = 2
n(Y(OH)2.XH2O) 1
C(OH-) = 2 x 0.0328
= 0.0656 moldm-3
(iii) Percentage purity of Y(OH)2.XH2O
% purity = mass of pure x 100
Mass of impure
= 1.30 x 100
6.50