Chemistry Reference
In-Depth Information
(a)
Explain why back titrations are sometimes used in volumetric
analysis.
(b)
Write balanced chemical equations for the reactions
expressed above, and hence calculate the weight of lithium
carbonate equivalent to 1 mL of 1
M
HCl (the equivalent
relationship).
(c)
This assay was carried out and the following results were
obtained
Weight of bottle
sample
11.7707 g
Weight of bottle
residual sample
10.7142 g
Volume of 1
M
(
f
0.9989) HCl added
50.00 mL
Burette readings, titrant 1
M
(
f
1.012) NaOH:
Initial volume
0.50 mL
Final volume
21.55 mL
(i)
Calculate the percentage weight in weight of
lithium
carbonate in the sample.
(ii)
What is the significance of an answer greater than 100%?
(iii)
Suggest an indicator for this assay, and explain your
reasoning.
Methyldopa (Figure 6.6) is a drug useful in the treatment of
hypertension. The BP assay for methyldopa is as follows.
Q6.2
COOH
HO
H
3
C
NH
2
HO
Figure 6.6
The structure of methyldopa, M
r
211.2.
Weigh about 0.2 g of sample accurately and dissolve in a mixture
of 15 mL of anhydrous formic acid, 30 mL of anhydrous acetic
acid and 30 mL of 1,4-dioxan. Titrate with 0.1
M
perchloric acid
using crystal violet solution as indicator.
(a)
State which technique of volumetric assay is used for
methyldopa, and explain why titrations of this type are
