Please explain... In the titration of a weak acid HA with 0.100 M NaOH, the stoi
ID: 1068546 • Letter: P
Question
Please explain...
In the titration of a weak acid HA with 0.100 M NaOH, the stoichiometric point is known to occur at a pH value of approximately 11. Which of the following indicators would be best to use to mark the endpoint of this titration?
a. an indicator with Ka = 10–13
b. an indicator with Ka = 10–14
c. an indicator with Ka = 10–10
d. an indicator with Ka = 10–8
e. an indicator with Ka = 10–11
An indicator HIn has Ka = 1 × 10–8. At pH = 6.0, what is the ratio HIn/In– ?
a. 1/100
b. 10/1
c. 1/1
d. 100/1
e. none of these
Explanation / Answer
a. an indicator with Ka = 10–13
weak acid + strong base ------------> resultant solution is more basic nature. in basic solution Ka range 10-12 to 10-13
b. Pka = -logKa
= -log10-8 = 8
PH = Pka + log[In-]/[HIn]
6 = 8 + log[In-]/[HIn]
log[In-]/[HIn] = 6-8
log[In-]/[HIn] = -2
[In-]/[HIn] = 10-2
HIn/In– = 100/1
d. 100/1