ph of 0.1 molar solution of any NaA is 8.92. calculate Pka ofHA is if a drop phenolphthalein whose pk=9.52 be added to the above solution . predict weather a pink color solution would be visible or not under the medical fact our eyes can see the pink color when the molar percentage ionized form of indicator is 25%or more
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Chemistry
pH of 0.1 M solution of NaA (sodium salt of a weak acid HA) is 8.92. Calculate pK
a
of HA. If a drop of HPh (pK
in
=9.52) be added to the above solution, predict whether the pink colour will visible or not under the medical fact that our eyes can see the pink colour if the mole % of ionized form of indicator is 25% or more.
Answer
Correct option is
A
Pink colour will be visible
A
⊖
0.1
0.1−x
+
H
2
O
⟶
HA
x
+
O
H
⊖
x
K
r
K
r
=
0.1−x
x
2
=
K
HAa
K
w
⇒ K
aHA
=K
w
(
x
2
0.1−x
)
[pK
a
=lK
w
−log(0.1−x)+2logx]−−−(1)
pH=−logx=8.92 ⇒ x=10
−8.92
putting this in (1) & joining x in (0.1−x)
as x<<0.1
pK
a
=pK
w
−log0.1+2logx
=14−8.92+1
=6.08
2
nd
part:
HPh+O
H
⊖
⟶P
H
⊖
+H
2
O
PK
in
=9.52 ⇒ K
in
=K
ax
n=10
−9.52
K
rxn
=10
−9.52
=
[HPh][OH
⊖
]
[Ph
⊖
][H
⊖
][OH
⊖
]
∴
[HPh]
[Ph
⊖
]
=10
−9.52+8.92
=10
−0.6
≅0.2512>0.25
∴ pink variable