Find the pH of a 0.1 M salt of weak acid and a strong
base at
25°C.
Given :- Ka of weak acid = 10-5
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Answer:
Correct option is
A
4
Since, pK
b
=7, K
b
=1×10
−7
The base dissociation equilibrium is as shown below.
B
−
+H
2
O⇌BH+OH
−
.
Let the hydroxide ion concentration be x M.
B
−
BH
OH
−
Initial concentration (M)
0.1
0 0
Equilibrium concentration (M)
0.1−x
x x
The expression for the equilibrium constant is K=
[B
−
]
[BH][OH
−
]
Substitue values in the above expression.
1×10
−7
=
0.1−x
x×x
Since, the value of the equilibrium constant is very small, the value of x is also very small.
Hence, 0.1−x≈=0.1
The equilibrium constant expression becomes
0.1
x
2
=1×10
−7
Hence, x=1×10
−4
.
Hence, the pOH of the solution is pOH=−log[OH
−
]=−log1×10
−4
=4.
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