Concept explainers
8-112 Consider an initial 0.040 M hypobromous acid (HOBr) solution at a certain temperature.
At equilibrium after partial dissociation, its pH is found to be 5.05. What is the acid ionization constant, Ka, for hypobromous acid at this temperature?

Interpretation:
The acid dissociation constant of hypobromous acid is to be calculated.
Concept Introduction:
Weak acids do not dissociate completely. Let HA be a weak acid. The dissociation of the weak acid can be represented by the chemical equation,
HA(aq)⇌H+(aq)+A−(aq)
The equation for acid dissociation constant can be written from this chemical equation.
Ka=[H+][A−][HA]
Here, [H+] is concentration of hydrogen ion, [A−] is concentration of conjugate base and [HA] is concentration of acid.
Answer to Problem 104P
The acid dissociation constant of hypobromous acid is Ka=1.9858×10−9.
Explanation of Solution
Hypobromous acid is a weak acid. Hence, it do not dissociate completely. The dissociation of the given weak acid can be represented by the chemical equation,
HOBr(aq)⇌H+(aq)+OBr−(aq)
The equation for acid dissociation constant can be written from this chemical equation.
Ka=[H+][OBr−][HOBr]
The concentrations of each of the ions at equilibrium can be obtained from the ICE table. Where ICE represents the Initial, Change and Equilibrium concentrations of the weak acid.
HOBr(aq)⇌H+(aq)+OBr−(aq) [HOBr] [H+] [OBr−]i 0.040 0 0 C −X +X +X E 0.040−X +X +X
The hydrogen ion concentration can be obtained from the given pH. The pH is defined as the negative logarithm of the hydrogen ion concentration.
pH=−log[H+]
The pH of the weak acid solution at equilibrium is 5.05. Thus, we can calculate the concentration of the hydrogen ion.
pH=−log[H+][H+]=10−5.05=8.9125×10−6 mol L−1X=[H+]=8.9125×10−6 mol L−1
We calculated the “x” which is the concentration of hydrogen ion. The concentration of the anion is also “x”. Thus,
X=[OBr−]=8.9125×10−6 mol L−1
Now, we need to calculate the concentration of [HOBr] = 0.040 − x. The value of x is very negligible hence, it can be ignored, and put just 0.040. Thus, the concentration of hypobromous acid is.
[HOBr] = 0.040.
The concentrations of the anion, hydrogen ion and hypobromous acid are used in the equation used for acid dissociation constant.
Ka=[H+][OBr−][HOBr]=[8.9125×10−6][8.9125×10−6]0.040Ka=1.9858×10−9
Thus, the acid dissociation constant of hypobromous acid is Ka=1.9858×10−9.
Weak acids do not dissociate completely. Each weak acid has a specific dissociation constant. Here, ICE table is made from the given chemical equation. Thus, the acid dissociation constant of hypobromous acid is Ka=1.9858×10−9.
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Chapter 8 Solutions
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