# Calculate the ph in an aqueous 0.120 m nitrous acid solution.

## Answers

- pH is 2.14.

Solution:- Nitrous acid, [tex]HNO_2[/tex] is a weak acid so first of all we solve for [tex]H_3O^+[/tex] and then figure out the pH.

the equation is written as:

[tex]HNO_2(aq)+H_2O(l)\leftrightharpoons H_3O^+(aq)+NO_2^-(aq)[/tex]

Initial concentration for the acid is given as 0.120 M. Let's say the change in concentration is x. Then the equilibrium concentrations would be as:

[tex]HNO_2=0.120-x[/tex]

[tex]H_3O^+[/tex] = [tex]x[/tex]

[tex]NO_2^-[/tex] = [tex]x[/tex]

Ka for nitrous acid is [tex]4.5*10^-^4[/tex] and the equilibrium expression for this would be written as:

[tex]Ka=\frac{[H_3O^+][NO_2^-]}{HNO_2}[/tex]

Let's plug in the values in it.

[tex]4.5*10^-^4=\frac{(x)^2}{0.120-x}[/tex]

To make the calculations easy we could ignore [tex]x[/tex] for the bottom and the expression becomes:

[tex]4.5*10^-^4=\frac{(x)^2}{0.120}[/tex]

On cross multiply:

[tex]x^2=4.5*10^-^4*0.120[/tex]

On taking square root to both sides:

[tex]x=7.3*10^-^3[/tex]

So, [tex][H_3O^+]=7.3*10^-^3M[/tex]

Now we could calculate the pH using the pH formula:

[tex]pH=-log[H_3O^+][/tex]

[tex]pH=-log(7.3*10^-^3)[/tex]

pH = 2.14

So, the pH of 0.120M nitrous acid is 2.14.

pH can be calculated from pKa of aniline hydrochloride

[tex]Calculate the ph in an aqueous 0.120 m nitrous acid solution.[/tex]

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