Question

Calculate the pH in an aqueous 0.120 M nitrous acid solution.

Answer 1

pH can be calculated from pKa of aniline hydrochloride

Answer 2

Answer:- pH is 2.14.

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

the equation is written as:

`HNO_2(aq)+H_2O(l)\leftrightharpoons H_3O^+(aq)+NO_2^-(aq)`

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:

`HNO_2=0.120-x`

`H_3O^+` =
`x`

`NO_2^-` =
`x`

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

`Ka=([H_3O^+][NO_2^-])/(HNO_2)`

Let's plug in the values in it.

`4.5*10^-^4=((x)^2)/(0.120-x)`

To make the calculations easy we could ignore
`x` for the bottom and the expression becomes:

`4.5*10^-^4=((x)^2)/(0.120)`

On cross multiply:

`x^2=4.5*10^-^4*0.120`

On taking square root to both sides:

`x=7.3*10^-^3`

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

Now we could calculate the pH using the pH formula:

`pH=-log[H_3O^+]`

`pH=-log(7.3*10^-^3)`

pH = 2.14

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