Answer : The correct option is, (B) 0.82 M
Explanation : Given,
The dissociation constant for formic acid = [tex]K_a=1.8\times 10^{-4}[/tex]
Concentration of formic acid (weak acid)= 0.72 M
pH = 3.80
First we have to calculate the value of [tex]pK_a[/tex].
The expression used for the calculation of [tex]pK_a[/tex] is,
[tex]pK_a=-\log (K_a)[/tex]
Now put the value of [tex]K_a[/tex] in this expression, we get:
[tex]pK_a=-\log (1.8\times 10^{-4})[/tex]
[tex]pK_a=4-\log (1.8)[/tex]
[tex]pK_a=3.745[/tex]
Now we have to calculate the concentration of sodium formate (conjugate base or salt).
Using Henderson Hesselbach equation :
[tex]pH=pK_a+\log \frac{[Salt]}{[Acid]}[/tex]
Now put all the given values in this expression, we get:
[tex]3.80=3.745+\log (\frac{[Salt]}{0.72})[/tex]
[tex][Salt]=0.82M[/tex]
Therefore, the concentration of sodium formate is 0.82 M.
