What molarity of a NH4NO3 solution has a pH of 5.220? Kb for NH3 is 1.8 x 10^-5.

Question

Choose one answer.
a. 6.31x 10-6.
b. 2.21 x 10-6
c. 5.56x 10-10.
d. 0.065

I started by taking the inverse log of the PH to get (6.02*10^-6) then I was thinking its not the way I should start but perhaps.....using the Kb

1.00*10^-14/1.8*10^-5 which gave me 5.5*10^-10 so I am completely lost on which direction I need to take.....

I got the answer incorrect because I picked C thinking it was that but its not.

Answer 1

You were going great guns--you just stopped too soon.
NH4^+ + HOH ==> NH3 + H3O^+
So your Ka is correct at 5.55 x 10^-10
Ka = (NH3)(H3O^+)/(NH4^+)

You have the (H3O^+) and (NH3) is the same as (H3O^+). solve for (NH4^+), the only unknown in the equation.