A solution of HI has a [H3O+] of 1x10^-4. What are the pH and its pOH? What are [OH-] and [HI]?

Question

Is the soluton acidic or basic? how would I do this problem and what are the numbers that tell you wether a solution is acidic or basic?

Answer 1

[H3O+] is also [H+]
pH = - log[H3O+] = 4
pH + pOH = 14 => pOH = 10

=> [OH-] = 10^-10M
=> [HI] = [H3O+] = 10^-4M

this solution is acidic

if pH of a solution is less than 7 , it is acidic
if pH of a solution is greater than 7, it is basic

Answer 2

H3O+ is usually written as H+ (in H2O).

pH=-log[H+]=-log(1x10^-4)=-(-4)=4. This solution is acidic because the pH is smaller than 7. When pH is 7, the solution is neutral. When pH is greater than 7, the solution is basic.

pOH=14-pH =14-4=10. pOH=-log[OH-], so [OH-]=1x10^-10 (M).

[HI] is zero because HI is a strong acid, which dissociate completely to H+ (in the form of H3O+) and I-(aq).