Question

The pOH of a solution is 10.75. What is the concentration of OH– ions in the solution?

Answer 1

1. Since you have the Ka for NH3 (I'm going to assume that's a typo, and you meant NH3), you can easily find Kb by using Ka x Kb = Kw. Kw is something you can look up online if you don't know it already.

2. The pH of the solution is 10.75. This means the pOH of the solution is 14-10.75. pOH is equal to -log[OH-], so to find [OH-], you use 10^(-pOH).

3. The expression for Kb is [HB+][OH-]/[B]. [B] is what you're trying to find. Because ammonia is a weak base, you can assume that the dissociated ion concentrations, [HB+] and [OH-], are the same. Now, this is a pretty simple problem. You have the Kb from step 1 and [OH-] from step 2. Since [HB+] and [OH-] are the same, you square the concentration of [OH-]. Now, it should be pretty easy to find [B], the concentration of the solution. Just plug everything in.

Answer 2

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