Calculate either [H3O+] or [OH-] for the solutions at 25°C.

Question

A) [OH-]=2.07×10^(-7) M

[H3O+]=

 

B)[H3O+]=8.77×10^(-9) M

[OH-]=

 

C)[H3O+]=0.000647 M

[OH-]=

 

Which of these solutioms are basic at 25°C?

Answer 1

From the definition of pH, you know that

 

[H₃O⁺][OH^-] = 10^-14

 

So if you are given [H₃O⁺] and you want to find [OH^-],

 

[OH^-] = 10^-14 / [H₃O⁺]

 

Similarly, if you are given [OH^-] and you want to find [H₃O⁺],

 

[H₃O⁺] = 10^-14 / [OH^-]

 

I will do the first one for you.  The others are solved similarly.

 

(a) 

 

[OH^-] = 2.07 × 10^-7

 

[H₃O⁺] = 10^-14 / [OH^-] = 10^-14 / (2.07 × 10^-7) = 4.83 x 10^-8

 

Find pH to determine if acid or base.

 

pH = -log[H₃O⁺] = -log(4.83 x 10^-8) = 7.32 > 7 ⇒ base (if < 7, acid)

 

Do the others the same way.