Question

This is a two part question. Calculate either [H3O+] or [OH−] for each of the solutions at 25 °C.

Solution A: [OH−]= 2.91×10−7 M Solution A: [H3O+]= M

Solution B: [H3O+]=9.81×10−9 M   Solution B: [OH−]= M

Solution C: [H3O+]=0.000669 M Solution C: [OH−]= M

Which of these solutions are basic at 25 °C? (You are able to pick more than one option). Solution C, Solution A, and Solution B are the three options.

Solution C: [H3O+]= 0.000669 M

Solution A: [OH−]= 2.91×10−7 M

Solution B: [H3O+]= 9.81×10−9 M

Answer 1

PART 1

Calculations for solution A

We have relation , [H3O+] [OH-] = Kw = 1 10 -14

Therefore, [H3O+] = 1 10 -14 / [OH-] = 1 10 -14 / 2.91 10 -07 = 3.44 10 -08 M

We have relation, pH = - log [H3O+]

pH of solution A = - log 3.44 10 -08= 7.46

pH of solution A > 7 , therefore solution A is Basic.

Calculations for solution B

We have, [H3O+] [OH-] = Kw =1 10 -14

Therefore, [OH-] = 1 10 -14 / [H3O+] = 1 10 -14 / 9.81 10 -09 = 1.02 10 -06 M

We have relation, pH = - log [H3O+]

pH of solution B = - log 9.81 10 -09= 8.01

pH of solution B > 7 , therefore solution B is Basic.

Calculations for solution C

We have, [H3O+] [OH-] = Kw =1 10 -14

Therefore, [OH-] = 1 10 -14 / [H3O+] = 1 10 -14 / 0.000669  = 1.49 10 -11 M

We have relation, pH = - log [H3O+]

pH of solution C = - log 0.000669= 3.17

pH of solution C < 7 , therefore solution C is Acidic.

PART 2

ANSWER : Solution A : [OH-] =2.91 10 -07 M and Solution B :  [H3O+] =9.81 10 -09 M