3.3 - Hydrogen Ions and pH

Question 1

Define the ion-product constant for water, Kw, and write its expression and value at 25 oC?

Answer 1

It is the value of the equilibrium constant expression for the self-ionization of water.

Kw= [H+][OH⁻]
At 25 oC, Kw= 1.0 × 10−14

Question 2

The concentration of hydrogen ions in a solution at 25oC is 4.10 ×10^-4. Calculate [OH⁻] ?

Answer 2

1.0 × 10^−14 = [H+][OH⁻]
[OH⁻] = 1.0× 10^−14 / 4.10 ×10^-4
[OH⁻] = 2.45 ×10^-11M

Question 3

At 298 K, the H+ ion concentration in a cup of coffee is 1.0 ×10^-5. What is the OH⁻ ion concentration in the coffee? Is coffee acidic, basic, or neutral?

Answer 3

1.0 × 10−14 = [H+][OH⁻]
[OH⁻] = 1.0 × 10^−14 / 1.0 ×10^-5
[OH⁻] = 1.0×10^-9M
Because [H+] > [OH⁻], the coffee is acidic.