Question

A solution has a pH of 2.5. Answer the following questions.
pOH =
[H+] =
[OH-] =

Answer 1

pH + pOH = 14  ⇒  pOH = 14 - pH
pOH = 14 - 2.5
pOH = 11.5

[H⁺] = 10^(-pH) = 10^(-2.5)
[H⁺] = 0.003 M
[OH⁻] = 10^(-pOH) = 10^(-11.5) = 3 × 10⁻¹² M
[OH⁻] = 3 × 10⁻¹² M


pH = 2.5 implies one significant digit

Answer 2

The pOH, hydrogen ion concentration, [H⁺] and hydroxide ion concentration [OH¯] of the solution are:

1. The pOH of the solution is 11.5

2. The hydrogen ion concentration, [H⁺] of the solution is 3.16×10¯³ M

3. The hydroxide ion concentration [OH¯] is 3.16×10¯¹² M

1. Determination of the pOH of the solution.

pH = 2.5

pOH =?

pH + pOH = 14

2.5 + pOH = 14

Collect like terms

pOH = 14 – 2.5

pOH = 11.5

Thus, the pOH of the solution is 11.5

2. Determination of the hydrogen ion concentration, [H⁺] of the solution.

pH = 2.5

Hydrogen ion concentration [H⁺] =?

pH = –Log [H⁺]

2.5 = –Log [H⁺]

–2.5 = Log [H⁺]

Take the antilog of –2.5

[H⁺] = antilog –2.5

[H⁺] = 3.16×10¯³ M

Thus, the hydrogen ion concentration, [H⁺] in the solution is 3.16×10¯³ M

3. Determination of hydroxide ion concentration [OH¯] in the solution

pOH = 11.5

Hydroxide ion concentration [OH¯] =?

pOH = –Log [OH¯]

11.5 = –Log [OH¯]

–11.5 = Log [OH¯]

Take the antilog of –11.5

[OH¯] = antilog –11.5

[OH¯] = 3.16×10¯¹² M

Therefore, the hydroxide ion concentration [OH¯] in the solution is 3.16×10¯¹² M

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