Question

If \([\text{H}_3\text{O}^+]\) of a solution is \(1\times 10^{-4}\). What is the value of pOH at \(298\,\text{K}\) ?

• A. \(4\)
• B. \(10\)
• C. \(12\)
• D. \(14\)

Hint:

First get pH from the hydronium concentration, then use pH + pOH = 14.

Answer 1

The Correct Option is B

Step 1: Alternative route
Use the hydroxide ion concentration. At 298 K, $K_w = 10^{-14}$.

Step 2: Compute
$[\text{OH}^-] = K_w/[\text{H}_3\text{O}^+] = 10^{-14}/10^{-4} = 10^{-10}$ M.

Step 3: pOH
$\text{pOH} = -\log(10^{-10}) = 10$.

Step 4: Meaning
Acidic solutions have very little hydroxide, so a large pOH is expected. The values 4, 12 and 14 do not match $[\text{OH}^-] = 10^{-10}$ M.

Final Answer:
Hydroxide concentration is 1e-10 M, so pOH is 10. This is option (B). \[ \boxed{\text{(B) }10} \]