Arrange the following solutions in the decreasing order of pOH :
(A) 0.01 M HCl
(B) 0.01 M NaOH
(C) 0.01 M CH₃COONa
(D) 0.01 M NaCl

Solution

Chemistry ma’am · 3 steps

The more basic the solution, the smaller its pOH, so rank from most acidic to most basic.

1. Read pOH correctly. pOH = -log[OH⁻], so more OH⁻ means a smaller pOH. The acidic solution has the largest pOH and the strongly basic one the smallest.
2. Hydrolysis of acetate. NaCl comes from a strong acid and a strong base, so it does not hydrolyse and stays neutral. CH₃COONa comes from a weak acid and a strong base. Its CH₃COO⁻ ion takes a proton from water and releases OH⁻, so the solution is mildly basic.

   pOH = 1/2(pK_w - pKₐ - log C) = 1/2(14 - 4.74 + 2) ≈ 5.63

   Solution	Nature	pOH
   (A) 0.01 M HCl	strong acid, pH = 2	14 - 2 = 12
   (D) 0.01 M NaCl	neutral, no hydrolysis	7
   (C) 0.01 M CH₃COONa	mildly basic by hydrolysis	about 5.6
   (B) 0.01 M NaOH	strong base, [OH⁻] = 10⁻² M	2

3. So. Decreasing pOH is 12 > 7 > 5.6 > 2, that is (A) > (D) > (C) > (B), which is option (b).

The idea to keep. A salt of a weak acid and a strong base gives a basic solution, so its pOH is below 7. Large pOH means acidic. Always check whether the question asks for pH or pOH before you rank.

Answer from the NTA final answer key. Solution by PrepWiser.