A-Level Chemistry October/November 2024 Q1(b)(i): Show that the concentration of hydroxide ions in solution F is 0.123moldm-3.

1 answer

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  The pH of solution F is given as 13.09.

  First, calculate the pOH of the solution using the relationship $pK_w = pH + pOH$ at 298 K. $pOH = pK_w - pH$ $pOH = 14.00 - 13.09 = 0.91$

  Next, calculate the concentration of hydroxide ions, $[OH^{-}]$, from the pOH. $[OH^{-}] = 10^{-pOH}$ $[OH^{-}] = 10^{-0.91}$ $[OH^{-}] = 0.123026... \text{ mol dm}^{-3}$

  Therefore, the concentration of hydroxide ions is $0.123 \text{ mol dm}^{-3}$ (to 3 s.f.).

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  How the marks are awarded

  • M1 — Correctly calculating the pOH of the solution by subtracting the given pH from pKw (14.00), as shown by the working pOH = 14.00 - 13.09 = 0.91.
  • A1 — Correctly using the calculated pOH to find the hydroxide ion concentration, as shown by the expression [OH-] = 10^-0.91.

  Common mistakes

  • Calculating [H+] as 10^-13.09 and then incorrectly stating this is the [OH-] concentration.
  • Making a subtraction error when finding pOH, e.g., 14.00 - 13.09 = 0.81.
  • Using an incorrect formula, such as trying to divide pH by pOH or vice versa.
  • Showing no working and just writing the final answer, which scores no marks in a 'show that' question.

  Examiner tip: Master the interconversion between pH, pOH, [H+], and [OH-] using the key relationships pH = -log[H+], pOH = -log[OH-], and pKw = pH + pOH.

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