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Define the term weak acid - NSC Physical Sciences - Question 7 - 2017 - Paper 2
Question 7
Define the term weak acid. Which acid, OXALIC ACID or CARBONIC ACID, is stronger? Give a reason for the answer. Oxalic acid ionises in water according to the follo... show full transcript
Worked Solution & Example Answer:Define the term weak acid - NSC Physical Sciences - Question 7 - 2017 - Paper 2
Step 1
Step 2
Which acid, OXALIC ACID or CARBONIC ACID, is stronger? Give a reason for the answer.
Answer
Oxalic acid is stronger than carbonic acid because it has a higher acid dissociation constant (Kₐ) value of 5.6 × 10⁻² compared to carbonic acid's Kₐ value of 4.3 × 10⁻⁷. A higher Kₐ indicates a greater tendency to dissociate and donate protons.
Step 3
Step 4
Calculate the pH of the solution.
Answer
To calculate the pH of the sodium hydroxide solution:
-
Calculate the concentration of hydroxide ions, [OH⁻]:
[OH⁻] = 0.1 mol.dm⁻³
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Now, calculate the pOH:
pOH = - ext{log}[OH⁻]
pOH = - ext{log}(0.1) = 1
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Since pH + pOH = 14, we find:
pH = 14 - pOH = 14 - 1 = 13
Step 5
Calculate the concentration of the oxalic acid solution.
Answer
Using the titration data:
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Calculate moles of NaOH used:
n(NaOH) = c × V = (0.1 mol.dm⁻³) × (0.0251 dm³) = 0.00251 mol
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The balanced reaction shows a 1:1 ratio between NaOH and (COOH)₂:
n(COOH)₂ = n(NaOH) = 0.00251 mol
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Now, calculate the concentration:
c(COOH)₂ = n/V = 0.00251 mol / 0.0142 dm³ ≈ 0.176 mol.dm⁻³
Step 6
Which ONE of the indicators above is most suitable for this titration? Give a reason for the answer.
Answer
Indicator B (pH range 6,0 - 7,6) is the most suitable because the pH at the equivalence point of this titration will be slightly above 7, due to the presence of the weak oxalic acid, making B appropriate for detecting the endpoint.