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Cyanidin is a plant pigment that may be used as a pH indicator - HSC - SSCE Chemistry - Question 20 - 2022 - Paper 1
Question 20
Cyanidin is a plant pigment that may be used as a pH indicator. It has four levels of protonation, each with a different colour, represented by these equilibria: H₃... show full transcript
Worked Solution & Example Answer:Cyanidin is a plant pigment that may be used as a pH indicator - HSC - SSCE Chemistry - Question 20 - 2022 - Paper 1
Step 1
What colour would the indicator be if added to a 0.75 mol L⁻¹ solution of hypoiodous acid, HIO (pKₐ = 10.64)?
Answer
To determine the color of the indicator in the given solution, we first note the pKₐ value of the acid. A solution of hypoiodous acid (HIO) with a pKₐ of 10.64 is considered relatively weak, indicating that the corresponding equilibrium will predominantly lean towards the undissociated form due to the low concentration of hydronium ions.
Next, we analyze the species at various pH levels based on the graph and the equilibria provided. The given pH of the solution can be estimated based on the pKₐ. Since the pKₐ is 10.64, the pH of the solution is expected to be on the acidic side, likely around 6-8, depending on the exact concentration.
From the graph, we observe the following:
- At low pH (around 2-4), the indicator is in the protonated form (H₃Cy⁺), which appears red.
- As the pH increases past the pKₐ, there is a transition of colors. Most relevantly, at a pH around 6-8, the species in relative amounts would shift toward the HCy⁻ and later to the Cy²⁻ forms.
- At pH levels closer to 6, the dominant species is HCy⁻, which corresponds to the color purple.
Given the expected pH in the solution and the transition states of the indicator, we conclude that the color of the indicator in a 0.75 mol L⁻¹ solution of hypoiodous acid would be C. Purple.