A 0.001 mol L⁻¹ solution of hydrochloric acid and a 0.056 mol L⁻¹ solution of ethanoic acid both have a pH of 3.0 - HSC - SSCE Chemistry - Question 21 - 2010 - Paper 1
Question 21
A 0.001 mol L⁻¹ solution of hydrochloric acid and a 0.056 mol L⁻¹ solution of ethanoic acid both have a pH of 3.0.
Why do both solutions have the same pH?
Worked Solution & Example Answer:A 0.001 mol L⁻¹ solution of hydrochloric acid and a 0.056 mol L⁻¹ solution of ethanoic acid both have a pH of 3.0 - HSC - SSCE Chemistry - Question 21 - 2010 - Paper 1
Step 1
Identify HCl as a Strong Acid
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Answer
Hydrochloric acid (HCl) is classified as a strong acid because it fully ionizes in aqueous solution. This means that in a solution of 0.001 mol L⁻¹ HCl, the concentration of hydrogen ions ( ext{[H⁺]}) will also be 0.001 mol L⁻¹.
Step 2
Identify Ethanoic Acid as a Weak Acid
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Answer
Ethanoic acid (CH₃COOH) is a weak acid, which means it does not fully ionize in aqueous solution. Its ionization degree is lower, resulting in fewer hydrogen ions compared to strong acids at the same concentration.
Step 3
Comparison of pH and [H⁺]
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Answer
Despite the lower concentration of HCl, both solutions have the same pH of 3.0. The pH can be calculated using the formula:
extpH=−extlog(ext[H+])
For both solutions, the concentration of hydrogen ions can be expressed as:
For HCl: ext{[H⁺]} = 0.001 mol L⁻¹ leading to pH = 3.
For CH₃COOH: The total concentration (0.056 mol L⁻¹) must be high enough to generate an equivalent [H⁺]. Therefore, by equilibrium calculations, ethanoic acid contributes enough [H⁺] to achieve the same pH as the strong acid.
