Investigating pH: Comparing the Strength, Proticity, and Concentration of Acids
Dot-Point 3: conduct an investigation to demonstrate the use of pH to indicate the differences between the strength of acids and bases (ACSCH102)
The pH of a solution can reveal important information about its acidity and the strength of the acid present. Through this investigation, we will explore how pH measurements highlight the differences between acids of varying strength, proticity, and concentration.
1. Comparing Acids of Different Strength
Solution 1: Strong Acid (HCl)
Concentration: 1.0 mol L⁻¹
Dissociation: Completely dissociates into [H+]
[H+]=1.0[H^+] = 1.0 mol L⁻¹
pH = -log(1.0) = 0.0
Solution 2: Weak Acid (CH₃COOH)
Concentration: 1.0 mol L⁻¹
Dissociation: Partially dissociates into [H+]
Given [H+]=0.020
pH = -log(0.020) = 1.70
Observation
The strong acid (HCl) has a much lower pH (0.0) than the weak acid (CH₃COOH) (1.70), even though both are at the same concentration. This is because HCl completely dissociates, producing a higher concentration of hydrogen ions.
2. Comparing Acids of Different Proticity
Solution 1: Monoprotic Acid (HCl)
Concentration: 1.0 mol L⁻¹
Dissociation: Fully dissociates into [H+][H^+].
[H+]=1.0[H^+] = 1.0 mol L⁻¹
pH = -log(1.0) = 0.0
Solution 2: Diprotic Acid (H₂SO₄)
Concentration: 1.0 mol L⁻¹
Dissociation:
The first hydrogen ion (H+H^+) dissociates completely.
The second hydrogen ion partially dissociates, adding additional [H+][H^+].
Given [H+]=1.2[H^+] = 1.2 mol L⁻¹
pH = -log(1.2) = -0.08
Observation
The diprotic acid H2SO4 produces a higher concentration of hydrogen ions [H+] due to its additional ionization, resulting in a slightly lower pH compared to HCl. However, this does not imply H2SO4 has greater strength—it simply produces more ions due to its diprotic nature.
3. Comparing Acids of Different Concentration
Solution 1: Dilute Strong Acid (HCl)
Concentration: 0.0010 mol L⁻¹
Dissociation: Fully dissociates into [H+][H^+].
[H+]=0.0010[H^+] = 0.0010 mol L⁻¹
pH = -log(0.0010) = 3.00
Solution 2: Concentrated Weak Acid (CH₃COOH)
Concentration: 1.0 mol L⁻¹
Dissociation: Partially dissociates into [H+][H^+].
Given [H+]=0.0042[H^+] = 0.0042 mol L⁻¹
pH = -log(0.0042) = 2.37
Observation
Despite being a weak acid, acetic acid (CH₃COOH) at 1.0 mol L⁻¹ has a lower pH (2.37) than the highly diluted HCl (0.0010 mol L⁻¹) with a pH of 3.00. This highlights how concentration can impact pH more significantly than acid strength in some cases.
