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>how can two solutions having the same pH differ in acidity?
Great follow-up question! Two solutions can have the same pH but still differ in acidity due to differences in the strength of the acids involved. Here’s how that works:
1. Strong vs. Weak Acids:
Even though pH measures the concentration of hydrogen ions ( H+ H + ) in a solution, the nature of the acid (whether it’s a strong acid or weak acid) can still make them behave differently in terms of their overall acidity.
Strong acids (like HCl, hydrochloric acid) dissociate completely in water. They donate all of their protons, leading to a high concentration of H+ H + ions in solution.
Weak acids (like acetic acid, found in vinegar) dissociate partially in water. They do not release all of their protons, and the concentration of H+ H + is lower compared to a strong acid at the same pH.
2. How Two Solutions Can Have the Same pH but Differ in Acidity:
Same pH, Different Acid Strength: Let’s say we have two solutions, both with a pH of 4.
Solution 1: Made with hydrochloric acid (HCl), a strong acid. Since HCl dissociates completely, it provides a large number of H+ H + ions for a given amount of acid.
Solution 2: Made with acetic acid (CH₃COOH), a weak acid. Even though the pH is the same as HCl, acetic acid only dissociates partially, so the concentration of H+ H + ions is much lower than in the HCl solution.
Both solutions may have the same pH (4), which means they have the same [H⁺] concentration, but the weak acid solution (acetic acid) is less "acidic" in terms of its ability to donate protons. This is because only a small fraction of acetic acid molecules release their protons into solution.
3. Acid Strength and pKa:
The acid dissociation constant (Ka) or pKa tells us how strong an acid is.
A high Ka (or a low pKa) means the acid is strong (it dissociates more readily).
A low Ka (or high pKa) means the acid is weak (it dissociates less).
Two solutions with the same pH could have different acid strengths, and the acid with the lower pKa would be considered stronger, even if both solutions have the same concentration of hydrogen ions.
4. Example:
Solution A: 0.1 M HCl (strong acid)
Completely dissociates into H+ H + and Cl− Cl − , so it provides a high concentration of hydrogen ions at pH 1.
Solution B: 0.1 M Acetic Acid (weak acid)
Only partially dissociates, so the concentration of H+ H + ions at pH 1 will be much lower, and the solution will be "less acidic" in its proton-donating ability.
So, while the pH of both solutions might be the same, the acid strength and the degree of dissociation (which relates to their acidity) differ because one is a strong acid (HCl) and the other is a weak acid (acetic acid).
TL;DR:
Two solutions with the same pH can differ in acidity because the acid strength (strong vs. weak) determines how fully the acid dissociates into hydrogen ions. A strong acid will have more dissociation and proton donation, even at the same pH, compared to a weak acid, which dissociates less.