VCE Stuff > VCE Chemistry
VCE Chemistry Question Thread
lzxnl:
Nope. None of that rubbish at all. I hated it too.
If you're keen, look through last year's exam for a guideline on what's on the course now.
Penguuu:
Ohhhh, I clearly need to practice more haha. Thanks both of youse ^^
Penguuu:
Hi guys, I'm stuck with this question.
A solution of hydrochloric acid of concentration 0.1M has a pH of 1. Ethanoic acid of the same concentration has a pH of approximately 3. Explain how this difference arises.
psyxwar:
--- Quote from: Penguuu on December 29, 2013, 10:56:56 am ---Hi guys, I'm stuck with this question.
A solution of hydrochloric acid of concentration 0.1M has a pH of 1. Ethanoic acid of the same concentration has a pH of approximately 3. Explain how this difference arises.
--- End quote ---
Ethanoic acid is a weak acid and unlike hydrochloric acid (a strong acid), does not completely ionise in water. Ionising less = less H+ in solution = higher pH. In a solution of HCl, most of the molecules are Cl- and H+, whereas in a solution of ethanoic acid, most of the molecules are ethanoic acid molecules (CH3COOH) that have not ionised.
DJA:
--- Quote from: Penguuu on December 29, 2013, 10:56:56 am ---Hi guys, I'm stuck with this question.
A solution of hydrochloric acid of concentration 0.1M has a pH of 1. Ethanoic acid of the same concentration has a pH of approximately 3. Explain how this difference arises.
--- End quote ---
Hydrochloric acid is a strong acid. A strong acid is one which ionises completely in water. We use a straight arrow to denote this
HCl + H2O -> H3O+ + Cl-
Ethanoic acid is a weak acid. A weak acid does not completely ionise in water, we use a reversible arrow to denote it.
CH3COOH + H2O ↔ H3O+ + CH3COO-
The H+ or H3O+ species is formed when an acid reacts with water. pH for practical purposes is the measure of the concentration of the hydronium ion H3O+ (the negative logarithm of the hydronium ion activity). Since ethanoic acid is not completely ionized, there is less than the maximum possible number of hydronium ions in the solution (there are un-ionized ethanoic acid molecules floating around) hence making it a higher pH than hydrochloric acid which is fully ionized and has more hydronium ions in the solution than ethanoic acid giving it a lower pH.
(Edit:psyxwar beat me to it but ill post anyway for the sake of completeness)
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