A 1.0 M solution of acetic acid (Ka = 1.8 × 10⁻⁵) and a 0.001 M solution of HCl are compared. Which statement is correct?
AHCl is the stronger acid because its solution is more concentrated
BAcetic acid is the stronger acid because its concentration is higher
CHCl is the stronger acid because it ionizes completely regardless of concentration
DThey are equally strong because Ka and concentration balance out
Acid strength refers to the degree of ionization, not concentration. HCl is a strong acid that ionizes essentially 100% in water regardless of how dilute it is. Acetic acid is a weak acid — it partially ionizes (Ka = 1.8 × 10⁻⁵ means very little dissociation). A dilute solution of HCl is still a stronger acid than a concentrated solution of acetic acid. These two concepts — strength and concentration — must be kept separate.
Question 2 True / False
In the reaction CH₃COOH + H₂O ⇌ CH₃COO⁻ + H₃O⁺, water is acting as an acid.
TTrue
FFalse
Answer: False
In this reaction, water accepts a proton from acetic acid, making water the Brønsted-Lowry base (proton acceptor), not the acid. Acetic acid (CH₃COOH) donates the proton, so it is the acid. Water is amphoteric — it can act as either an acid or a base depending on its reaction partner — but in this specific reaction it acts as the base.
Question 3 Short Answer
HCl ionizes completely in water, producing Cl⁻ as its conjugate base. Why is Cl⁻ an extremely weak base?
Think about your answer, then reveal below.
Model answer: Because Cl⁻ has almost no tendency to accept a proton from water. The stronger an acid, the weaker its conjugate base — since HCl is a very strong acid, it releases its proton very easily and has essentially no desire to reclaim it.
Conjugate acid-base strength is inversely related. A strong acid ionizes completely because the bond to H⁺ is weak — the conjugate base has little affinity for the proton. Cl⁻ is so stable after losing H⁺ that it essentially never recaptures one in aqueous solution, making it a negligible base. This is why HCl solutions don't revert to HCl once dissolved.