Which of the following weak acids would produce more hydronium ion when placed in water?
hcho2 (ka = 1.8 x 10–4)
hf (ka = 7.2 x 10–4)
hno2 (ka = 4.5 x 10–4)
hc2h3o2 (ka = 1.8 x 10–5)

The HF produces more Hydronium ions

Explanation:

The strength of an acid is asociated to several factors. One of them, is the pH of them. However we don't have enough information to get the pH of every one of the acids.

We only have information of the Ka. This values indicates the capacity of each acid to dissociate in hydronium ions and to react with a strong or weak base, and also indicates if the acid is strong or weak, so it's pH should be lower. The lower the pH, the greater it's concentration in Hydronium is. And if the hydronium is high, means that the Ka shoould be higher too, and we can associate the Ka with the pKa which is a number that indicates in a better way, the acidic of a solution, if this number is low, then the acid is more strong and therefore, the hydronium is higher.

So let's calculate the pKa of each of these acids, the one with the lowest value is the strongest.

pKa = -logKa

so with all the acids here, let's calculate the pKa:

pKa1 = -log(1.8x10^-4) = 3.74

pKa2 = -log(7.2x10^-4) = 3.14

pKa3 = -log(4.5x10^-4) = 3.35

pKa4 = -log(1.8x10^-5) = 4.74

Therefore, HF with the lowest pKa is the one with more hydronium ions in water.