# Comparison of organic and inorganic acid strengths

So I know how different factors affect acid/base strength, but I am not sure when they are all in the same question, which affects acid/base strength more.

So, for example this question:

How do I know that $\ce{HBr}$ is the strongest acid...because I also have the inductive electron withdrawal from Fluorine in answer choice B.

You will be likely be expected to memorize approximate pKa values for different functional groups. Molecules containing protons with lower pKa values are more acidic.

In this case, these are the functional groups you have:

• a. alcohol (pKa ~16)
• b. alkyl halide and carboxylic acid (pKa ~4-5)
• c. hydrobromic acid (pKa ~-9)
• d. carboxylic acid (pKa ~4-5)
• e. ammonia (pKa ~35)

I would have to suspect that b is slightly more acidic than d, but they are going to be in similar ball parks. What you're looking for is the most and the least acidic compounds, and the pKas for those compounds are distinct in this question. Since the acidity of a group is inversely proportional to its pKa, the compound with the lowest pKa among its groups is the most acidic and the compound with the highest pKa among its groups is the least acidic. Things get a bit more complicated than that when multiple groups are involved, but this rule usually holds for simple cases.

Hydrobromic acid has the lowest pKa, so it is the most acidic. Ammonia has the highest pKa, so it is the least acidic.

pKa value is great indicator of acid base strength, lower the pKa value stronger is tha acid and conjugate base of that acid is week base due to high pKa value.

HBr has pKa value (-9) strongest acid after HI (pKa -10).