# what makes an acid a strong acid or weak acid? [duplicate]

When my teacher referenced nitric acid as a strong acid, I thought it was strong because it donates more than one H+ per molecule, but then I saw the equation is $\ce{HNO_3}$ and it only donates one. Acetic acid is also monoprotic, but it is considered a weak acid. What makes an acid strong or weak? If an acid donates more than one proton, does it make it stronger? Feel free to make any formatting changes to formulas as well.

• I recall at least 2 definitions from school: (1) the dividing line between weak vs. strong acids is at p$K_{\rm a}$ = 0, with weak ones having p$K_{\rm a}$ > 0 and strong ones with p$K_{\rm a}$ < 0; and (2) an arbitrary cutoff defined by $\ce{HF}$ as the weak acid, and anything with a p$K_{\rm a}$ < the p$K_{\rm a}$ of $\ce{HF}$ being a strong acid. I have no idea where (2) came from but I recall it. – Todd Minehardt Sep 9 '15 at 18:40
• I feel it would be appropriate to advice the topic starter to read something (there are quite a few reliable sources out there, Wiki is as good as any other), and then come back and ask what's left unclear. – Ivan Neretin Sep 9 '15 at 18:58
• – Mithoron Sep 9 '15 at 20:03