Acetic acid has a Ka = 1.75 x 10-5 and formic acid has a Ka = 1.8 x 10-4. Which is the stronger acid?

1 answer

You COULD go through the math and calculate the H^+ of each and the larger H^+ would be the stronger. However, you should be able to reason it through without the math.
........HAc ==> H^+ + Ac^-
1.75E-5 = (H^+)(Ac^-)/(HAc)

.........Hform ==> H^+ + form^-
1.8E-4 = (H^+)(form^-)/(Hform)

Which is the larger Ka? Hform of course. Larger Ka expression means products are larger than reactants. So larger Ka has larger H^+. Which is stronger?