[Blonde]

Which is the weakest acid?

H3BO3, Ka= 6 x 10 [-10]
H2S, Ka= 9 x 10 [-8]
CH3COOH, Ka= 2 x 10 [-5]
H3PO4, Ka= 7 x 10 [-3]

[Will]

What do you think is the weakest acid?
Remember- the lower the K_a, the weaker the acid.

[Donaldson Tan]

Ref: http://www.chemicalforums.com/index.php?topic=9037.0
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HA <-> H⁺ + A^-
K_a = [ H⁺ ] [ A^- ] / [ HA ]

Since K_a is the ratio of the product of concentration of the product to that of the reactants, than the higher the value of K_a, the more likely the acid dissociates to produce H⁺ ions in solution. Hence, the higher the value of K_a, the stronger it is as an acid.