Carmen V.

asked • 05/16/18

Which of the following is the weakest acid?

A.  HF (KA for HF is 7.2 x 10-4)
B.  H3BO3 (Ka for H3BO3 is 5.4 x 10 -10)
C.  HCNO (ka for HCNO is 2 x 10-4)
D.  HCN (ka for HCN is 4.9 x 10-10)

2 Answers By Expert Tutors

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J.R. S. answered • 05/16/18

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The weakest acid has the smallest Ka. In this problem that would be (D) HCN. 
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May J.

Thank you, I was wondering why it would be this and not B.? Just because they have the same exponent but the first number is what I don't understand. I know the exponent is smaller but what about the number in front?
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05/16/18

J.R. S.

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So look at them as though they are both x10^-11. You’d have 54 and 49 x10^-11, and 49 is smaller than 54. Or another way is just forget the exponent since they are the same, and just compare 49 to 54. 
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05/16/18

Jackie L. answered • 05/16/18

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In general, the more positive the pKa is, the weaker the acid. The values in the answer choices are in Ka. pKa=-log(Ka), so the smaller the Ka value is, the weaker the acid. In your specific question, the answer would be  C (HCNO).
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J.R. S.

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Jackie,
I believe you have this backwards. 
An acid with Ka = 1x10^-2 is stronger than an acid with a Ka = 1x10^-3. 
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05/16/18

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