Posted by Saphire on Friday, January 20, 2012 at 4:00pm.
You COULD work each one for pH and see which is the lowest but that isn't necessary. I'll show you how to do one of them, HF since it is first.
.............HF ==> H^+ + F^-
initial......1.0.....0.....0
change.......-x......x......x
equil......1.0-x.....x.......x
Look up Ka. In my text Ka is listed as 7.2E-4
Ka = (H^+)(F^-)/(HF)
7.2E-4 = (x)(x)/(1-x)
Solve for x = 0.0268 M = (H^+) and pH = 1.57.
In a similar manner the pH of the other acids can be calculated, then compared to pick the one with the lowest pH.
BUT you can reason which will have the lowest pH. Low pH means highest acidity and highest acidity means the one with the largest Ka which means the strongest acid. (Note the M is the same and all have just one H atom/molecules.)
So
Ka HF = 7.2E-4
Ka HCN = 2.1E-9
Ka HCOOH = 1.77E-4
Ka CH3COOH = 1.8E-5
Based on these values I would pick HF as the strongest acid leading to the lowest pH. However, you should check in your text for Ka for each acid and pick the one with the largest Ka value.
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