Worksheet Dec 1 2011 key

Worksheet Dec 1 2011 key - Quiz Section Worksheet Dec 1...

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Quiz Section: ____ Name:____KEY______ Worksheet Dec 1, 2011 Score: ____ out of 10 points 1. a.) Calculate the pOH of a 0.30 M solution of NaF. The acid HF has a dissociation constants of 7.2x10 -4 . K b = K w / K a = 10 -14 / (7.2 * 10 -4 ) = 1.4 * 10 -11 K b = [OH - ][HF] / [F - ] F - HF + OH - I 0.30 - - C -x +x +x E 0.30 - x x x K b = x * x / (0.30-x) x 2 / 0.30 [OH - ] = x = (0.30 * 1.4 * 10 -11 ) 1/2 = 2.0 * 10 -6 M pOH = -log [OH - ] = -log (2.0 *10 -6 ) = 5.69 b.) Calculate the pH of a buffer solution that is made with 0.30 M [NaF], that also has 0.30 M of [HF] in it. Approximate method is OK. K a = [H + ][F - ] / [F] HF F - + H + I 0.30 0.30 - C -x +x +x E 0.30 - x 0.30 + x x K a = x * (0.30+x) / (0.30-x) x * (0.30) / (0.30) ( x is tiny compared to 0.30) [H + ] = x K a = 7.2 *10 -4 pH = -log[H + ] = -log(7.2 *10 -4 ) = 3.14 Alternate method: This is more easily solved with the Henderson-Hasselbach equation: pH = pK a + log([base]/[acid]) = -log(7.2x10 -4 ) + log(0.30 / 0.30) =
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Worksheet Dec 1 2011 key - Quiz Section Worksheet Dec 1...

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