Question

A solution of NaOH(aq) contains 7.7 grams of NaOH in 93 mL of solution. Calculate the pH at 25◦C.
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7.7 grams x (1 mol/39.997 grams) = 0.19 mol
0.19 mol/0.093 L = 2.07 M
pOH = -log(2.07) = -0.316
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Am I doing anything wrong? Why am I getting a negative pOH/pH greater than 14?

Answer 1

MOST of the time the H^+ or OH^- is less than 1 so the log is negative but the - sign in the definition changes that to a + number. In fact, that's why the - sign was placed there in the definition so that the usually negative numbers would become positive. Here, however, the OH is >1 so the - sign changes that + log number to a negative number. Nothing wrong so far.
Some examples.
OH = 1; pOH = 0
OH = 0.1; pOH = -*-1 = +1
OH = 0.01; pOH = -*-2 = +2
BUT if OH = 10, pOH = -*1 = -1
etc.