9. A 0.10 M solution of a weak acid, HB, is found to have $[\; H+]$ of $1.0\; x\; 10-5$ M. The value of $K$_a for this acid is which of the following?

a) $1.0\; x\; 10-11$
b) $1.0\; x\; 10-10$
c) $1.0\; x\; 10-9$
d) $1.0\; x\; 10-5$
e) $1.0\; x\; 10-3$

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