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Hydronium and Molarity

APCHEM-9IEQGU

Which of the following is the hydronium ion concentration of a 0.100 M $HA$ solution if $K_a$ for $HA$ =$1 × 10^{-9}$?

A

$1 × 10^{-10}$

B

$1 × 10^{-9}$

C

$1 × 10^{-5}$

D

$1 × 10^{-1}$