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Strongest Base

APCHEM-URIE7F
Substance $K_a$ $\hspace{3cm}$
Hydrogen carbonate ion — bicarbonate ion ($HCO_3 \ ^-$) $4.8 × 10^{-11}$
Hydrogen phosphate ion ($HPO_4 \ ^{-2}$) $3.6 × 10^{-13}$
Water ($H_2O$) $1.0 × 10^{-14}$
Hydrogen sulfide ion ($HS^-$) $1.0 × 10^{-19}$

$\ $
Using acid dissociation constants above, which of the following substances is the strongest base?

A

$CO_3 \ ^{-2}$

B

$PO_4 \ ^{-3}$

C

$OH^-$

D

$S^{-2}$