Here is a $0.400\;M$ solution of $HN_3$
(hydrazoic acid, $K_a = 1.9\cdot 10^{-5}$)
Equilibrium of hydrolysis:
$HN_3$ $+$ $H_2O$ 
$H_3O^+$ $+$ $N_3^-$
What are the changes of the concentrations of the these species?
a) $+x\;M$, $-x\;M$, $+x\;M$ b) $-x\;M$, $+x\;M$, $+x\;M$ c) $+x\;M$, $+x\;M$, $+x\;M$ d) $-x\;M$, $+x\;M$, $-x\;M$
The answer b) is correct, because when one mole dissociates, one mole of each ion is produced.