$n_X = \dfrac{9}{60} = 0,15(mol)$
CTCT X : $HCOOCH_3$
$HCOOCH_3 + KOH \to HCOOK + CH_3OH$
$n_{HCOOK} = n_X = 0,15(mol)$
$m = 0,15.84 = 12,6(gam)$
Theo gt ta có: $n_{X}=0,15(mol)$
Vì chỉ chứa muối nên X là $CH_3COOH$
Ta có: $n_{CH_3COOK}=0,15(mol)\Rightarrow m_{CH_3COOK}=14,7(g)$