Answer
a) $NH_3$
b) $CH_3CN$
c) $CH_3COOH$
Work Step by Step
a) $NH_3$ is more soluble than $PH_3$ in water because the electronegativity difference between nitrogen and hydrogen is greater than that between phosphorous and hydrogen; therefore, $NH_3$ is more polar than $PH_3$ due to its more polar bonds and is more soluble.
b) $CH_3CN$ is more soluble than $CH_3CH_3$ in water because $CH_3CN$ is polar while $CH_3CH_3$ is nonpolar, and water dissolves polar molecules better than nonpolar molecules.
c) $CH_3COOH$ is more soluble than $CH_3COOCH_3$ in water because its oxygen atoms (the most electronegative atom in the molecule) are further from the center of the molecule than the oxygen atoms in $CH_3COOCH_3$, making $CH_3COOH$ more polar.
