HOOC-CH2-COOH malonic acid
CH3-CH2-COOH propanoic acid
Both have 3 carbons and both have polar carboxyl groups (-COOH). Also, they both will exhibit hydrogen bonding. But notice that malonic acid has TWO carboxyl groups and this makes it more polar than propanoic acid. Thus, malonic acid should be more soluble in water than propanoic acid.