CH3CH2CH2COOH,CH3CH2COOCH3smel

1.
CH3CH2CH2COOH is butanoic acid, which has "rotten butter smell";
CH3CH2COOCH3 is methyl propanoate, which is an ester of propanoic acid, which also has very bad-smell.


2.
do you mean than butanoic acid is more bad-smelling than methyl propanoate?
smelling is dependent on concentration and varies on different persons.
my guess is, acids also possess acrid (acidic) smelling which the ester doesn't. this may be more irritating to some people.

my question is that why we can smell more pungent smell when mix CH3CH2CH2COOH and CH3CH2COOCH3 together