Why benzoic acid is stronger than p-methoxy benzoic acid.plzz reply today it's very urgent for me.

Two reasons:

1. The methoxy group is situated at the para position, so the oxygen atom can directly donate one of its lone pairs into the benzene ring through conjugation and make it available to the carboxy group on the other side. The methoxy group thus shows +R effect.

2. The carbonyl carbon of the -COOH group ‘receives’ the donated electrons and it becomes less polar (since the two oxygens are exerting -I effect on it). Since the carbonyl carbon now has more negative charge on it, the oxygen of the -OH group can withdraw these electrons with -I effect and even that oxygen becomes less polar. Hence the O-H bond strengthens.

Strong O-H bond => less acidity. But the least confusing reason is the +R effect of the methoxy group

 

Dear student 

I have one example to show a particuar case,
Acidic strength increases if there is less electron density on a particular compound.
Since,there is nitro group on 4 nitrobenzoic acid,which is electron withdrawing group which decreases
electron density,and on the other hand methyl group is present which is a electron releasing group,so it increases electron
density on  4-methoxybenzoic acid.And,as we know greater the electron density on the compound,lesser acidic it will be.
Therefore,4 nitrobenzoic is stronger acid then benzoic acid and 4-methoxybenzoic is weaker acid then benzoic acid