Question

Write the correct order of acidity:

P.

Q.

R.

S.

a.) P > Q > R > S
b.) Q > P > R > S
c.) Q > R > S > P
d.) S > R > Q > P

Answer 1

Hint: Acidity is the tendency of a compound to act as ${ H }^{ + }$ donor. The carboxylic acid is acidic as it has hydrogen in the ${ -COOH }$ group. The compound whose carboxylate ion is the more stable will be more acidic.

Complete step by step solution:
Carboxylic acids are acidic in nature due to resonance stabilization of carboxylate ion and hydrogen of ${ -COOH }$, which is found to be capable of ionization. As we know, oxalic acid easily releases its H atom to form ${ CH_{ 3 }COO }^{ - }$ ion whereas formic acid needs more energy. Hence, the Strength of oxalic acid is more than formic acid.

So, Oxalic acid (COOH-COOH) is more acidic than formic acid (${ HCOOH }$). Malonic acid, (${ COOH-CH }_{ 2 }{ -COOH }$) is less acidic than oxalic acid because of the intervening presence of ${ -CH }_{ 2 }$ group and succinic acid (${ COOH-CH }_{ 2 }{ { -CH }_{ 2 }-COOH }$) which is much weaker than (${ COOH-CH }_{ 2 }{ -COOH }$). Malonic acid due to the greater distance created by ${ -CH }_{ 2 }$ group.
Hence, the order of acidity will be:

COOH-COOH > HCOOH > COOH-CH$_2$-COOH > COOH-CH$_2$-CH$_2$-COOH
So, the correct order is QP R S and hence, the correct option is B.

Additional Information:
Carboxylic acids are important in the manufacture of greases, plastics, and crayons.
Carboxylic acids exhibit strong hydrogen bonding between the molecules.

Note: The possibility to make a mistake is that you can choose option D. ${ COOH-COOH }$ is more acidic than ${ COOH-CH }_{ 2 }{ -COOH }$ due to +I effect which causes a decrease in acidity.