Question

Which of the following orders of the relative strength of acids is correct?
(A) $C{H_3}COOH > HCN > {H_2}O > {C_2}{H_5}OH$
(B) $C{H_3}COOH < HCN < {H_2}O < {C_2}{H_5}OH$
(C) $C{H_3}COOH > HCN < {H_2}O < {C_2}{H_5}OH$
(D) $C{H_3}COOH < HCN < {H_2}O > {C_2}{H_5}OH$

Answer 1

Hint: As we know that relative strength of acids is basically the power using which the acids can ionise in the aqueous solution, a strong acid completely dissociates in the aqueous solution whereas a weak acid do not dissociates completely into the aqueous solution.

Complete answer:As we have already learnt that the relative strength of acids is basically their strength or their power to use which they can ionize in the aqueous solution. We can determine the relative acidic strength by measuring the equilibrium constants of acids in the aqueous solution. As we know that stronger acids ionizes more easily and to a greater extent than a weak acid, hence the stronger acids will have a larger ionization constant and as the value of ionisation constant increases, the acidic strength increases.
So, we are given with acetic acid and hydrogen cyanide which are weak acids but acetic acid is stronger than hydrogen cyanide, so it will ionize partially in aqueous solution but to a greater extent than hydrogen cyanide and therefore it will have a stronger conjugate base and thus has a higher acidic strength than hydrogen cyanide.
Next we have water and ethanol, we know that the alkyl group present in ethanol possesses a positive inductive effect which decreases the stability of its conjugate base whereas water does not have any alkyl group and thus it is more acidic than ethanol.
 We also know that hydrogen cyanide has the ability to release its hydrogen ion more easily than water and thus forms a stable conjugate of base and thus is more acidic than water.

Therefore from the above explanation the correct order is option (A).
$C{H_3}COOH > HCN > {H_2}O > {C_2}{H_5}OH$

Note:Since the strength of acidity depends on the ease of release of hydrogen ions and further the stability of the conjugate base that is formed. So, stability of the conjugate base of an acid determines its acidity. And acidic strength is inversely proportional to the $p{K_a}$ value.