(a) (i) First we find out the molecular formula of the compound 
% of O₂ = 100 - (% of C + % of H) = 100 - (69.77 + 11.63) 
= 100 - (81.40)= 19.6%
Empirical formula of the compound = C₅H₁₀O
Empirical formula mass = 5 × 12 + 10 × 1 + 16 × 1 = 86
but given that molecular mass = 86
n = molar mass Emperical formula = 8686 = 1 molar mass Emperical formula = 8686 = 1
∴ molecular formula of the compound (C₅H₁₀O),
= C₅H₁₀O
(b) (i) The strength of an acid is indicated by pK_a value, where, pK_a = – log K_a
Since monochloroethanoic acid is weaker than dichloroethanoic acid so it has lower value of dissociation constant K_a.
Therefore, it has higher value of pK_a.
(ii) The —COOH group in benzoic acid is attached to sp² - carbon of the phenyl ring and is more acidic than acetic acid in which —COOH group is attached to sp³ – carbon atom of CH₃ group. So, benzoic acid is stronger than acetic or acetic acid is weaker acid than benzoic acid.

(a) (i) Clemmensen Reduction
The carbonyl group of aldehydes and ketones is reduced to the CH₂ group on treatment with zinc amalgam and concentrated hydrochloric acid. This is known as Clemmensen reduction.

(ii) Hell-Volhard-Zelinsky (HVZ )reaction.
Carboxylic acids having a hydrogen are halogenated at the  position on treatment with chlorine or bromine in the presence of small amount of red phosphorus to give  halocarboxylic acids. The reaction is known as Hell-Volhard-Zelinsky reaction.

(b) (i) Propanal (CH₃CH₂CHO) can be distinguished from propanone (CH₃COCH₃) by iodoform test. 
Being a methyl ketone, propanone on treatment with I₂/NaOHundergoes iodoform reaction to give a yellow ppt. of iodoform.
(ii) (ii) Benzaldehyde (C₆H₅CHO) and acetophenone (C₆H₅COCH₃) can be distinguished by iodoform test.
(iii)  Acetophenone, being a methyl ketone on treatment with I₂ /NaOH undergoes iodoform reaction to give a yellow ppt. of iodoform. On the other hand, benzaldehyde does not give this test. 

(b) (i)  Ethanal and Propanal
Iodoform test – Methyl ketones give a positive iodoform test.
Ethanal has one methyl group linked to the carbonyl carbon atom and therefore, responds to this test.
Propanal does not respond to this test.
(ii)  Benzaldehyde and Acetophenone
Tollen’s test – aldehydes give a positive Tollen’s test.
Benzaldehyde is an aldehyde. Therefore, it reduces Tollen’s reagent to give a reddish-brown precipitate.
Acetophenone is a ketone and therefore gives a negative test.
(iii)  Phenol and Benzoic acid
Ferric chloride test – phenols give a violet coloration with this test.
Phenol reacts with neutral ferric chloride to form an iron-phenol complex, which gives a violet coloration.
Whereas, benzoic acid reacts with neutral ferric chloride to give a buff coloured precipitate of ferric benzoate.