Is SnCl₄ a Lewis acid or a base???

Arrange the following Bronsted Acids in increasing acidic strength,Give Reason for your choice :

1)HCl,HBr ,HI,CH₃COOH,HCO₃,H₂O

2)HCOOH,C₆H₅COOH,CH₃COOH

SnCl₄ is a lewis acid. This is because chlorine being the more electronegative atom, pulls the shared pair of electrons towards itself, making tin electron deficient. As a result, Sn can accept electron pairs from molecules like NH₃, thereby forming a coordinate bond. 

According to the Bronsted-Lowry concept, an acid is a substance that accepts a proton while a base is a substance that accepts a proton. The smaller is an anion attached with a proton, the more strongly it will hold the H⁺ ion and hence will furnish it less readily. Thus the smaller is the anion attached to the H⁺ ion, the weaker is the acid. Hence, the increasing order of bronsted acids for the given compounds is as follows :

1) H₂O < H₂CO₃ < CH₃COOH < HCl < HBr < HI

2) For carboxylic acid , the increasing order of acidic strength is as follows : 

CH₃COOH < C₆H₅COOH < HCOOH

The acidity in carboxylic acids is basically due to the resonance stabilised carboxylate group in which the negative charge is well accommodated between the two electronegative oxygen atoms. However, in case of benzoic acid, the benzene ring is attached directly to the carbonyl group. Thus there is additional resonance between the carbonyl group and the benzene ring, which disturbs the resonance in the carboxylate ion. In other words, the negative charge in the carboxylate group of formic acid is accommodated in a much better manner than in carboxylate group of benzoic acid. Hence formic acid furnishes H⁺ ions more easily than benzoic acid. 

In case of acetic acid, an electron releasing group is attached to the COOH group which reduces the acidity of the carboxyl group. You will learn more about it in grade 12.