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What is the correct order of acidic strength?

Which acid is the strongest?

Arrange these carboxylic acids in decreasing order of ease of esterification with $CH_3OH/H^{+}$:
$(I)$ $CH_3-CH(CH_3)-COOH$
$(II)$ $CH_3-CH(CH_3)-CH_2-COOH$
$(III)$ $CH_3-C(CH_3)_2-COOH$
$(IV)$ $(CH_3-CH_2)_3C-COOH$

Identify the product obtained in the following reaction:
$(CH_3 CO)_2 O \stackrel{H_2 O}{\longrightarrow} \text{Product}$

Which of the following reagents will convert a carboxylic acid into an ester?