Question

Which of the following acids, whose pKa values are given next to them, would have the strongest conjugate base? pKa a) H2SO4 b) CH3COOH 4.8 c) HS 12.9 d) NH* 2.0 9.2 Reaction of a carboxylic acid with an alcohol produces: a) an alkane b) an ester c) a ketone d) an amide During the neutralisation of a weak acid by a strong base the pH of the solution equals the pKa of the acid: a) at the beginning of the neutralisation b) at half neutralisation c) at the equivalence point d) just after the equivalence point Bromophenol blue is yellow at pH 2, purple at pH 3 and blue at pH 4. What colour will this indicator be in a solution made by diluting 1 mL of 10 mol L - HCl to 1 litre? a) yellow b) purple c) blue d) between yellow and purple The function of a buffer is to a) maintain a neutral pH b) act as a strong acid c) be a strong base d) maintain the pH of a solution

Answer 1

1 )option c (HS-).The weakest acid gives strogest conjugate base and weakest base gives strogest conjugate acid.As pka value increases acidic strength decreases.here HS- is acid having high pka, thus low acidic strength.weak acid give strongest conjugate base.

2) option b, ester.

Esters are produced when carboxylic acids are heated with alcohols in the presence of an acid catalyst. The catalyst is usually concentrated sulphuric acid. Dry hydrogen chloride gas is used in some cases, but these tend to involve aromatic esters (ones containing a benzene ring). If you are a UK A level student you won't have to worry about these.

The esterification reaction is both slow and reversible. The equation for the reaction between an acid RCOOH and an alcohol R'OH (where R and R' can be the same or different) is:

R-C R-C O=0 + R'OH + H2O O-H O-R

3) option b, at half of nuetralisation

4) 1 mL 10 mol/L diluted to 1L

10 mol/L means 10 moles in 1000 mL

Thus, 10/1000= 10^(-2) moles in 1 mL

When diluted to 1L molarity =10^(-2)/1L

. = 10^(-2) M

Concentration of H+ = 10^(-2) M

PH= -log(10^(-2))= 2

So colour of bromophenol blue is yellow( pH=2)

5)The fuction of buffer is maintain the pH of solution.option is d is correct answer.

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