Why on dilution the `Lambda_(m)` of `CH_(3)COOH` increases drastically, while that of `CH_(3)COONa` increases gradually?

When CH_(3)COONa is added to an aqueous solution of CH_(3)COOH

pK_(a) of CH_(3)COOH is 4.74 . The pH of 0.01 M CH_(3)COONa IS

Assertion (A): The pH of an aqueous solution of CH_(3)COOH remains unchanged on the addition of CH_(3)COONa . Reason (R) : The pH of an aqueous solution of CH_(3)COOH remains unchanged on the addition of CH_(3)COONa . Reason (R) : The ionisation of CH_(3)COOH is supressed by the addition of CH_(3)COONa .