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Definition of Salt of Weak acid and Strong Base

Chemistry Formulas

What is salt of weak acid and strong base

Such salts give alkaline solutions in water. Some of such salts are : CH3 COONa, Na2 CO3 , K2 CO3 , KCN etc . For our discussion, we consider CH3 COONa (sodium acetate) in water. When CH3 COONa is put in water, it completely ionises to give CH3 COO- (acetate) ions and Na+ ions. For More Chemistry Formulas just check out main pahe of Chemsitry Formulas. 

Now acetate ions (CH3 COO- ) absorb some H+ ions from weakly dissociated H2O molecules to form undissociated CH3 COOH. Na+ remains in ionic state in water.

Now for Kw (ionic product) of water to remain constant, H2O further ionises to produce more H+ and OH- ions. H+ ions are taken up by CH3 COO- ions leaving OH- ions in excess and hence an alkaline solution.

Let BA represents such a salt. As it is put in water ;

salt of weak acid and strong base

Degree of Hydrolysis

It is defined as the fraction of total salt that has undergone hydrolysis on attainment of equilibrium. It is denoted by h.

Let c be the concentration of salt and h be its degree of hydrolysis.

degree of hydrolysis

chemistry

Example of Salt of Weak acid and Strong Base

Example 1.

A Solution of weak acid HA was titrated with base NaOH. The equivalent point was reached when 40ml of 0.1M NaOH has been added. Now 20ml of 0.1M HCl were added to titrated Solution . The pH was found to be 5. What will be the pH of the Solution obtained by mixing 20ml of 0.2M NaOH and 20ml 0.2M HA?

A. 7
B. 4
C. 9
D. 5
 

Ans. C

Example 2.

A Solution of weak acid HA was titrated with base NaOH. The equivalent point was reached when 40ml of 0.1M NaOH has been added. Now 20ml of 0.1M HCl were added to titrated Solution . The pH was found to be 5. What will be the pH of the Solution obtained by mixing 20ml of 0.2M NaOH and 20ml 0.2M HA?

A. 7
B. 4
C. 9
D. 5
 

Ans. C

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