K a because value of log is zero. Thus if we take molar concentration of acid and salt (conjugate base) same, the pH of the buffer solution will be equal to the p K a of the acid. So for preparing the buffer solution of the required pH we select that acid whose p K a is close to the required pH. For acetic acid p K a value is .
, therefore pH of the buffer solution formed by acetic acid and sodium acetate taken in equal molar concentration will be around . . A similar analysis of a buffer made with a weak base and its conjugate acid leads to the result, pOH= p +log [Conjugate acid,BH+] [Base,B] b K ( . ) pH of the buffer solution can be calculated by using the equation pH + pOH = .
We know that pH + pOH = p K w and p K a + p K b = p K w . On putting these values in equation ( . ) it takes the form as follows: - pH= p [Conjugate acid,BH ] [Base,B] w w K K K a log or pH= p [Conjugate acid,BH ] [Base,B] K a + + log ( . ) If molar concentration of base and its conjugate acid (cation) is same then pH of the buffer solution will be same as p K a for the base.
p K a value for ammonia is s9. ; therefore a buffer of pH close to . can be obtained by taking ammonia solution and ammonium chloride solution of same molar concentration. For a buffer solution formed by ammonium chloride and ammonium hydroxide, equation ( .
) becomes: pH= [Conjugate acid,BH ] [Base,B] . log + + pH of the buffer solution is not affected by dilution because ratio under the logarithmic term remains unchanged. . SOLUBILITY EQUILIBRIA OF SPARINGLY SOLUBLE SALTS We have already known that the solubility of ionic solids in water varies a great deal.
Some of these (like calcium chloride) are so soluble that they are hygroscopic in nature and even absorb water vapour