65 One liter of a 01 M Tris buffer see Table 24 is prepared

65. One liter of a 0.1 M Tris buffer (see Table 2-4) is prepared d adjusted to a pH of 2.0. (a) What are the concentrations of the conjugate base and weak acid at this pH? (b) What is the pH when 1.5 mL of 3.0 M HCl is added to 1.0 L of the buffer? Has the buffer functioned effectively? Explain.

Solution

Given we have a buffer solution of Tris of 0.1 M concentration,and we need to find out its concentration of conjugate base and weak acid

a. Thus using Henderson\'s eqaution to get the relation between conjugate base and weak acid as

pH=pKa+ log([A^-]/[HA])

From standard table pKa for tris buffer=8.3

so for maintaiing pH=2

2=8.3+log([A^-]/[HA])

-6.3=log([A^-]/[HA])

[A^-]/[HA]=10^-6.3=5*10^-7

Then finding out % of A^- ,we get

%A^-=(5*10^-7/(1+5*10^-7))*100=5*10^-5 %

Thus,[A^-]=5*10^-5*0.1/100=5*10^-8 M

and [HA]=0.1 M(appraoximately same as concentration of A^- is very small)

b.

MilliMoles of HCl added=1.5*3=4.5 mmoles=4.5*10^-3 moles

Now we can see here that as the concentration of conjugate base A^- is very less ,so there will be no effect of buffer and the buffer will not function efficiently here as there is no role of conjugate base here which is negligible in this case.So pH will not be adusted properly.