The Foundations of Chemistry

19-3 Preparation of Buffer Solutions 805

Substitution into the Kbexpression for aqueous ammonia gives

Kb1.8 10 
^5 

NH 4 Cl is 100% dissociated, so x1.4 10
^5. Then (x1.4 10
^5 )
x.

1.8 10 
^5 x0.13 M[NH 4 ]MNH 4 Cl

Now we calculate the number of moles of NH 4 Cl that must be added to prepare 500. mL
(0.500 L) of buffer solution.

_?_ mol NH 4 Cl0.500 L0.065 mol NH 4 Cl

_?_ g NH 4 Cl0.065 mol3.5 g NH 4 Cl

You should now work Exercise 37.

We have used Kaor Kbexpressions or the Henderson–Hasselbalch equation in its acid–
salt or base–salt form to find the pH of buffered solutions. Each of these approaches

involves a ratioof concentrations, for instance. Although the ratios are

[conjugate base]



[acid]

53.5 g NH 4 Cl



mol NH 4 Cl

0.13 mol NH 4 Cl



L

(x)(1.4 10 
^5 )



0.10

(x1.4 10 
^5 )(1.4 10 
^5 )



0.10
1.4 10 
^5

[NH 4 ][OH
]



[NH 3 ]

NH 4 Cl NH 4 

(0.10 
 1.4  10 
^5 ) M 1.4  10 
^5 M 1.4  10 
^5 M

x Mx Mx M

NH 3 H 2 ONH 4 

Cl

OH



Total [NH 4 ]  (x  1.4  10 
^5 ) M

100%

Here xdoes notrepresent a changein

concentration, but rather the initial

concentration of NH 4 Cl. We do not

assume that x1.4 10 
^5 , but

rather the reverse.

Preparation of the buffer solution in Example 19-6. We add 3.5 grams of NH 4 Cl to a
500-mL volumetric flask and dissolve it in a little of the 0.10 MNH 3 solution. We then
dilute to 500 mL with the 0.10 MNH 3 solution.