Question

What volumes of 0.55 M HNO2 and 0.55 M LiNO2 must be mixed to prepare 1.00 L of a solution buffered at pH = 3.78? HNO2? LiNO2 Incorrect: ?

Answer 1

Use the David Hasselhof equation.

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

pKa for nitrous acid is 3.30.

pH = 3.78

[A(-)] / [HA] = 10^(3.78 - 3.30) = 3.019

Your volumes are V(acid) and V(base)

V(acid) + V(base) = 1.0 L

Since everything is in the same solution, a ratio of moles of acid to moles of base is the same as the ratio of their concentrations.

moles(base) / moles(acid) = 3.019

[V(base)*(0.55)]/[V(acid)*(0.55)] = 3.019

Substitute V(base) = 1.0 - V(acid) and just call V(acid) "V" from now on,

[(1.0 - V)/V] = 2.799

1.0 - V = 2.799 V

1.0 = 3.799V

V = 0.26 L (that's volume of acid)

V(base) = 0.74 L

Now check your work

0.26 L x 0.55 M = 0.143 mol HA
0.74 L x 0.55 M = 0.407mol A(-)

pH = 3.3 + log (0.407 / 0.143) = 3.78