Untitled - Kelly Walsh High School

Buffers and Other Equilibria 243
Quick Tip
The K a expression is:
HNO 2(aq) K H (aq) NO 2 (aq)
0.150 x x x
K a
We can now enter the concentrations of the various substances:
The next step is to find the K a from the pK a.
Ka 103.35 4.4668 104 10 (unrounded)
pKa Ka 4.4668 10-4 [x] [x]
[HNO2 ] [0.150 x]
Solving this relationship for x gives x 8.1855 10 3 M (unrounded). This is
the hydrogen ion concentration, so we can use it to determine the pH:
pH log [H ] log (8.1855 10 3 ) 2.08695 2.09
The initial pH of this weak acid is, as expected, acidic.
b. 25.00 mL This will be in the second region of the titration curve. (We usually
cannot predict this by simple inspection.) To do this, and all later steps, we
will require the balanced chemical equation for the reaction and the moles of
each reactant.
The reaction is:
[x] [x]
Ka
[HNO2 ] [0.150 x]
[H ] [NO 2 ]
[H ] [NO 2 ]
[H ] [NO 2 ]
[HNO 2 ]
HNO 2(aq) NaOH(aq) l Na (aq) NO 2 (aq) H2O(l)
It is usually helpful to write the products in ionic form.