Question

For which of the following are we permitted to make the assumption that the equilibrium concentration of the acid or base is the same as the initial concentration when  we calculate the pH of the solution specified?

(a)

0.020

M

HC

2

H

3

O

2

(c)

0.002

M

N

2

H

4

(b)

0.10

M

CH

3

NH

2

(d)

0.050

M

HCHO

Answer 1

To do this, you need to know the Ka or Kb values of each of this species. If the value of Ka is lower than 1x10^-3 order, then you can assume that the concentration in equilibrium is the same as the innitial, basically because the Ka or Kb values indicates the constant of dissociation of the acid or base. When an Acid or base is a strong one, this dissociation constant is complete and near to one. But when the acid or base is weak, then, it does not have the strength enough to dissociates it's proton. It's like if you have a 0.2 M concentration of an acid with a constant of 1x10^-8 means that the dissociation is small too, so it's probably dissociates something like 0.0000005.that's practically so little that compared to 0.2 is almost nothing.

Now, for these compounds, the Ka and Kb values are as follows:
a) Ka = 1.8x10^-5 Means that we can assume concentration in equilibrium = concentration innitial
b) Kb = 3.7x10^-4  Means that we can assume concentration in equilibrium = concentration innitial
c) Kb = 3x10^-6  Means that we can assume concentration in equilibrium = concentration innitial
d) This compound is a neutral compound. It does not have a Ka or Kb specific. So with this we can't assume the same concentration.

Hope this helps