[nothincomin]

Chloroacetic acid has a relatively large equilibrium constant, so at low acid concentrations it is necessary to use the quadratic equation in order to calculate the concentrations of the aqueous species. Ka for ClCH2COOH is 1.4 X 10^-3. For the problems that follow, consider 1 liter of a solution that originally had 0.02 mol of chloroacetic acid.

What is the equilibrium concentration of [ClCH2COOH]? (please show me how you got your answer)

What is the pH of this chloroacetic acid solution?

What is the % dissociation of the weak acid in this solution?

Thanks for your *delete me*

[MrTeo]

Show us your attempts first

[nothincomin]

ClCH2COOH + H2O -> H3O+ + ClCH2COO-

X^2/ (0.02- X) = 1.4 X 10^-3

X^2 + (1.4 X 10^-3) - (2.8 X 10^-5) = 0

I used quad formula to solve for x.
x= 0.0093

After this I got confused on what to do next.

[MrTeo]

I think you meant to write:

x²+(1.4e-3)x-(2.8e-5)=0

But solving it I find x=0.0046, try to check your work...
Anyway you only need to focus on what you've found: compare the quadratic equation you wrote to the expression of the equilibrium in this case:



considering also that:

[nothincomin]

Ok I just rechecked my work and found out I made an error. It is 0.0046 as you said, but this answer isn't given in the multiple choice part. The options are A) 0.01  B) 0.012  C) 0.015  D) 0.017  E) 0.019

[MrTeo]

You have to find the concentration on ClCH₂COOH: what is the x you have found? (Again, look at the equilibrium constant expression, even if I used acetic acid instead of chloroacetic  ).

[nothincomin]

Ah, okay didn't see that.
0.02- 0.0046 = 0.0154 making C) the answer.

2) -log(0.0046)= 2.33 pH

Btw how do you find the percent dissociation?

[MrTeo]

The degree of dissociation is defined as (where HA is a generic acid and A^- its conjugate base):

$$ \alpha = \frac{\left[A^-\right]}{\left[HA\right]} /$$

To find the percent dissociation simply multiply it by 100:

$$ \alpha_{\%} = \alpha \cdot 100 /$$

[nothincomin]

I have a few other questions. Can you help me confirm if they are correct?

1) For which of the following reactions is the equilibrium constant called a base ionization constant, Kb? My answer was A).

A) (CO3)2- (aq) + H2O -> HCO3- (aq) + OH- (aq)

B) HCO3- (aq) + OH- (aq) -> (CO3)2- (aq) + H2O

C) H3O+ (aq) + OH- (aq) -> 2H2O

D) HOCl (aq) + H2O -> H3O+ (aq) + OCl- (aq)

E) HCOOH (aq) + NH3 -> HCOO- + (NH4)+ (aq)


2) The symbol Ka (HC2O4-) is the equilibrium constant for which reaction? My answer was A) as well.

A) HC2O4- + H2O -> H3O+ + (C2O4)2-

B) H2C2O4 + H2O -> H3O+ + HC2O4-

C) HC2O4- + H2O -> H2C2O4 + OH-

D) HC2O4- + OH- -> (C2O4)2- + H2O

E) HC2O4- + H3O+ -> H2C2O4 + H2O

[MrTeo]

I think they're both correct