The pH of a Mixture of Weak Acids

In summary, two solutions of 2.00M chlorus acid and 2.00M formic acid were mixed together. The Ka values for chlorus acid and formic acid are 1.1×10-2 and 1.78×10-4, respectively. Using the quadratic equation, the concentration of H+ ions for chlorus acid was found to be 0.143. For formic acid, the concentration of H+ ions was calculated to be 0.0189. The pH of the resulting solution was determined to be either 0.845 or 1.09, depending on whether the concentrations were added or averaged. However, due to the small difference in pKas and the weak
  • #1
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Homework Statement


2.00M chlorus acid and 2.00M formic acid are mixed. What is the pH?


Homework Equations


Ka of chlorus acid is 1.1×10-2
Ka of formic acid is 1.78×10-4


The Attempt at a Solution


For chlorus acid with quadratic equation to find x:
x²+0.011x-0.022
(-0.011[tex]\pm[/tex][tex]\sqrt{}0.000121-4(1)(-0.022)[/tex])/2
(-0.011[tex]\pm[/tex]0.297)/2
The positive number of the two choices is 0.143

For formic acid, and the x being small enough to neglect, x is:
x²/2=0.000178
x²=0.000356
x=0.0189

When mixing weak acids, I think I was told that only the acid with the larger Ka value matters, meaning taking the negative log of chlorus acid's H ion gets -log0.143=0.845.

I wasn't sure this was correct so then I tried adding the two H ion concentration from each acid to find their pH. I did 0.143+0.0189=0.162 and negative log that for 0.791.
I'm not sure it is added this way, since I got a bit confused about the idea of adding molarity together, so I divided by 2 first then negative log for 1.09.

Still I'm not sure this is correct so I think I took the wrong steps. How is this correctly done?
 
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  • #2
312213 said:
When mixing weak acids, I think I was told that only the acid with the larger Ka value matters, meaning taking the negative log of chlorus acid's H ion gets -log0.143=0.845.

It isn't entirely true - you can do it if the difference between both acids pKas is large enough and if the weaker acid is weak enough. That's the case here and 0.84 is a correct answer.

Unfortunately, it is hard to be more specific.
 
  • #3


I would like to clarify a few things before providing a response. First, it is important to note that the given statements do not specify the volume of the mixture. This is a crucial factor in determining the pH of a solution. Additionally, the equations and calculations provided do not take into account the dissociation of water, which also affects the pH of a solution.

Assuming that the volume of the mixture is 1 liter, we can proceed with the following steps:

1. Calculate the molar concentrations of chlorus acid and formic acid in the mixture. Since the initial concentrations are given in terms of molarity (M), we can directly use these values. Thus, the final concentration of chlorus acid is 2.00M and the final concentration of formic acid is also 2.00M.

2. Calculate the dissociation constants (Ka) for each acid. This step is not necessary as the given values for Ka are already provided in the statement.

3. Use the Henderson-Hasselbalch equation to calculate the pH of the mixture. The Henderson-Hasselbalch equation is given as pH = pKa + log ([A-]/[HA]), where pKa is the negative logarithm of the dissociation constant, [A-] is the concentration of the conjugate base, and [HA] is the concentration of the acid. In this case, we have two acids in the mixture, so we need to consider the concentrations of both the conjugate base and the acid for each acid.

For chlorus acid:
pH = 2.95 + log ([ClO2-]/[HClO2]), where [ClO2-] is the concentration of the conjugate base (chlorite ion) and [HClO2] is the concentration of the acid (chlorus acid). Since both acids have the same concentration, the ratio [ClO2-]/[HClO2] will be 1, and thus the log term will be 0. Therefore, the pH of the mixture of chlorus acid is 2.95.

For formic acid:
pH = 3.75 + log ([HCOO-]/[HCOOH]), where [HCOO-] is the concentration of the conjugate base (formate ion) and [HCOOH] is the concentration of the acid (formic acid). Again, since both acids have the
 

What is the pH of a mixture of weak acids?

The pH of a mixture of weak acids is determined by the concentrations of each acid present and their respective acid dissociation constants.

How do you calculate the pH of a mixture of weak acids?

The pH can be calculated using the Henderson-Hasselbalch equation, which takes into account the concentrations of the weak acids and their respective dissociation constants.

What factors can affect the pH of a mixture of weak acids?

The pH of a mixture of weak acids can be affected by changes in the concentration of the acids, changes in temperature, and the addition of strong acids or bases.

Is the pH of a mixture of weak acids always acidic?

No, the pH of a mixture of weak acids can be acidic, basic, or neutral depending on the concentrations of the acids and their respective dissociation constants.

What is the relationship between the pH and pKa of a mixture of weak acids?

The pH of a mixture of weak acids is equal to the pKa of the dominant acid at the half-equivalence point, where the concentrations of the acid and its conjugate base are equal.

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