Solving pH for HOCl in a 1 L Solution

In summary, pH of a solution containing hypochlorous acid is calculated to be -log(x) + log(acid/base), where x is the concentration of HClO.
  • #1
Gaunt101
8
0

Homework Statement



Determine the pH when 0.39 mol HOCl is added to sufficient water to make 1.000 L of solution. (Ka (HOCl) = 3.700 x 10-8)

Homework Equations



Ka = Kw/Kb
pH = -log(pKa) + log(acid/base)

The Attempt at a Solution



For this question first we need the concentration: c= n/v hence the concentration of HOCL is 0.39M.

At this point if you write the balanced equation I think it's HOCl + H2O <---> H3O + ClO
so there is a positive increase in H3O and ClO, hence:

Ka = [H3O]+[ClO]- / [HOCL]
by letting H3O be x, then both the concentrations of H3O and ClO will be x^2 since they increase the same.

So I figure Ka = x^2 / [HOCL]

x = 1.170 e-4

so the pH = -log(x) + log(acid/base)
pH = 3.932 + log(acid/base)

I'm not sure what values I'd put in for acid / base, and I'm not sure if what I've done so far is correct.

Thank you very much for your time!

Gaunt.
 
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  • #3
Hey Borek,

thanks for the sites, but I"m a little confused. Chembuddy says "the sum of concentrations of all acid forms present in the solution must be identical to the concentration of acid added."

Does that mean for the log(acid/base), the acid value would be the Ka and the base just the Kb?
 
  • #4
No, it is just a mass balance. HClO dissociates, so solution contains both HClO and ClO-. In your case mass balance for hypochloric acid means that

[HClO] + [ClO-] = 0.39M

--
 
  • #5
[HClO] + [ClO-] = 0.39M

So if you were to do log(acid / base), it would simply be 0.39 since all the sums of the acid must be equal to the original concentration? hence the answer is just 0.39 + 3.932?
 
  • #6
No, you don't know what are concentrations of HClO and its conjugate base ClO-, so you can't calculate their ratio and log of their ratio. You have to calculate them first. Simplest approach is to calculate in reverse - that is, calculate pH of the solution, then use it to calculate ratio of concentrations.

log(acid/base) is a way of dealing with buffers, not with solutions of pure acid.
 

Related to Solving pH for HOCl in a 1 L Solution

1. What is the formula for calculating the pH of a 1 L solution of HOCl?

The formula for calculating pH is pH = -log[H+], where [H+] is the concentration of hydrogen ions in the solution.

2. How do I determine the concentration of HOCl in a 1 L solution?

The concentration of HOCl can be determined by using the Henderson-Hasselbalch equation: pH = pKa + log([HOCl]/[OCl-]). The pKa value for HOCl is 7.5.

3. Can I use a pH meter to measure the pH of a 1 L solution of HOCl?

Yes, a pH meter can be used to measure the pH of a 1 L solution of HOCl. However, it is important to calibrate the pH meter and use a standardized solution to ensure accurate results.

4. How does temperature affect the pH of a 1 L solution of HOCl?

Temperature can affect the pH of a solution as it can impact the dissociation of molecules, including HOCl. As temperature increases, the dissociation of HOCl increases, resulting in a decrease in pH.

5. Can I use a universal indicator to determine the pH of a 1 L solution of HOCl?

No, a universal indicator is not suitable for accurately measuring the pH of a 1 L solution of HOCl. This is because the color change of the indicator may not be sensitive enough to detect small changes in pH at the expected range for HOCl (between 6-8).

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