Problem: Calculate the pH of a solution that is 0.20 M HOCl and 0.90 M KOCl. In order for this buffer to have pH = pKa, would you add HCl or NaOH? What quantity (moles) of which reagent would you add to 1.0 L of the original buffer so that the resulting solution has pH = pKa?

🤓 Based on our data, we think this question is relevant for Professor Dixon's class at UA.

FREE Expert Solution

We are asked to calculate the pH of a solution that is 0.20 M HOCl and 0.90 M KOCl.


Use buffer equation: 

pH = pKa + log[A-][HA]

Given: 

pKa = 7.53 (from standard tables)

[A-] →  KOCl

[HA] → HOCl


Calculate pH: 

pH = 7.53 + log[0.90][0.20]

pH = 8.18 


The pH of a solution that is 0.20 M HOCl and 0.90 M KOCl is pH = 8.18


The ratio [A-]/[HA] = 1 so that pKa = pH. 

  • Our Initial [A-]/[HA] = 4.5
  • This means we need to remove the conjugate base and produce more acid to lessen the ratio. 
  • This means we need to add more HCl.


Calculate used HCl: 



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Problem Details

Calculate the pH of a solution that is 0.20 M HOCl and 0.90 M KOCl. In order for this buffer to have pH = pKa, would you add HCl or NaOH? What quantity (moles) of which reagent would you add to 1.0 L of the original buffer so that the resulting solution has pH = pKa?

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What scientific concept do you need to know in order to solve this problem?

Our tutors have indicated that to solve this problem you will need to apply the Buffer concept. You can view video lessons to learn Buffer. Or if you need more Buffer practice, you can also practice Buffer practice problems.

What professor is this problem relevant for?

Based on our data, we think this problem is relevant for Professor Dixon's class at UA.

What textbook is this problem found in?

Our data indicates that this problem or a close variation was asked in Chemistry: An Atoms First Approach - Zumdahl 2nd Edition. You can also practice Chemistry: An Atoms First Approach - Zumdahl 2nd Edition practice problems.