Titration of strong base + weak acid

In summary, the conversation discussed a titration problem involving benzoic acid and calcium hydroxide. The equivalence point was determined to be 56.25 mL of Ca(OH)2, and the question focused on finding the pH after adding 60.00 mL of Ca(OH)2. The solution involved calculating the moles of excess Ca(OH)2, its concentration, and using fundamental principles to find the pOH and pH. The resulting pH was calculated to be 12.13.
  • #1
UKWildcat
4
0

Homework Statement


Consider a titration in which 50.00 mL of .450 M benzoic acid, C6H5COOH, is reacted with 0.2000 M Ca(OH)2. What will the pH be when 60.00 mL of Ca(OH)2 has been added?

Homework Equations


2C6H5COOH + Ca(OH)2 -> 2 H2O + Ca(C6H5COO)2

The Attempt at a Solution


I have determined that the equivalence point is at 56.25 mL, so this is past that. The thing that is throwing me off is the mol ratio of acid:base. I try doing an ICF chart and keep getting that there is still acid left and no base. I thought that after the equiv. point, there is no acid left and you determine the pH based on the remaining base.

Can anyone help me fill in the ICF chart so that I can find the pH?
I need moles of acid and base to do this.
 
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  • #2
Your first calculation is correct, that equivalence point needs 56.25 ml. of 0.2000 Ca(OH)2.

Now think in steps: How many moles of extra Ca(OH)2 was added beyond equivalence?
How many moles of hydroxide is this? What is the concentration of excess hydroxide and therefore what is the pOH?
 
  • #3
How many moles of extra Ca(OH)2 was added beyond equivalence?

This is the part I am stuck on. When I plug into my ICF chart I get that there are .0225 moles of C6H5COOH.
Then I calculate the moles of Ca(OH)2 using .060L X 0.200mol/L and get .012 moles of Ca(OH)2.
Do I divide that by 2 because it is a 2:1 reaction?
Even if I don't, when I go to subtract the smaller amount of moles, the Ca(OH)2 comes out as the limiting reagent, which means it is all used up and I know this is not right.
What am I missing?
 
  • #4
Forget your ICF chart. Use fundamental principles. You know you passed the equivalence point for neutralization so you have no unneutralized C6H5COOH. How many moles beyond the equivalence point have you gone, and what is the concentration of the hydroxide (in moles per liter), and from this, what is pOH?
 
  • #5
Think in terms of limiting reagents only. And compare with this pH of acid/base mixture calculation example.

In a way question is idiotic - there is no such thing as 0.2M calcium hydroxide, its solubility is about 10 times smaller if memory serves me well. No idea about benzoic acid solubilityk, but with bulky phenyl it won't be easily soluble as well.

--
 
  • #6
I am not sure if I came up with the right answer, but I got a pH of 12.13. I calculated that there were .0015 mols of Ca(OH)2 remaining.
 
  • #7
Yes.
 

1. What is the purpose of titration in the context of a strong base and weak acid?

The purpose of titration is to determine the unknown concentration of an acid or base by using a known concentration of the opposite type. In the case of a strong base and weak acid titration, the goal is to find the concentration of the weak acid by using a strong base to neutralize it.

2. How is a titration of strong base and weak acid performed?

A titration of strong base and weak acid is performed by adding a known volume of the strong base to a measured volume of the weak acid solution. An indicator, such as phenolphthalein, is used to determine the endpoint of the reaction, which is when the solution changes color from acidic to basic. The volume of the strong base used to reach the endpoint is then used to calculate the concentration of the weak acid.

3. What is the difference between a strong base and a weak acid in terms of titration?

A strong base is a substance that completely dissociates in water, meaning that all of its molecules break apart into ions. This makes it easier to determine its concentration through titration. On the other hand, a weak acid only partially dissociates in water, making it more difficult to determine its concentration through titration.

4. How is the equivalence point determined in a titration of strong base and weak acid?

The equivalence point is determined by the point at which the number of moles of the strong base added is equal to the number of moles of the weak acid present in the solution. This is also known as the stoichiometric point, where all of the acid has been neutralized by the base. It is indicated by a sudden change in pH, as measured by the indicator.

5. What are some potential sources of error in a titration of strong base and weak acid?

Some potential sources of error in a titration of strong base and weak acid include incorrect measurement of volumes, improper calibration of equipment, and failure to properly mix the solutions. Additionally, the choice of indicator can also affect the accuracy of the results, as some indicators have a larger margin of error. It is important to carefully control these variables in order to obtain accurate results.

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