Why Doesn't Adding Water Change the pH of Acid During Titration?

In summary, the conversation discusses the use of water in titration and its potential effects on the pH and equilibrium constant. The conclusion is that while adding water may change the pH, it does not significantly affect the titration results as the amount of acid is calculated from the neutralization stoichiometry.
  • #1
alingy1
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Homework Statement



Ok, so, I've been thinking about this for a while now.

When I was doing titration back in high school, we had to put water (so that the solution would have a larger volume to work with) in the acid in which we were going to pour base from a buret.

Now, my question is this:

Why doesn't the water change the pH of the acid? You add more water which dilutes the concentration of H+. So, then, there is the equilibrium formula:

K=[H+][A-]/[HA]

Since you dilute the solution (let's assume by doubling the volume of the solution), the K changes:
[0.5H+][0.5A-]/[0.5HA]

So, equilibrium constant is not the same anymore. Shouldn't this cause changes in the concentration of the acid and thereby false the titration?

I feel there is something that doesn't click here.
 
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  • #2
alingy1 said:
Why doesn't the water change the pH of the acid?

It does.

You add more water which dilutes the concentration of H+.

It also dilutes all other things present in the solution.

Since you dilute the solution (let's assume by doubling the volume of the solution), the K changes:
[0.5H+][0.5A-]/[0.5HA]

No, K doesn't change. Equilibrium shifts till K has exactly the same value it had before.

So, equilibrium constant is not the same anymore. Shouldn't this cause changes in the concentration of the acid and thereby false the titration?

K didn't change, but even if it would, it would not change the titration result. Amount of acid (note: amount, not concentration!) is calculated from the neutralization stoichiometry, not from pH.
 

FAQ: Why Doesn't Adding Water Change the pH of Acid During Titration?

1. What is titration in chemistry?

Titration is a laboratory technique used to determine the concentration of a substance in a solution. It involves adding a known volume of one solution (called the titrant) to a known volume of another solution (called the analyte) until the reaction between the two is complete. This allows for the determination of the unknown concentration of the analyte.

2. Why is pH important in titration?

pH is a measure of the acidity or basicity of a solution. In titration, the goal is often to determine the concentration of an acidic or basic solution. pH is important because it indicates the endpoint of the titration, which is when the reaction between the titrant and analyte is complete. This is typically when the solution reaches a neutral pH of 7.

3. How do you calculate the concentration of an unknown solution using titration?

To calculate the concentration of an unknown solution using titration, you first need to know the volume of the titrant and the volume of the analyte. Then, using the balanced chemical equation for the reaction, you can determine the mole ratio between the titrant and analyte. From there, you can use the formula M1V1 = M2V2 to calculate the concentration of the unknown solution, where M1 is the molarity of the titrant, V1 is the volume of the titrant, M2 is the molarity of the unknown solution, and V2 is the volume of the unknown solution.

4. What is an indicator in titration?

An indicator is a substance that changes color when the pH of a solution changes. In titration, an indicator is used to signal when the reaction between the titrant and analyte is complete. Common indicators used in acid-base titrations include phenolphthalein, methyl orange, and bromothymol blue.

5. How do you choose the appropriate indicator for a titration?

The choice of indicator for a titration depends on the type of reaction being performed. For acid-base titrations, the indicator should have a pH range that falls within the expected endpoint of the titration. This can be determined by looking at the indicator's color change range on a pH scale. In general, the indicator should change color at a pH that is close to the expected endpoint of the titration for the most accurate results.

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