Titration of Weak Acid HCN: Calculating pH on Zumdahl's Chemistry (9th edition)

In summary, on page 736 of Zumdahl's Chemistry (9th edition), it is shown that when a 50ml sample of 0.100M HCN (a weak acid with a Ka value of 6.2x10^-10) is titrated with 0.100M NaOH, the resulting pH after adding 8.00mL of NaOH is 8.49. This may seem counterintuitive as such a small amount of base leads to a basic pH. This is because during titration, CN- (a strong base) is produced and consumes H+ from the autodissociation of water. This results in a low concentration of H+ (3.3x
  • #1
alingy1
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Hello, on Zumdahl's Chemistry (9th edition), p.736:
HCN, weak acid (Ka=6.2x10^-10), is dissolved in water. 50ml sample of 0.100M HCN is titrated with 0.100M NaOH. Calculate pH after 8.00 mL of 0.100M NaOH has been added.

The steps are given. Then final answer is 8.49. That makes no sense to me! You added so little base to get a basic pH!
The concentration of H+ is 3.3x10^-9 M. However, this value is really low. When you do the calculations, you assume that H+ concentration is 0 in this problem. But, that doesn't make any sense since the concentration you get is lower than the normal concentration of H+ in water (10^-7M). WHAT IS GOING ON?
 
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  • #2
While titrating you produce CN-. CN- is a quite strong base consuming H+ from the water autodissociation.
 

1. What is the purpose of titrating a weak acid?

The purpose of titrating a weak acid is to determine its concentration or the concentration of another substance that reacts with the acid. This is done by adding a known concentration of a strong base to the acid and measuring the amount of base required to neutralize the acid.

2. How does titration of a weak acid differ from titration of a strong acid?

Titration of a weak acid differs from titration of a strong acid in the type of indicator used and the endpoint of the titration. Weak acids have a lower pH and a smaller change in pH during titration, so a more sensitive indicator is needed. The endpoint of a weak acid titration also occurs at a pH higher than 7, whereas strong acid titrations typically have an endpoint at a pH lower than 7.

3. What is the importance of selecting the right indicator for titration of a weak acid?

The right indicator is crucial in titration of a weak acid because it must be able to detect the subtle pH changes that occur during the titration. If an indicator with a color change at a pH of 7 is used, it may not accurately detect the endpoint of the titration, resulting in an incorrect determination of the acid's concentration.

4. How does the initial concentration of the weak acid affect the titration results?

The initial concentration of the weak acid affects the titration results as it determines the amount of base needed to neutralize the acid. A higher concentration of the weak acid will require more base to reach the endpoint, while a lower concentration will require less base. Therefore, it is important to accurately measure and record the initial concentration of the acid for precise titration results.

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

Potential sources of error in titration of a weak acid include inaccurate measurement of the acid's initial concentration, improper calibration of the pH meter or burette, and incorrect selection or use of an indicator. Other factors such as temperature changes or contamination of solutions can also affect the accuracy of the titration results.

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