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

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SUMMARY

The discussion centers on the titration of a weak acid, HCN, with NaOH as outlined in Zumdahl's Chemistry (9th edition). The specific scenario involves a 50 mL sample of 0.100 M HCN being titrated with 0.100 M NaOH, where the pH is calculated to be 8.49 after the addition of 8.00 mL of NaOH. The confusion arises from the low concentration of H+ ions calculated (3.3x10^-9 M), which is below the neutral concentration of H+ in water (10^-7 M). The production of CN- during the titration is highlighted as a significant factor, as it acts as a strong base that consumes H+ ions, leading to a higher pH.

PREREQUISITES
  • Understanding of weak acid dissociation and the concept of Ka (acid dissociation constant).
  • Familiarity with titration techniques and calculations.
  • Knowledge of pH calculations and the relationship between H+ concentration and pH.
  • Basic principles of acid-base chemistry, including the role of conjugate bases.
NEXT STEPS
  • Study the principles of weak acid titration and how to calculate pH at various points in the titration curve.
  • Learn about the impact of conjugate bases on pH, specifically focusing on CN- as a strong base.
  • Explore the concept of hydrolysis of anions and its effect on pH in titration scenarios.
  • Review examples of titration involving weak acids and strong bases to solidify understanding of pH changes.
USEFUL FOR

Chemistry students, educators, and anyone interested in mastering acid-base titration concepts and calculations, particularly in the context of weak acids and their conjugate bases.

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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While titrating you produce CN-. CN- is a quite strong base consuming H+ from the water autodissociation.
 

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