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

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In the titration of 50 mL of 0.100 M HCN with 0.100 M NaOH, after adding 8.00 mL of NaOH, the calculated pH is 8.49, which raises concerns due to the low concentration of H+ ions at 3.3x10^-9 M. The confusion stems from the assumption that H+ concentration is negligible, despite it being lower than the typical concentration in pure water. The formation of CN- during titration acts as a strong base, which consumes H+ ions from water's autodissociation, affecting the overall pH. This interaction complicates the calculations, leading to a basic pH even with a small amount of added base. Understanding the role of CN- is crucial for accurately interpreting the pH results in weak acid titrations.
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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