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Homework Help: Titration of a Weak Acid

  1. Jan 8, 2014 #1
    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?
  2. jcsd
  3. Jan 8, 2014 #2


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    Staff: Mentor

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