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.
Then final answer is 8.49. That makes no sense to me! 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?

Question

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.
 Then final answer is 8.49. That makes no sense to me! 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?

aaronq · Answer

have you tried finding the concentrations of HCN and CN^- and used the henderson-hasslebalch equation?

vincent-lyon.fr · Answer

HCN is an extremely weak acid <=> CN– is a relatively strong base.

So, even in a solution in which there are more HCN molecules than CN–, the strength of CN– takes over and the resulting solution is alkaline.