What is the pH? Please help me! It's so hard!?

Question

What is the pH of a 0.300M solution of HCN in which the CN- concentration has been adjusted to 0.00100 M?

Answer 1

You will need to look up the Ka.

Ka = [H+][CN-]/[HCN]

Ka = (x)(0.001+x)/(0.3-x)

the x will be so small, that 0.001+x = 0.001 and 0.3-x = 0.3, so

Ka = (x)(0.001)/(0.3)

solve for x and do the -log(x) to find pH

When you drop the x like that, always confirm it is small.  If not, then go back the the first equaion and solve it including the x.