[H] concentration?

Question

Calculate the [H+] for 0.50 M HCN solution. K subscript a=4.0*10^-10
Calculate the % dissaciation.

I have no idea how to do this, so if you could walk me through it step by step, and provide an answer at the end it would be much appreciated.

Also, how would I solve it if it asked me to calculate the [H3O+] or the [OH-]


thanks soo much!

Answer 1

HCN ----> H+ + HCN-

H+ : HCN
1 : 1
[H+] = 0.50
another way: if you know the ph it will help because:
pH= -log[H+] the same for oh-

Answer 2

[H+] = root (Ka x concentration)

% dissociation = 100 x root (Ka/concentration)

[H+] and [H3O+] are one and the same thing.

[OH-] = 1 x 10-14/[H+]