*Question by Harmony S*: CHEMIStry*?

what is [H3O] in 0.70M HCN?

answer is 1.9×10^-5, how do you figure out?

**Best answer:**

*Answer by Dennis M*

You need Ka to solve it. My book (Zuhmdahl, 5th ed.) says it is 6.2 X 10^-10.

HCN = H+ + CN-

Let “X” equal [H+] and [CN-].

Ka = [H+][CN-] / [HCN] = 6.2 X 10^-10

X^2 / 0.70 = 6.2 X10^-10

X^2 = 4.34 X 10^-10

X = ( 4.34 X 10^-10)^0.5

X = 2.08 X 10^-5

So [H+] = [H3O+] = 2.08 X 10^-5 M

The slight difference in answers is probably because your textbook has a different Ka value for HCN.

**Give your answer to this question below!**

Ka for HCN = 6.0×10^-10

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

after X moles/l have dissociated the concentrations are-

[H+] = X

[CN-] = X

[HCN] = 0.70 – X

Since Ka is very small, we know X << 0.70, so we can assume [HCN] = 0.70 (to 2 significant figures) 6.0x10^-10 = X^2 / 0.70 4.2x10^-10 = X^2 X = 2.0x10^-5 Perhaps your textbook gives a different value of Ka. 1.9x10^-5 would be the correct answer if the value of Ka was 5.2x10^-10 1.9x10^-5 would be the correct answer if Ka = 6.0x10^-10 and the concentration of HCN was 0.60 M (not 0.70 M)