Chemistry Help please :) Acids and Bases....?

Question

Any help would be greatly appreciated.
1. Write the pH of the following solutions:
a. [HNO3=1.0 x 10 -3M
b. [H3O+]=2.5 x 10 -4M
c. 1.0 x 10 -2M KOH
d. [OH-]=0.00037M

2. Hydrocyanic acid, HCN, reacts with water to form a solution containing CN- and hydronium ion. Write the chemical equation to show the reaction of hydrocyanic acid with water. List the formula of the conjugate base of HCN; List the chemical formula of each acid in the equation. Then write the Ka expression for this reaction.

3. The value of the acid ionization constant, Ka, for HCN is 4.4 x 10 -10. What is the concentration of H3O+ in a 1.65M solution of hydrocyanic acid? Show set-up.

4. What is the pH of the 1.65M HCN solution?
Thanks in advance!!

Thanks for the help, really :)

Answer 1

To find pH from molarity, If the molarity is a x 10-b M, the pH is [b]-Log a.

The rxn is HCN + H2O = H3O+ + [CN]-
cyanide ion is the conjugate base.

The Ka expression is used to solve this problem.
From the equation above, if x is the amount of HCN converted to H3O+ and [CN]-, x is the amount of [H3O]+ and [CN]-. Then,
[x][x]/ [1.65-x] = 4.4x10^-10.
Compared to 1.65, x will be quite small. Then, as a good appxn. x^2/1.65 = 4.4x10^-10. x is appx 8.4x10^-5 mole/liter.
pH from above is 5-log 8.4 = 4.08 appx.