Please help....chemistry.?

Question

I have a review and I'm having some trouble, can anyone please help?

#1: What is the pH of an aqueous solution containing CH3COOH and NaCH3COO in a molar ratio of 4.0 to 1.0?

#2: A solution that is .20M in NH3 is also .30M in NH4Cl. What is the [OH-] in this solution?

#3: What is the [H3O+] of a solution that is .010M in aqueous NH3 and 0.030M in NH4NO3?

Answer 1

pH = pKa + log (base/acid)  Henderson-Hassalbalch Equation

#1

pH = pKa + log (base/acid)

pH = -log(1.8 x 10^-5) + log (1/4)

pH = 4.74 -.060 = 4.14

#2

The Ka of NH4+ = Kw/Kb(NH3) = 1.00 x 10^-14 / 1.8 x 10^-5 = 5.56 x 10^-10

pH = pKa + log (base/acid)

pH = -log(5.56 x 10^-10) + log(.20/.30)

pH = 9.26 - 0.52 = 8.73

pH + pOH = pKw

8.73 + pOH = 14.00

pOH = 5.27

[OH-] = 10^-5.27 = 5.4 x 10^-6 M

#3

pH = pKa + log (base/acid)

pKa of NH4+ (from above) = 9.26

pH = 9.26 + log (0.010 / 0.030)

pH = 9.26 -0.48 = 8.78

[H3O+] = 10^-8.78 = 1.66 x 10^-9 M