I can't figure out these chemistry problems

Question

1. When 1.0 M solutions of the substances listed below are arranged in order of decreasing pH the order is

NH3 HCl NaOH CH3COOH

 

1. NaOH, NH3, CH3COOH, HCl
2. NH3, NaOH, HCl, CH3COOH
3. CH3COOH, HCl, NaOH, NH3

HCl, CH3COOH, NH3, NaOH

2. What is the pH of a solution of 0.020 M HF solution, which has a pKa of 3.14.

Answer 1

1). Decreasing pH means going from high pH to low pH or going from most basic (alkaline) to most acidic.

Option 1. would be the correct answer because NaOH is a strong base, NH3 is a weak base, CH3COOH is a weak acid and HCl is a strong acid. None of the options are what you have listed, and you have listed them in exactly the reverse order.

2). HF is a weak acid and will dissociate as follows: HF <==> H⁺ + F^-

Ka = [H^+][F^-]/[HF] and Ka = 1x10^-3.14 = 7.2x10^-4

7.2x10^-4 = (x)(x) / 0.02 - x and if we assume x is small relative to 0.02 we can ignore it

x2 = 1.4x10^-5

x = [H+] = 3.7x10^-3 and this is about 19% of 0.02 so our above assumption is not valid. We must use the quadratic equation to solve for [H+] as follows:

x² = 1.4x10^-5 - 7.2x10^-4x

x² + 7.2x10^-4x - 1.4x10^-5 = 0

x = [H+] = 0.0034 M

pH = -log [H+] = -log 0.0034

pH = 2.5