0.100 M solution of a weak acid, HX, is known to be 15% ionized. The weak acid has a molar mass of 72 g/mol.

1. What is Ka for the weak acid?

2. What is the pH of the buffer prepared by adding 10.0 g of the sodium salt of the acid (NaX) to 100.0 mL of 0.250 M HX

3 answers

  1. 0.1M x 0.15 ionized = 0.015 concn.
    ...........HX --> H^+ +X^-
    Ka = (H^+)(X^-)/(HX)
    Substitute from the ICE chart and solve for Ka.
    For #2, use the Henderson-Hasselbalch equation.

  2. I am a little confused with the Henderson-Hasselbalch eqn on 2, #1 worked great, thanks!

  3. You have 10.0 g of the Na salt. The acid has a molar mass of 72. We remove 1 for H and add 23 for Na for the molar mass of the NaX to be 94.
    mols NaX = 10.0/94 = 0.106
    mols acid = 0.1L x 0.25M = 0.025
    The HH equation is
    pH= pKa + log (base)/(acid)
    pH = pKa from your value in #1.
    mols base = 0.106
    mols acid = 0.025
    Crunch the numbers and solve for pH.

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