Use activity coefficients to find the concentration of hydrogen ions in a solution of 90.0 mM butanoic acid and 0.05 M NaCl. The ionization constant of butanoic acid is Ka = 1.52 × 10-5. Take the size of C3H7COO– to be 500 pm. How do I calculate the ionic strength and activity coefficients if I don't know the concentration of H+?