The intrinsic solubility of a drug, sulfathiazole, is ~0.002 M and its pKa is 7.12. What
are the lowest pHs at which a 1% and 5% solution of Na sulfathiazole (mol wt = 304) are completely soluble?

I tried calculating the molarity of each and subtracting it from pkA. The formula I used was pH=pka- log [A-]/[HA]

I thus replaced the concentration portion with the total (solubility-(intrisnic solubility/ intrisnic solubility))

But my answer isn't correct!

The correct answers are 8.31 and 9.03

Help :(