I want to measure the Bismuth molar concentration in the OTC drug "pepto bismol".

  • 525 mg bismuth subsalicylate (C7H6BiO4) / 30 mL (17.5 mg/mL)
  • molecular weight 363.10054 g/mol

If I convert to mM (17.5 mg/mL / 363.10054 g/mol * 1000), I get 48.2 mM. However, this value seems very high. Am I missing something obvious?

I am sorry if this question seems basic to you. I can't find the answer anywhere else though. Thank you so much in advance!


Your calculation is correct. You are not missing anything obvious. Your set up, which converts mL to L and then mg to mmol is correct and you get the correct units.

$$\require{cancel} \mathrm{\dfrac{17.5\ \cancel{mg}}{\cancel{mL}}\times \dfrac{1000\ \cancel{mL}}{L}\times \dfrac{mmol}{364.10054\ \cancel{mg}} =48.2\ mM}$$

This concentration is not "too high". It is what it is. 48.2 mM is 0.0482 M, which is a pretty low concentration.

For comparison, your 17.5 mg per mL is at most 2% by mass (low-balling the density of the suspension to be around 1). 1 M hydrochloric acid (which perhaps seems like a high concentration) is around 3% by mass. Concentrated HCl is 36% by mass and about 12 M.

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