Calculate assay of lisinopril dihydrate based on the anhydrous form – How to

When you weigh your reference standard and use the mass in the assay calculations of your formulation, you must correct for the water since the reference standard is the di-hydrate form.

You know the molecular weight of lisinopril and of the two water molecules so you would apply a mass correction factor of MW anhydrous form / MW di-hydrate form.

Using this gives the mass of anhydrous lisinopril in the reference standard portion that was weighed.