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.

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