How many grams of a 0.0021 m aqueous solution of nalorphine (C19H21NO3) are needed to obtain a dose of 2.25 mg?

First, convert the amount of C₁₉H₂₁NO₃ to moles.

Since carbon has a molar mass of 12.01 g/mol, hydrogen has a molar mass of 1.01 g/mol, nitrogen has a molar mass of 14.01 g/mol, and oxygen has a molar mass of 16 g/mol, the molar mass of C₁₉H₂₁NO₃ is:

Molar mass C₁₉H₂₁NO₃ = 19*12.01 + 22*1.01 + 14.01 + 3*16 = 312.42 g/mol.

Then, 2.25 mg = 0.00225 g and:

Moles C₁₉H₂₁NO₃ = 0.0025/312.42 = 8 x 10^-6 mol.

By the definition of molarity:

M = m/V

==> 0.0021 M = (8 x 10^-6 mol)/V

==> V = (8 x 10^-6 mol)/(0.0021 M) = 0.0038 g.

I hope this helps!