Question

The concentration of the stock solution she needs is 100 milli molar (mm) and she needs to make 1.2 milli liters (ml) solution of the drug-a. the drug is available in a salt form with a molecular weight of 181.6 grams / mole. what is the amount (quantity in grams) of drug-a will she have to weigh-out in order to make the stock solution

Answer 1

Molarity is defined as the number of moles of solute in 1 L of solution 
molarity of stock solution to be prepared - 100 x 10⁻³ mol/L
volume of stock solution to be prepared - 1.2 mL 
Therefore number of moles in 1.2 mL - 100 x 10⁻³ mol/L x 1.2 x 10⁻³ L 
number of moles of drug - 1.2 x 10⁻⁴ mol 
mass of drug required - 1.2 x 10⁻⁴ mol x 181.6 g/mol = 21. 8 mg
21.8 g of drug is required to make the stock solution