Question

How many ml of absolute alcohol (100%) must be used to make 400ml of a 15% W/V drug solution. Sp gravity of absolute alcohol 0.8; Sp gravity of 15% solution of drug made in alcohol 0.9). when the drug is mixed in alcohol the sp gravity of the solution is different from that of absolute alcohol.
the answer is 375 .

Answer 1

To Find = volume of alcohol required to make desired solution

Volume of desired solution = 400 ml (Given in question)

Concentration of solution = 15% W/V

%(W/V) =

of solute) x 100 (volume of solution) .equation(1) mass

Here, Drug and alcohol forms a binary solution in which Drug is solute and alcohol is solvent.

solute Drug, solventAlcohol

So, from equation 1 we have

15 of drug x 100) (volume of solution)

(mass of drug x 100)(400) = 15

= (15 x 400) 100 mass of drug

So , Mass of drug required is 60 g

Now, specific gravity of solution is 0.9 (Given in question)

so , density of solution is 0.9 g/ml

Mass of solution = (density) * (volume of solution)

= 0.9 * 400

Mass of solution = 360 g

But we know that (mass of solution) = (mass of drug) + (mass of alcohol)

mass of alcohol = mass of solution - mass of drug

360 60

so , Mass of alcohol required = 300 g

if specific gravity of pure alcohol is 0.8 (Given in question)

then , density of pure alcohol will be 0.8 g/ml   

volume of alcohol of alcohol)(den sity of pure alcohol) mass

volume of alcohol (300)(0.8) 375

Hence, Volume of alcohol required will be 375 ml.