This is MCQ # 905 of 1000 Pharmacy MCQs

Pharmaceutical Calculation Problem solution


905. How much of a 2% stock solution of a certified green dye should be used to prepare 6000 milliliters of a mouthwash that is to contain 2:20,000 w/v of the certified green dye as the coloring agent?
a- 10 milliliters 
b- 20 milliliters 
c- 30 milliliters 
d- 60 milliliters 
e- 120 milliliters 


Simple Solution:

2:20,000 w/v means 0.01%
6000 milliliters / 2 x 0.01 = 30 milliliters 

Detailed Solution:

To calculate the amount of the 2% stock solution of certified green dye needed, we can use the dilutions formula:

C1V1 = C2V2

where,
C1 is the concentration of the stock solution, 
V1 is the volume of the stock solution to be used, C2 is the desired concentration of the dye in the mouthwash, 
and V2 is the final volume of the mouthwash.

First, we need to convert the desired concentration of the dye in the mouthwash from w/v (weight/volume) to percent. 

2:20,000 w/v means 2 grams of dye in 20,000 milliliters of the solution, or 0.01% (2/20,000 x 100%) concentration.

Now we can plug in the values:

C1 = 2%
V1 = ?
C2 = 0.01%
V2 = 6000 mL

2% V1 = 0.01% x 6000 mL
V1 = (0.01/2) x 6000
V1 = 30 mL

Therefore, we need to use 30 mL of the 2% stock solution of certified green dye to prepare 6000 milliliters of mouthwash that contain 2:20,000 w/v of the dye as the coloring agent.

Thus the correct answer for MCQ # 905 is c.