How much water should be added to 120 cubic centimeters of the medicine if the final mixture is to contain only 20% alcohol

Question

A medicine contains 25% alcohol

Answer 1

Hello Te'a

 

The question is asking you to perform a dilution where you are starting with 120 cm³ of the medicine with an alcohol concentration of 25% and decrease it to 20% by adding a certain amount of water, which we need to calculate as per your question.  The equation to use is:

 

C_iV_i = C_fV_f  where the subscripts "i" is the initial amount and "f" is the final amount.

 

25% x 120 cm³ = 20% x V_f

 

V_f = (25% x 120 cm³) / 20% = 150 cm³

 

Amount of water to add = V_f - V_i = 150 - 120 = 30 cm³