Question

You have 10 ml of ethanol and want to use it all to make a 10-fold dilution with water. How would you do this, including the final volume? Show your calculations and give complete instructions.

Answer 1

You should add water to the 10 ml ethanol until you have a 100-ml solution. For this, pour the 10 ml ethanol in a 100-ml volumetric flask and add water until the mark that indicates that you have a volume of 100 ml. Don´t forget to mix before you reach the mark.

Step-by-step explanation:

You can assume that your ethanol has a concentration of 1X.

if you have to dilute it ten times, the final concentration will be 1X / 10 = 0.1X

The number of moles of ethanol present in the solution will remain the same after the dilution because you are not adding nor substracting ethanol, that means:

mol ethanol before dilution = mol ethanol after dilution

The number of moles can be expressed in this form:

mol ethanol = volume of the solution * concentration of ethanol

then, to find how much water you have to add you have to solve the final volume (Vf) from this equation:

mol ethanol before dilution = mol ethanol after dilution

Vi * Ci = Vf * Cf

where:

Vi = volume of the solution before dilution.

Ci = concentration of the solution before dilution

Vf = volume of the final solution

Cf = concentration of the final solution

Replacing with the data:

10 ml * 1X = Vf * 0.1X

10 ml *1X / 0.1X = Vf

100 ml = Vf