Question

How can I calculate the percent concentration of a solution?

Answer 1

The percentage concentration of any solution is most commonly expressed as mass percent:

Mass % of any component of the solution =
(Mass of the component in the solution / Total mass of the solution) x 100

Other methods are:

1. Volume percentage:

Volume % of a component =
(Volume of the component/Total volume of the solution) x 100

1. Mass by volume percentage:
   It is the mass of solute dissolved in 100 mL of the solution.

i.e. Mass by Volume percentage =
(Mass of solute in grams/Volume of solution in mL) x 100

Here's a point to be kept in mind :
Whenever we say mass or volume of the solution, you need to add the respective masses and volumes of ALL the components of the solution. Do NOT commit the error of taking the mass or volume of only the solute or solvent in the denominators of the above expressions.

The concentration of a solution is most of the time expressed as the number of moles of solute present in 1 Liter of the solution (also called molarity )

(There are also other ways to express concentration. Please follow this link. )

EXAMPLE:
(a) If 25 moles of NaCl are present in 100 L of a solution wherein H2O is the solvent, then the concentration of the solution is `25/100=0.25 "mol·L"^-1`.

(b) What is the molarity of a solution prepared by dissolving 15.0 g of sodium hydroxide in enough water to make a total of 225 mL of solution?

Solution

• Calculate the number of moles of solute present.

Moles of NaOH = 15.0 g NaOH × `(1"mol NaOH")/(40.00"g NaOH")` = 0.375 mol NaOH

• Calculate the number of litres of solution present.

Volume = 225 mL × `(1"L")/(1000"mL")` = 0.225 L soln

• Divide the number of moles of solute by the number of litres of solution.

Molarity = `(0.375"mol")/(0.225"L")` = 1.67 mol/L