5 Steps to a 5 AP Chemistry

Mass (Sometimes Called Weight) Percentage

The mass percentage of a solution is the mass of the solute divided by the mass of the solu-

tion, multiplied by 100% to get percentage. The mass is commonly measured in grams.

mass % =(grams of solute/grams solution) ×100%

For example, a solution is prepared by dissolving 25.2 g of sodium chloride in 250.0 g

of water. Calculate the mass percent of the solution.

Answer:

A common error is forgetting to add the solute and solvent masses together in the

denominator.

When solutions of this type are prepared, the solute and solvent are weighed out separately

and then mixed together to form a solution. The final volume of the solution is unknown.

Mass/Volume Percentage

The mass/volume percent of a solution is the mass of the solute divided by the volume of

the solution, multiplied by 100% to yield percentage. The volume of the solution is gener-

ally expressed in milliliters.

mass/volume % =(grams solute/volume of solution) × 100%

When mass/volume solutions are prepared, the grams of the solute are weighed out and

dissolved and diluted to the required volume.

For example, a solution is prepared by mixing 125.0 g of benzene with 250.0 g of

toluene. The density of benzene is 0.8765 g/mL, and the density of toluene is 0.8669 g/mL.

Determine the mass/volume percentage of the solution. Assume that the volumes are

additive.

Answer:

First, determine the volume of the solution.

solution volume =(125.0 g benzene)(mL/0.8765 g benzene)

+(250.0 g toluene)(mL/0.8669 g toluene)

=431.0 mL

Then

Notice that it is not necessary to know the chemical formula of either constituent.

A common error is forgetting to add the solute and solvent volumes together.

Volume/Volume Percentage

The third case is one in which both the solute and solvent are liquids. The volume percent

of the solution is the volume of the solute divided by the volume of the solution, multiplied

by 100% to generate the percentage.

volume % =(volume solute/volume solution) ×100%

ma ss %

(125.0g benzene)

431.0mL solution

=×100%==29 00.%

ma ss %

(25.2g solute)

(25.2+250.0)g solutio

=

nn

×= 100 %.%9 16

Solutions and Colligative Properties  181

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