CK-12-Chemistry Intermediate

http://www.ck12.org Chapter 16. Solutions

Step 1: List the known quantities and plan the problem.

Known

• mass of solute = 28.60 g C 6 H 12 O 6


• mass of solvent = 250. g = 0.250 kg


• molar mass of C 6 H 12 O 6 = 180.18 g/mol

Unknown

• molality = ?m

Convert grams of glucose to moles and convert the mass of the solvent from grams to kilograms.

Step 2: Solve.

28 .60 g C 6 H 12 O 6 ×

1 mol C 6 H 12 O 6

180 .18 g C 6 H 12 O 6

= 0 .1587 mol C 6 H 12 O 6

0 .1587 mol C 6 H 12 O 6

0 .250 kg H 2 O

= 0. 635 mC 6 H 12 O 6

Step 3: Think about your result.

The answer represents the moles of glucose per kilogram of water and has three significant figures.

Practice Problem

5. Calculate the molality of a solution containing 2.25 g of LiNO 3 dissolved in 600. g of water.

Molality and molarity have very similar values for dilute aqueous solutions because the density of those solutions
is relatively close to 1.0 g/mL. This means that 1.0 L of solution has a mass of just about 1.0 kg. As the solution
becomes more concentrated, its density will not be as close to 1.0 g/ml, and the molality value will be different than
the molarity. For solutions with solvents other than water, the molality will be very different than the molarity. Make
sure that you are paying attention to which quantity is being used in a given problem.

Lesson Summary

• Concentrated and dilute are qualitative terms that reflect the relative concentration of a solution, which is the
  amount of solute dissolved in a given amount of solvent.


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


• The molarity of a solution is the moles of solute divided by the volume of the solution in liters. Molarity is
  the most commonly used way to express the concentration of a solution that is used in a chemical reaction.


• Dilution is the process of adding solvent to a solution in order to decrease its concentration. The dilution
  equation can be used to calculate the molarity of diluted solutions or to determine the volume of a stock
  solution needed for a dilution.


• Molality is the moles of solute divided by the kilograms of solvent.