Chemistry Reference
In-Depth Information
A. number of moles of solute = 3.0 M × 0.500 L = 1.5 moles of NaCl
B. mass of sample = number of moles × molar mass
= 1.5 moles × 58.5 g/mole = 87.75 g
Final answer: 88 g of NaCl would be required
Another useful way of indicating the concentration of a solution is called
“molality.” The molality of a solution is a measure of the number of moles
of solute dissolved in each kilogram of solvent. The formula for molality is
given here:
Molality (m) =
number of moles of solute
kilograms of solvent
The fact that the words molality and molarity are so similar, and the
respective formulas and symbols are also similar, can lead to confusion.
The main difference is that molarity is based on the volume of the whole
solution, whereas molality is based on the mass of just the solvent. Molality
is measured in moles/kilogram, but it is given the derived unit “m,” which
stands for “molal.”
Let's begin with a basic molality calculation.
Example 4
What would be the molality of a solution made from 6.0 moles of
NaOH dissolved in 2.0 kg of water?
Given: number of moles of solute = 6.0 moles;
kilograms of solvent = 2.0 kg
Find:
molality
Formula
Answer
number of moles of solute
kilogra
ms of solvent
6.0 moles
2.0 kg
Molality =
=
= 3.0 molal
As with the molarity calculations, molality problems often incorporate
the formula for determining the number of moles that a sample repre-
sents. Let's suppose you knew the mass of both the solute and the solvent
that went into the solution. Would you be able to calculate the molality?
Here's an example.
Example 5
Determine the molality of a solution made with 120. g of CaCl
2
and
1.5 kg of water.
