Molality
Objectives: define molality, calculate molality, and calculate the mass of solute
The Definition
Calculate Molality
Ex. What is the molality of 5.30 g of Na2CO3 dissolved in 400.0 g H2O?
5.30 gx
1mole
 .05mole / .400 kg  .125m
106.0 g
Calculate Mass of Solute
Ex. Calculate the mass of solute needed to prepare a .50m solution of sodium chloride if
you use 1000 grams of water
1kg
.50mole 58.45 g
1000 gx
x
x
 29 g
1000 g
1kg
1mole
Add 29 grams of solute and 1000 grams of water together and stir to
create a homogeneous mixture.
Can you tell the difference between M and m ???
Because the solute amount and solvent amount are added together—the
final volume is greater than the solvent!
PRACTICE
Ex. Calculate the molality of a solution if 13.75 grams of calcium hydroxide are added to
250 g of water
1mole
13.75 gx
 .186 mole / .250 kg  .744 m
74.1g
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Ex. Calculate the mass of solute needed to prepare a .75m solution of sodium hydroxide if
you use 1000 g of water
.75mole 40.0 g
1000 gx
x
 30.g
1000 g 1mole
Add 30. grams of solute and 1000 grams of water together and stir to
create a homogeneous mixture.
Ex. Calculate the mass of solute needed to prepare a .45m solution of hydrochloric acid if
you use 500 g of water
.45mole 36.45 g
500 gx
x
 8.2 g
1000 g 1mole
Add 8.2 grams of solute and 500 grams of water together and stir to
create a homogeneous mixture.
One to GO on…
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250 gx
Calculate the mass of solute needed to prepare a .010 molal solution of sodium phosphate if 250
grams of water are available
.010 mole 164.0 g
x
 .41g
1000 g
1mole
Add .41 grams of solute and 250 grams of water together and stir to
create a homogeneous mixture.
Minute Paper
How do I get from where I am to where I’m going?
T. O’Toole
10 Minute Makeover
Customize your learning by choosing from the following options each night
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Complete the appropriate section on the Portfolio Page
Review the calculation video on the website
Review the PPT file on the website
Read pages 520-521 in the book
Answer questions on page 521 # 29. Answers are in Appendix C.
Check out the tutorial sources in the LiveBinder
Complete the SYS Assignment
Review the skills from today’s Train Your Brain
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