Senior High School Department
STEM – Chemistry 2
LEARNING PACKET
Week 2.4.4– Molality
Learning Goals
By the end of this lesson, I can…
1. Calculate solution concentrations using Molality
Opening prayer
As I go back to school, Lord, keep my lamp burning in my studies and relationships. Arm me with
strength so that I may be able to handle the different challenges that we face at school. Give me the
courage to face each day with grace and love. In Jesus' name, I believe and pray, Amen.
To start our day
Here’s the next installment on how to express qualitatively the concentration of solute in a solution.
Lesson Proper
To refresh your understanding of our topic for today, read the lecture notes and highlight important
information.
Topic:
✓ Molality
Molality definition
Molality (m), or molal concentration, is the amount of a substance dissolved in a certain mass of solvent.
It is defined as the moles of a solute per kilograms of a solvent.
Molality formula and units
The units of molality are m or mol/kg.
Molality equation
https://www.technologynetworks.com/analysis/articles/molarity-vs-molality-formula-and-definitions-334119#:~:text=Molarity%20is%20the%20ratio%20of,the%20kilograms%20of%20a%20solvent.
Key Points
Molality differs from molarity only in the denominator. While molarity is based on the liters of solution,
molality is based on the kilograms of solvent.
Concentrations expressed in molality are used when studying properties of solutions related to vapor
pressure and temperature changes. Molality is used because its value does not change with changes in
temperature. The volume of a solution, on the other hand, is slightly dependent upon temperature.
https://chem.libretexts.org/Bookshelves/Introductory_Chemistry/Book%3A_Introductory_Chemistry_(CK-12)/16%3A_Solutions/16.11%3A_Molality
Page 1 of 3
Prepared by: Josephine T. Atlas
Week 2.4.4 – Molality | Chemistry 2 (SY 2021-2022)
jtatlas@ssam.edu.ph
Let’s try these!
• What is the molality of a solution that contains 48 grams of sodium chloride and 250 mL of
water?
Given:
Molality = x
Mass NaCl = 48 grams
Volume of water = 250 mL
Solution:
a) Convert mass of NaCl to mole
48 grams x 1 mol = 0.8276 mol
58 g
b) Convert volume of water to mass (kg)
250 mL x 1 g H2O x 1 kg H2O = 0.25 kg H2O
1 mL H2O 1000 g H2O
c) Compute for molality
m = mol solute
kg of solvent
= 0.8276 mol
0.25 kg
= 3.3104
= 3.3 mol/kg or 3.3 m
• What is the molality of NaCl in an aqueous solution which is 4.20 molar? The density of the
solution is 1.05 x 103 g/L.?
Given:
Molarity of NaCl solution = 4.20 mol/L
Density of solution = 1.05 x 103 g/L
Molality of solution = ?
Solution:
a) Solve for the mol of solute c) Solve for the mass of the solute
Molar (M) = mol solute 4.20 mol NaCl x 58 g = 243.6 g NaCl
L of solution 1 mol
4.20 M = 4.20 mol NaCl
d) Solve for the mass of solvent
1L
1.05 x 103 g – 243.6 g = 806.4 g
b) Solve for the mass of solution
1 L solution = 1.05 x 103 g/L e) Solve for Molality
1.05 x 103 g = x m = mol of solute
L 1L kg of solvent
3
X = 1.05 x 10 g/L = 4.20 mol NaCl
1L 0.8064 kg
X = 1.05 x 103 g solution = 5.208333
͌ 5.21 m
Practice on these!
• You are given 450.0 g of a 0.7500 molal solution of acetone dissolved in water. How many grams
of acetone are in this amount of solution?
Answer: 18.78 g
• What is the molality of NaCl in an aqueous solution in which the mole fraction of NaCl is 0.100?
Answer: 6.17 m
Page 2 of 3
Prepared by: Josephine T. Atlas
Week 2.4.4 – Molality | Chemistry 2 (SY 2021-2022)
jtatlas@ssam.edu.ph
In summary …
When to Use Molality and When to Use Molarity
Molality is used any time you expect the solute may interact with the
solute and in the following situations:
• To determine a boiling point
• To determine a melting point
• When working with colligative properties (boiling point
elevation,
freezing point depression)
Molarity is used for dilute aqueous solutions held at a constant
temperature. In general, the difference between molarity and molality
for aqueous solutions near room temperature is very small and it
won't really matter whether you use a molar or molal concentration
https://www.thoughtco.com/differences-between-molality-and-molarity-609192
Closing Prayer
Prayer for Openness
Jesus, you open us to new life, new hope, new possibilities.
You conquer the way we close up, close down, and close in.
Jesus, lover of us all, open our hearts to feel your compassion for people we would rather walk by;
open our ears to hear the cries of the suffering deep inside where we cannot fail to be moved;
open our understanding to new ways of living that do not diminish anyone; open our eyes to see a vision
of this world as you would have it. May these prayerful words on our lips become the meditations of our
hearts as we live in ways that are acceptable to you. O God, our Rock and our Redeemer. Amen.
That in all things, God may be glorified!
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Prepared by: Josephine T. Atlas
Week 2.4.4 – Molality | Chemistry 2 (SY 2021-2022)
jtatlas@ssam.edu.ph
