Moles and Molar Mass

Learning Targets/Content Objectives:

I can calculate the molar mass of a compound knowing its molecular formula.
I can convert between number of moles and mass of a compound using moles to mass, mass to moles conversion factors.
I can calculate quantities of a substance or its relative number of particles using dimensional analysis and the mole concept.

Vocabulary

Molar mass- mass - moles- grams- Conversion factor- Dimensional Analysis- Avogadro's Number- particles- atoms.

The Mole

The mole is an amount unit similar to familiar units like pair, dozen, gross, etc. It provides a specific measure of the number of atoms or molecules in a bulk sample of matter. A mole is defined as the amount of substance containing the same number of discrete entities (such as atoms, molecules, and ions) as the number of atoms in a sample of pure ¹²C weighing exactly 12 g.

1 mole of any element contains the same number of atoms as 1 mole of any other element. The masses of 1 mole of different elements, however, are different, since the masses of the individual atoms are drastically different. The molar mass of an element (or compound) is the mass in grams of 1 mole of that substance, a property expressed in units of grams per mole (g/mol). (See figure 4)

Dimensional Analysis and Conversion Factors

Dimensional analysis (also called factor label method or unit analysis) is used to convert from one set of units to another. This method is used for both simple (feet to inches) and complex (g/cm3 to kg/gallon) conversions and uses relationships or conversion factors between different sets of units. Below are useful examples in Chemistry.

Video on how to convert atoms to grams

You can use the molar mass to convert from grams to moles or from moles to grams. See example 1.

You can use the Avogadro's number to convert from moles to particles and from particles to moles. See example 2.

You can use both molar mass and Avogadro's number to convert from grams to particles or atoms and from particles/atoms to grams see videos below for more help.

Video on how to convert grams to atoms

Check for Understanding

Determine the number of moles of CO₂ in 454 grams of CO₂

First, look up the atomic masses for carbon and oxygen from the periodic table. The atomic mass of C is 12.01, and the atomic mass of O is 16.00. The formula mass of CO₂ is:

12.01 + 2(16.00) = 44.01

Thus, one mole of CO₂ weighs 44.01 grams. This relation provides a conversion factor to go from grams to moles. Using the factor 1 mol/44.01 g:

moles CO₂ = 454 g x 1 mol/44.01 g = 10.3 moles

2. Determine the mass in grams of H₂SO in 3.60 mol of H₂SO

First, look up the atomic masses for hydrogen, sulfur, and oxygen from the periodic table. The atomic mass is 1.008 for H, 32.06 for S, and 16.00 for O. The formula mass of H₂SO₄ is:

2(1.008) + 32.06 + 4(16.00) = 98.08

Thus, one mole of H₂SO₄ weighs 98.08 grams. This relation provides a conversion factor to go from grams to moles. Using the factor 98.08 g / 1 mol:

grams H₂SO₄ = 3.60 mol x 98.08 g / 1 mol = 353 g H₂SO₄

3. How many moles of potassium chloride (KCl) are in 4.0 L of a 0.65 M solution?

0.65 moles/L x 4.0 L= 2.6 moles KCl

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