Molar Mass and Percent Composition

Counting Atoms

Counting Atoms

Molar Mass

• The molar mass, also known as molecular weight, is the sum of the total mass in grams of all the atoms that make up a mole of a particular molecule.

• Measured in the unit: grams/mole

How to Calculate Molar Mass

• 1. Count How many atoms you have of each element.

• 2. Find the atomic mass of each element and multiply it by their number of atoms.

• 3. Add up all the masses.

Example: Find the molar mass of Magnesium Nitrate Mg(NO₃)₂!

• Count Number of atoms.

• Find atomic mass from periodic table and multiply it by number of atoms. (use Dynamic Periodic Table)

• Add up all of the masses.

Multiple Choice

What is the Molar Mass for Calcium Hydroxide Ca(OH)₂?

• 74.1 g/mol

• 114.14 g/mol

• 58.09 g/mol

Multiple Choice

Find the Molar Mass for (NH₄)₂SO₄ Ammonium Sulfate!

• 132.16 g/mol

• 114.11 g/mol

• 118.15 g/mol

• 146.17 g/mol

Multiple Choice

Find the molar mass of Sulfur Heptafluoride SF₇!

• 133 g/mol

• 146.06 g/mol

• 165.06 g/mol

• 127.06 g/mol

Topic 4.8

Percent Composition

What portion of the overall mass of something is a given part?

Image on the right describes the percent compositions for specific elements of the human body...

The human body is 65% oxygen atoms by mass and only 3.3 % nitrogen atoms by mass.

(consider that we are mostly water)

Topic 4.8

Percent Composition

What portion of the overall mass of something is a given part?

Image on the right describes the percent compositions for specific elements of the human body...

The human body is 65% oxygen atoms by mass and only 3.3 % nitrogen atoms by mass.

(consider that we are mostly water)

Percent Compositions in terms of mass.

(i.e. what portion of the overall mass is each element)

Example 1 in detail

Example 2 in detail

Example 3 in detail

Percent Composition Formula

Perhaps a simpler formula is this. (From back page of your Reference Tables)

"Mass of part" is the mass contribution from the specific element of interest. (but can also be a polyatomic ion or H₂O)

"Mass of whole" is the total mass of the formula (a.k.a... gram formula mass)

Multiple Choice

Considering the generic formula for percent composition below, what number would go in the numerator if you were attempting to solve for the percent composition of Hydrogen in $$CH_4$$  ?

$$\%\ comp\ =\frac{Mass\ contribution\ from\ the\ specific\ element}{Total\ mass\ in\ formula}\ x\ 100$$  

• 16.05 grams

• 4.04 grams

• 12.01 grams

• 48.04 grams

The mass contribution from hydrogen , H, in CH₄ is 4 x 1.01 grams = 4.04 grams

subscript of 4 indicates there are 4 hydrogen atoms in the formula and each one contributes its atomic mass of 1.01 u , just switched into grams.

Multiple Choice

Considering the generic formula for percent composition below and the answers to the two previous questions, what is the percent composition of Hydrogen in $$CH_4$$  ?

$$\%\ comp\ =\frac{Mass\ contribution\ from\ the\ specific\ element}{Total\ mass\ in\ formula}\ x\ 100$$  

• 80 %

• 4.04 %

• 0.25 %

• 25.17 %

• 74.83 %