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.

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)

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.