FORMULA MASS FORMULA MASS

17.2 Conservation of mass

Lavoisier showed that a closed system must be used when studying chemical reactions.

When chemicals are reacted in a closed container, you can show that the mass before and after the reaction is the same.

17.2 Formula mass

The sum of the atomic mass values of the atoms in a chemical formula is called the formula mass.

Ex 1

Find the formula mass of O2

Ex 2

Find the formula mass of CO2

Atomic & Molecular Weights

Chemical formulas and chemical equations have quantitative significance.

H2O means 2 atoms of hydrogen and 1 atom of oxygen.

C3H8 + 5O2 → 3CO2 + 4 H2O

means 5 moles of oxygen gas

Definitions:

Molecular Formula: the actual number of atoms of each element in the compound.

Molar mass: the mass of one mole of any substance

Atomic mass unit: amu

17.2 Molar Mass

• The mass (in grams) of one mole of a compound is called its molar mass.

What is the molar mass of one mole of CaCO3?

1. Looking for: – … molar mass of CaCO3

2. Given– … chemical formula

3. Relationships:– no. amu in formula = molar mass in grams

Solving Problems

4. Solution

Solving Problems

Formula mass CaC03 = 100.09 amu

1 mole CaC03 = 100.09 g CaCO3

17.2 Avogadro’s Number

The Avogadro number was named in honor of Amedeo Avogadro who discovered that a mole of any gas under the same conditions has the same number of molecules.

Johann Josef Loschmidt, a German physicist, named and discovered the Avogadro number.

Loschmidt realized that a mole of any substance—be it a gas, liquid, or solid—contains 6.02 x 1023 atoms or molecules.

Avogadro’s Number

Avogadro’s number states that for one mole of any substance, whether element or compound, there 6.02x1023

Particles present in the sample.

It is also the number of molecules in the formula mass of

a compound when these masses are expressed in grams.

One set of 6.02x1023 atoms or molecules is also referred to as a mole of that substance.

Remember

1 gram = 6.02214 x1023 amu

The atomic mass unit(amu) is assigned by

assigning exactly 12 amu to the 12C isotope

Elements occur in Nature as isotopes.

1 oxygen atom = 16.0grams

1mole of oxygen atoms=6.02x1023atoms=16.0grams

Formula Weight & Molecular Weight

Formula Weight – Ionic Compounds

Molecular Weight – Molecular/Covalent Compounds

****We tend to use interchangeably****

To find the Formula/Molecular Weight

Simply find the sum of the atomic weights of each atom in the compound.