Formula Weight & Molecular Weight

• The FORMULA WEIGHT of a compound equals the SUM of the atomic masses of the atoms in a formula.

• If the formula is a molecular formula, the formula weight is also called the MOLECULAR WEIGHT.

The MOLE

• Amount of substance that contains an Avogadro’s number (6.02 x 10 23)of formula units.

The MOLE

• The mass of 1 mole of atoms, molecules or ions = the formula weight of that element or compound in grams. Ex. Mass of 1 mole of water is 18 grams so molar mass of water is 18 grams/mole.

Formula for Mole

Mole = mass of element formula weight of

element

Sample Mole Calculations

1 mole of C = 12.011 grams» 12.011 gm/mol

• 0.5 mole of C = 6.055 grams» 12.011 gm/mol

Avogadro’s Number

•Way of counting atoms

• Avogadro’s number = 6.02 x 1023

Point to Remember

One mole of anything is 6.02 x 1023 units of that substance.

And……..

• 1 mole of C has the same number of atoms as one mole of any element

Formula Weight & Molecular Weight

• The FORMULA WEIGHT of a compound equals the SUM of the atomic masses of the atoms in a formula.

• If the formula is a molecular formula, the formula weight is also called the MOLECULAR WEIGHT.

Summary

• Avogadro’s Number gives the number of particles or atoms in a given number of moles

• 1 mole of anything = 6.02 x 10 23 atoms or particles

Sample Problem 2

• Compute the number of atoms and moles of atoms in a 10.0 gram sample of aluminum.

Solution

• PART I:• Formula for Mole:

–Mole = mass of elementatomic mass of element

Solution (cont.)

• Part II: To determine # of atoms

• # atoms = moles x Avogadro’s number

Problem # 2

• A diamond contains 5.0 x 1021

atoms of carbon. How many moles of carbon and how many grams of carbon are in this diamond?

Molar Mass• Often referred to as molecular mass–Unit = gm/mole

• Definition: –mass in grams of 1 mole of the

compound

Example Problem

• Determine the Molar Mass of C6H12O6

Solution

• Mass of 6 mole C = 6 x 12.01 = 72.06 g• Mass of 12 mole H = 12 x 1.008 = 12.096 g• Mass of 6 mole O = 6 x 16 = 96.00 g

•Mass of 1 mole C6H12O6

= 180.16 g

Problem #3

•What is the molar mass of (NH4)3(PO4)?

Molar Mass• Often referred to as molecular mass–Unit = gm/mole

• Definition: –mass in grams of 1 mole of the

compound

Sample Problem• Given 75.99 grams of (NH4)3(PO4), determine the ff:• 1. Molar mass of the compound• 2. # of moles of the compound• 3. # of molecules of the compound• 4. # of moles of N• 5. # of moles of H• 6. # of moles of O• 7. # of atoms of N• 8. # of atoms of H• 9. # of atoms of O