# Chapter 2 Atoms, Molecules and Ions. Formula Weight & Molecular Weight The FORMULA WEIGHT of a compound equals the SUM of the atomic masses of the atoms.

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Chapter 2 Atoms, Molecules and Ions

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 10 23

Point to Remember One mole of anything is 6.02 x 10 23 units of that substance.

1 gram = 6.02 x 10 23 a.m.u

Avogadro’s Number and the Mole If one mole of anything is 6.02 x 10 23 units of that substance, then: 1 mole of oranges = 6.02 x 10 23 oranges

And…….. 1 mole of C has the same number of atoms as one mole of any element

Also….. 1 mole of sand = 6.02 x 10 23 particles

An Even Better Analogy….. 1 dozen = 12 entities a dozen apples has the same number of entities as a dozen oranges

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 element atomic 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 10 21 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 C 6 H 12 O 6

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 C 6 H 12 O 6 = 180.16 g

Problem #3 What is the molar mass of (NH 4 ) 3 (PO 4 )?

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 (NH 4 ) 3 (PO 4 ), 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

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