# Mass Relationships in Chemical Reactions Chapter 3.

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Mass Relationships in Chemical Reactions Chapter 3

Formula Weight (FW) Sum of the atomic weights for the atoms in a chemical formula. e.g., formula weight of calcium chloride, CaCl 2 Ca: 1(40.1 amu) + 2Cl: 2(35.5 amu) 111.1 amu Formula weights used for ionic or molecular compounds

Molecular Weight (MW) Sum of the atomic weights of the atoms in a molecule. For the molecule ethane, C 2 H 6, the molecular weight would be C: 2(12.0 amu) 30.0 amu + 6H: 6(1.0 amu) Formula weights used for molecular compounds only

Percent Composition Percentage mass of a compound that comes from each of the elements in the compound: % element = (number of atoms)(atomic weight) (FW of the compound) x 100%

Percent Composition For the molecule ethane, C 2 H 6, with MW = 30.0 amu %C = (2)(12.0 amu) (30.0 amu) 24.0 amu 30.0 amu = x 100% = 80.0% C in ethane

The mole (mol) - amount of a substance that contains as many particles as there are atoms in exactly 12.00 grams of 12 C 1 mol = 6.02 x 10 23 Avogadro’s number (N A ) Micro World (atoms & molecules) Macro World (grams)

Avogadro’s Number 1 carbon atom has a mass of 12.0 amu 1 mole of 12 C has a mass of 12.0 g Micro World (atoms & molecules) Macro World (grams)

Using Moles Moles provide a bridge from the molecular scale to the real-world scale.

How many moles of calcium (Ca) are in 5.0750 g of calcium? (Z = 20) From periodic table: 1 mol Ca = 40.078 g Ca

Molar mass - the mass of 1 mole of in grams atoms molecules ions 1 mole 12 C atoms = 6.022 x 10 23 atoms = 12.00 g 1 mole S atoms = 32.066 g S 1 mole lithium atoms = 6.941 g of Li For any element: atomic mass (amu) = molar mass (grams) formula units

(Average) atomic mass (6.941)

Do You Understand Molar Mass? How many moles are in 0.551 g of potassium (K) ? 1 mol K = 39.0983 g K 0.551 g K 1 mol K 39.0983 g K x = 1.41 x 10 -2 mol K

Molecular mass (or molecular weight) - the sum of the atomic masses (in amu) in a molecule. 1S32.07 amu 2O + 2 (16.00 amu) SO 2 64.07 amu For any molecule: molecular mass (amu) = molar mass (grams) 1 molecule SO 2 = 64.07 amu 1 mole SO 2 = 64.07 g SO 2 SO 2

Do You Understand Molar Mass? How many moles are in 72.5 g of C 3 H 8 O ? 1 mol C 3 H 8 O = (3 mol C) (12.0 g/mol) + (8 mol H) (1.0 g/mol) + (1 mol O) (16.0 g/mol) 1 mol C 3 H 8 O = 60.0 g C 3 H 8 O (72.5 g C 3 H 8 O) 1 mol C 3 H 8 O 60.0 g C 3 H 8 O = 1.21 mol C 3 H 8 O

Do You Understand Molar Mass? What is the mass of 0.433 mol of Ca(NO 3 ) 2 ? 1 mol Ca(NO 3 ) 2 = (1 mol Ca) (40.078 g/mol) + (2 mol N) (14.01 g/mol) + (6 mol O) (16.00 g/mol) =164.1 g Ca(NO 3 ) 2 0.433 mol Ca(NO 3 ) 2 · 164.1 g Ca(NO 3 ) 2 1 mol Ca(NO 3 ) 2 = 71.1 g Ca(NO 3 ) 2