The Mole Calculating -Molecular Weight -Formula Weight -Molar Mass

Formula Weight (FW) Formula weight - sum of the atomic weights for the atoms in a chemical formula – generally reported for ionic compounds The FW of an ionic compound is the mass of one formula unit (the empirical formula of an ionic compound) – Ex: FW of one formula unit of calcium chloride, CaCl 2 Ca: 1( amu) + Cl: 2( amu) amu 2

Molecular Weight (MW) A molecular weight is the sum of the atomic weights of the atoms in a molecule – another name for formula weight, but it is usually only called MW for molecules For the molecule ethane, C 2 H 6, the molecular weight would be C: 2( amu) amu + H: 6( amu) 3

Example Determine the formula weight (FW) or molecular weight (MW) for the following: a) C 5 H 6 O b) Al 2 (CO 3 ) 3 4

Mole Even tiny samples in the lab contain MANY molecules, atoms, or ions – Ex: 1 teaspoon of water = 2 x water molecules! So chemists devised a counting unit to relate numbers of molecules, atoms, etc to lab-sized amounts…. THE MOLE – Examples of other counting units (dozen = 12 objects, gross = 144 objects) mole (abbr. mol) – the amount of matter that contains as many objects as there are atoms in 12 g of 12 C – number of atoms = 6.02 x = “Avogadro’s number” = N A – objects = atom, molecules, ions, or whatever object you’re counting… people, apples, etc 5

Avogadro’s Number 6 One mole of particles contains Avogadro’s number of those particles “particles” are whatever you’re looking for (or are given) the number of. -usually atoms, ions, or molecules 6.02 x particles = 1 mol particles

Avogadro’s Number: A Conversion Factor! – Avogadro’s number allows us to interconvert: Examples: – 1 mol Ag atoms = 6.02 x Ag atoms or – 1 mol H 2 O molecules = 6.02 x H 2 O molecules or – 1 mol NO 3 - ions = 6.02 x NO 3 - ions or 6.02 x Ag atoms 1 mole Ag atoms 6.02 x H 2 O molecules 1 mole H 2 O molecules 6.02 x NO 3 - ions 1 mole NO 3 - ions x particles 1 mole of those same particles 1 mole Ag atoms 6.02 x Ag atoms 1 mole NO 3 - ions 6.02 x NO 3 - ions 1 mole H 2 O molecules 6.02 x H 2 O molecules (the two conversion factors) 6.02 x particles = 1 mol particles 1 mole of those same particles 6.02 x particles number of particlesmoles of those same particles

Avogadro’s Number: Examples 8 1.How many Cu atoms are in 0.50 mole of Cu? 2.How many moles of CO 2 are in 2.50 x molecules CO 2 ? 3.0 x Cu atoms 4.15 moles of CO 2

The subscripts in a formula show 2 things: 1.the number of each type of atom in one molecule (molecular level) 2.the moles of each element in 1 mole of compound (molar level) Glucose C 6 H 12 O 6 In 1 molecule of C 6 H 12 O 6 :6 atoms C12 atoms H6 atoms O In 1 mole of C 6 H 12 O 6 :6 moles C12 moles H6 moles O Subscripts: Molecular Level and Molar Level

The subscripts allow us to interconvert: In 1 mole of C 6 H 12 O 6 :6 moles C12 moles H6 moles O We can write 3 conversion factors from the subscripts of C 6 H 12 O 6 : Or we can flip them upside-down: Subscripts: Another Conversion Factor! 6 mol C 1 mol C 6 H 12 O 6 6 mol O 1 mol C 6 H 12 O 6 12 mol H 1 mol C 6 H 12 O 6 6 mol C 1 mol C 6 H 12 O 6 6 mol O 1 mol C 6 H 12 O 6 12 mol H moles of atoms or ions moles of the compound the atoms or ions are contained in

Subscripts: Example 11 How many moles of O are in mole of aspirin, C 9 H 8 O 4 ? mole of O

Example: Avogadro’s Number + Subscripts Calculate the number of H atoms in 2.00 mol H 2 O. Our given is in units of moles We want the number of particles (H atoms in this case) The only way to relate number of particles to moles of those same particles is by using Avogadro’s number. -Remember: 6.02 x particles = 1 mole of those same particles -So for this problem, we have: 6.02 x H atoms = 1 mol H atoms We can also relate the number of moles of H 2 O to number of moles of H atoms (by using the subscripts) -So, 1 mol H 2 O = 2 mol H atoms 2.00 mol H 2 O x 2 mol H atoms 1 mol H 2 O 6.02 x H atoms 1 mol H atoms x = 2.41 x H atoms 12

Examples: Avogadro’s Number + Subscripts 1.Calculate the number of O atoms in 2.10 mol Al 2 (CO 3 ) 3. 2.Calculate the number of PO 4 3- ions in 3.0 mol Mg 3 (PO 4 ) 2. 3.How many moles of Mg 3 (PO 4 ) 2 are in x formula units of Mg 3 (PO 4 ) 2 ? x O atoms 3.6 x PO 4 3- ions x formula units

Molar Mass A mole is always the same number (6.02 x ), like a dozen is always 12, but 1-mole samples of different substances have different masses. The molar mass is the mass of 1 mole of an element or compound (unit g/mol) – Equals the atomic weight (for an element) that we find on the periodic table – molar mass of Li is g/mol – Or we can write, g Li = 1 mole of Li the formula weight (for a compound) – molar mass of H 2 O is g/mol – Or we can write, g H 2 O = 1 mole of H 2 O – The formula weight (in amu) is same number as the molar mass (in g/mol), but different units 14

Molar Mass: Another Conversion Factor! Molar mass allows us to interconvert: g Li = 1 mole of Li g H 2 O = 1 mole of H 2 O g Li 1 mol Li g Li g H 2 O 1 mol H 2 O g H 2 O mass of a substancemoles of that same substance

Molar Mass: Example 1.How many moles of H 2 SO 4 are in g of H 2 SO 4 ? 2.What is the mass in grams of 2.13 x moles of CO 2 ? x mol H 2 SO g CO 2

Using Moles Moles provide a bridge from the molecular scale to the real-world scale 17

Examples: Avogadro’s Number + Subscripts + Molar Mass 1.How many H atoms are in 3.01 x mg of H 2 O? 2.What is the mass in grams of Na + ions in a sample of Na 2 O containing 1.02 x10 20 Na 2 O formula units? x H atoms 7.79 x g Na +