Ch3-1-Formula Mass and Moles PDF

2/2/2025 3.1 Formula Mass and Molecular Mass Formula mass is the average mass of one formula unit of an ionic compound in amu. Molecular mass (sometimes called molecular weight) is the average mass of one molecule of a molecular compounds. You can calculate the formul...

2/2/2025 3.1 Formula Mass and Molecular Mass Formula mass is the average mass of one formula unit of an ionic compound in amu. Molecular mass (sometimes called molecular weight) is the average mass of one molecule of a molecular compounds. You can calculate the formula mass and molecular mass by adding atomic masses of all atoms in the formula (from the periodic table). molecular mass of CO2 = 12.011 amu + 2 (16.00 amu) = 44.01 amu formula mass of CaCl2 = 40.09 amu + 2 (35.45 amu) = 110.99 amu 1 Avogadro’s number (NA) Avogadro’s number is defined as the number of atoms in exactly 12 g of pure 12C. NA = 6.02214076  1023 Avogadro’s number is a calculated number (precisely to 8 decimal places. i.e. 9 SF) To keep it simple, we will use 6.022 x 1023 in calculations but remember that it has more significant figures than 4. 2 1  2/2/2025 Mole A mole is the amount of material that contain 6.022 1023 number of particles (Particles are atoms, molecules, or ions). Example: 1 mol C = 6.022 x 1023 C atoms 1 mol CO2 = 6.022 x 1023 CO2 molecules 1 mol Na+ = 6.022 x 1023 Na+ ions A mole is large enough to measure in grams. mol is the SI unit for the amount of a substance. (When written as a unit, write mol) 3 Converting between moles and number of particles Avogadro’s number can be used as a Conversion Factor For a mole of C 1 mol C = 6.022  1023 atoms C conversion factors 6.022 x 1023 atoms C or 1 mol C 1 mol C 6.022 x 1023 atoms C You can write similar relationships for any substance using atoms, molecules, or ions. Avogadro’s number converts moles to number of particles (atoms, molecules, ions) of the substance. number of particles of a substance to its moles. 4 2  2/2/2025 Ex: Calculate the number of moles in a sample of cobalt containing 5.00 × 1020 atoms. Given: 5.00 × 1020 atoms of Co Need: ? mol Co NA atoms of Co mol Co 1 mol Co = 6.022  1023 atoms Co 1 mol Co moles of Co = 5.00 × 1020 atoms Co × 6.022 × 1023 atoms Co = 8.30 ×10−4 mol Co 5 Molar mass Molar mass is the mass of one mole of a compound or an element. It has units of g/mol. For an element, it is the atomic mass expressed in g/mol. For a compound, it is the sum of molar masses of all the elements in the formula. Example: molar mass of carbon is 12.011 g/mol. molar mass of O is 16.00 g/mol molar mass of CO2 = 12.011 g/mol + 2 (16.00 g/mol) = 44.01 g/mol 6 3  2/2/2025 Converting between mass and moles You can use molar mass as a conversion factor to convert between mass and moles. Molar mass conversion factor converts grams of an element or a compound into moles. moles of an element or a compound into grams. Molar mass of C = 12.011 g/mol As an equality: 1 mol C = 12.011 g Conversion factors: 12.011 g C and 1 mol C 1 mol C 12.011 g C 7 Ex: Allyl sulfide C6H10S is a compound that has the odor of garlic. How many moles of C6H10S are in 225 g C6H10S? Given: 225 g C6H10S Need: mol C6H10S g C6H10S mol C6H10S molar mass Calculate the molar mass of C6H10S. (6 x 12.01) + (10 x 1.008) + (1 x 32.07) = 114.21 g/mol 1 mol C6H10S = 114.21 g C6H10S Set up the calculation using the molar mass conversion factor. 225 g C6H10S x 1 mol C6H10S = 1.97 mol C6H10S 114.21 g C6H10S molar mass conversion factor 8 4  2/2/2025 Subscripts as conversion factors Glucose C6H12O6 Ratio between atoms in a formula is related to In 1 molecule: 6 atoms C 12 atoms H 6 atoms O their mole ratio In 1 mol: 6 mol C 12 mol H 6 mol O For glucose, C6H12O6, conversion factors can be written as: 6 mol C 12 mol H 6 mol O 1 mol C6H12O6 1 mol C6H12O6 1 mol C6H12O6 1 mol C6H12O6 1 mol C6H12O6 1 mol C6H12O6 6 mol C 12 mol H 6 mol O ***Subscript factor converts between moles of a compound and moles of an element in that compound. 9 Example: How many moles of O are in 0.150 moles of aspirin C9H8O4? given unit = 0.150 mol C9H8O4 needed unit = ? mol O path mol C9H8O4 mol O (subscript factor) equality 1 mol C9H8O4 = 4 mol O 0.150 mol C9H8O4 x 4 mol O = 0.600 mol O 1 mol C9H8O4 10 5  2/2/2025 Relationship between moles, mass and atoms (or molecules) To convert between mass and no. of atoms (or molecules), convert into moles first. mass (g) moles No. of atoms (or molecules) Molar mass Avogadro’s number no. of atoms (or molecules) moles mass (g) Avogadro’s number Molar mass moles of the moles of an no. of atoms of mass of a compound (g) compound the element element Molar mass Subscript factor Avogadro’s number OR mass of a compound (g) moles molecules no. of atoms of Subscript one element Molar mass Avogadro’s number 11 Ex: Bees release about 1 × 10–6 g of isopentyl acetate (C7H14O2) when they sting. How many atoms of carbon are present in a typical bee sting? g C7H14O2 → ? C atoms Avogadro’s g C7H14O2 → → N molecules C7H14O2 mol C7H14O2 number → atoms C Molar mass A Subscript factor 1 mol C7H14O2 = 6.022  1023 molecules C7H14O2 1 mol C7H14O2 = 130.18 g C7H14O2 1 molecule C7H14O2 = 7 atoms C Molar mass = 84.07 g C + 14.11 g H + 32.00 g O = 130.18 g/mol 1 mol C7 H14 O2 6.022 × 1023 molecules 7 carbon atoms atoms C = 1 ×10−6 g C7 H14 O2 × × × 130.18 g C7 H14 O2 1 mol C7 H14 O2 1 molecule = 3 × 1016 carbon atoms 12 6  2/2/2025 From Textbook Read: 3.1 Problems to try: 1, 3, 5, 9, 13, 15, 17, 19, 21, 23, 25, 31 13 7