Molar Mass and Mole Conversions

AP Chemistry Name Molar Mass and Mole Conversions Period UNIT Date 2 – STOICHIOMETRY Remember that a mole represents the quantity 6.022 x 1023, also called Avogadro’s number. We count molecules in groups of moles because molecules are so tiny. Molar Mass is the mass (in grams) of one mole (6.022 x 1023 particles) of any element or substance. The molar mass is equal to the mass from the periodic table. Calculating Molar Mass: To calculate molar mass, simply add the mass of each atom in the compound. Remember that the units are grams per mole. Examples: CaF2 = 1 Ca + 2 F = 40.078 + 2(18.998) = 78.074 g/mol (NH4)2S = 2(N) + 8(H) + S = 2(14.007) + 8(1.008) + 32.066 = 68.144 g/mol FeSO4 ● 6 H2O = FeSO4 + 6(H2O) = 151.907 + 6(18.015) = 259.997 g/mol Practice: Answers are on the back of this sheet! 1) 2) 3) 4) (NH4)2CO3 _________________ 5) CaCO3 • 2 H2O __________________ 6) CuCl2 • 6 H2O ________________ 7) ammonium sulfide ______________ *Hint: write formula first! NaBr ___________________ Answers: Molar Mass: 1) 102.894 g/mol 2) 303.262 g/mol PbSO4 ___________________ 3) 74.092 g/mol 4) 96.086 g/mol 5) 136.116 g/mol 6) 242.542 g/mol Ca(OH)2 ___________________7) 68.144 g/mol Converting between Moles and Mass: We can use molar mass to convert between mass of a substance and the number of moles of that substance. There are two ways. 1. Use the molar mass in grams per mole as a conversion factor, then use dimensional analysis to convert between grams and moles. Example: If I have 44.0 grams of Helium (Molar Mass 4.00 g/mol), how many moles do I have? 44.0 g × 2. Use the formula n= 1 mole =11.0 moles Helium 4.00 g m where n = number of moles, m = mass, and M = molar mass. This equation is provided M on the equation sheet for AP chemistry. Example: If I have 44.0 grams of Helium (Molar Mass 4.00 g/mol), how many moles do I have? n= 44.0 g =11.0 moles Helium g 4.00 mol Practice: Convert each of the following quantities. 1. How many moles are in 36.8 g of water? 36.8 g n= =2.04 mol g 18.02 mol 2. How many moles are in 5.91 g of H2O2? n= 5.91 g =0.174 mol g 34.02 mol 3. How many moles are in 385.6 g of PbSO4? n= 385.6 g =1.272 mol g 303.26 mol 4. What is the mass of 0.56 moles of CH4? m=0.56 mol ×16.042 g =9.0 g mol 5. What is the mass of 2.45 moles of CO2? m=2.45 mol × 44.01 g =108 g mol 6. What is the mass of 13 moles LiCl? m=13 mol × 42.39 g =550 g mol 7. If 6.5 moles of a compound has a mass of 89.5 grams, what is it’s molar mass? Molar mass= 8.95 grams =1.4 g/mol 6.5 moles 8. A sample of a specific element weighs 145.2 grams and contains 4.53 moles. What element is it? Molar mass= 145.2 grams =32.0 g/mol 4.53 moles The element is sulfur 9. A sample of a diatomic gas weighs 160. Grams and contains 5.0 moles. What is the diatomic gas? Molar mass= 160. grams =32 g/mol 5.0 moles The gas is oxygen, O2. Calculating Molar Quantities Without a Calculator: Try these calculations without a calculator. 1. What is the mass of 0.10 mole of O2 (MM = 32. g/mol)? m=0.10 mol × 32. g =3.2 g mol 2. How many moles are in 4.0 grams of Argon (MM = 40. g/mol)? n= 4.0 g =0.10 mol g 40. mol 3. What is the mass of 0.005 moles of uranium (MM = 238. g/mol)? m=0.005 mol × 238 g =1.19 g mol 4. How many moles are in 0.46 grams of sodium (MM = 23. g/mol)? n= 0.46 g =0.02 mol g 23. mol 5. What is the density of a substance with a mass of 3.40 g and a volume of 0.5 L? D= 3.40 g =6.80 g/ L 0.5 L 6. What is the mass of a substance that has a density of 2.5 g/L and a volume of 0.3 L? mass=D ×V =2.5 g ×0.3 L=0.75 g L