Comparison of masses9.5 MOLAR VOLUME Mass to L conversionsCHAPTER 9: THE MOLE CONCEPT Problems: 1-20, 23-26, 29-46, 77-78 9.1 AVOGADRO'S NUMBER The mole is a collective quantity Examples of collective quantities: 1 dozen = 12, 1 pair = 2 1 mole = 6.02 x 1023 particles particles = atoms, ions, molecules, etc. How big is this?  The volume of 6.02 ×1023 softballs is about equal to the volume of the Earth. ¾ This extremely huge # is known as Avogadro's number. ⇒ The mole has also been defined as the # of C atoms in exactly 12 grams of Carbon-12. 9.2 MOLE CALCULATIONS I  Avogadro's number is a useful conversion factor. E.g. moleatomsx11002.623 or molemoleculesx11002.623 Example. How many atoms are in 10.0 moles of Pb? Example. How many moles of C atoms are present in a sample of 1.25×1024 C atoms? 9.3 MOLAR MASS ¾ Recall atomic mass is average mass for one atom of an element. (units = amu) ¾ For problems involving masses, find the mass of the element on the periodic table. ⇒ E.g. atomic mass of Cl = 35.45 amu Molar Mass - mass in grams of 1 mole of substance. (units = g/mol) ⇒ Numerically same as atomic mass, but units are different. Comparison of masses 1 Carbon atom weighs 12.01 amu 1 H2O molecule weighs 18.02 amu 1 mole of Carbon atoms weighs 12.01 g 1 mole of H2O molecules weighs 18.02 g molar mass of C = 12.01 g/mol; molar mass of H2O = 18.02 g/mol CHM 130: Chapter 9 page 1 of 4CHM 130: Chapter 9 page 2 of 4 Example. Calculate the molar mass for the following elements or compounds: A. O2: 2 (molar mass of O) = 2 (16.00 g/mol) = 32.00 g/mol B. NaCl: C. CO2: D. K2SO4: E. (NH4)3PO4: 9 .4 MOLE CALCULATIONS II • Molar Mass is also a useful conversion factor between grams to moles ⇒ Use the unit analysis method & write molar mass as a fraction - make sure that the units cancel and give you the units that you are solving for. Example. What is the mass in grams of 0.0235 moles of Na? Example. What is the mass in grams of 3.75 moles of FeCl3? Example. How many moles are in 12.6 grams of H2O? 9.5 MOLAR VOLUME Avogadro's Law: An equal number of gas molecules at the same temperature and pressure will occupy the same volume. Molar volume is the volume occupied by 1 mole of a gas at a given T & P. STP conditions: P = 1 atm, T = O °C At STP, 1 mole of any gas occupies 22.4 L. Question: Write 2 unit factors for the molar volume at STP:Gas Density ¾ Gas densities are about 1000 times lower than densities for solids and liquids ¾ The densities for gases are generally reported in grams per liter (g/L). ⇒ Can use molar volume to find density at STP: Density of gases: d = volumemass= VolumeMolarMassMolar Find the density of N2 gas at STP. Find the density of CO2 gas at STP. 9.6 Mole Calculations III Mass to L conversions ¾ Can use molar volume & molar mass to convert between g – L Problem. Find the volume in Liters for 0.500 g O2 gas at STP. Problem. Find the mass in grams for 1.5 L of NO2 gas at STP 9.7 PERCENTAGE COMPOSITION PERCENT COMPOSITION - percent by mass of each element present in a compound % element = ⎟⎟⎠⎞⎜⎜⎝⎛×CompoundofMassMolarelementofMassMolaratoms#x 100% CHM 130: Chapter 9 page 3 of 4CHM 130: Chapter 9 page 4 of 4 E.g. Calculate the % composition of Al2(SO4)3 MM Al2(SO4)3 = % Al = % S = % 0 = E .g. Calculate the % H, % C and % O in acetic acid. MM HC2H3O2 = % H = % C = % O