UNIT V The Mole Concept

V.1 AVOGADRO’S HYPOTHESIS Avogadro’s hypothesis : Equal volumes of different gases contain the same number of particles (at the same temperature and pressure). If 1L of gas A reacts with 1L of gas B, then the formula for the compound is AB If 2 L of gas A reacts with 1L of gas B, then the formula for the compound is A 2 B If 2 L of gas A reacts with 3L of gas B, then the formula for the compound is A 2 B 3 Questions: p. 78 #2-5

V.2 THE MOLE 1. The Mole the number of carbon atoms in exactly 12 g of carbon. = 6.02 x carbon atoms in 12 g of C (Avogadro’s number) Ex: 1 mol of Mg atoms = ______________ atoms and weighs _________ g the number 6.02 x is an accepted standard for the MOLE

V.2 THE MOLE 2. Molar Mass the mass of ONE MOLE of particles. molar mass is also known as atomic mass the periodic table shows the atomic mass of elements these are measured in amu (atomic mass units) the masses shown on the periodic table can be used directly if they are measured in GRAMS in other words, the atomic mass of an element is the same as how many grams of it there would be in one mole (we call this molar mass).

V.2 THE MOLE Ex: ElementAtomic MassMolar Mass Ca N Since the molar mass is the MASS (g) of ONE MOLE (mol) of particles: the units of molar mass are g/mol Ex: 1 mol of Carbon = 12 grams, so molar mass of carbon = 12.0 g/mol

V.2 THE MOLE More examples: Chlorine (Cl) = 35.5 g/mol Oxygen (O 2 ) = Sodium (Na) = Sodium oxide (Na 2 O) = Tin (II) cyanide (Sn(CN) 2 ) = Questions: p. 80 #6,7 (odds)

V.2 THE MOLE 3. Mass Conversions Molar Mass as a Conversion Factor Molar mass allows us to calculate or convert: mass → # of moles # of moles → mass Molar mass is given in grams per mole (g/mol); for example Carbon has a molar mass of 12.0 g/mol Therefore, 1 mol OR 12.0 g are conversion factors 12.0 g 1 mol

V.2 THE MOLE Ex: How many moles of Carbon are there in 22 grams of Carbon? Ex: How many grams of Carbon are there in 3.00 mol of Carbon?

V.2 THE MOLE Ex: What is the mass of 4.85 mol of Nitrogen? Ex: What is the mass of 2.90 mol of H 2 ?

V.2 THE MOLE Ex: How many moles of H 2 SO 4 are there in 4.0 grams of H 2 SO 4 ? Ex: How many grams are there in 3.00 x moles of CaCl 2 ? Questions: p. 82 # (odd)

V.2 THE MOLE 4. Moles and the Volume of Gas Recall Avogadro’s hypothesis: Equal volumes of different gases contain the same number of particles (at the same temperature and pressure). Standard Temperature and Pressure Chemists have a set STANDARD for temperature and pressure: STP = standard temperature and pressure STP = 0°C and kPa

V.2 THE MOLE 5. Molar Volume The molar volume of a gas is the VOLUME occupied by ONE MOLE of the gas. It is a fact that at STP, 1 mol of any gas occupies 22.4 L of volume this is known as ‘ molar volume’ We then get another CONVERSION FACTOR 1 mol OR 22.4L 22.4 L1 mol BUT ONLY AT STP

V.2 THE MOLE Ex: How many L of space will 2.3 mol of He (g) occupy at STP? Ex: How many mol of Ne (g) will fill a 400L space at STP? Questions p 83 #11, 12

V.2 THE MOLE The mole is the fundamental unit in chemistry for measuring the AMOUNT OF SUBSTANCE. We can convert from moles to number of particles. 1 mole = 6.02 x CONVERSION FACTOR: 1 mol OR 6.02 x x mol ** there are no units for Avogadro’s number because it is simply a number. It’s like a dozen (a dozen = 12). Think of it as the chemist’s dozen (but a much bigger number!)

V.2 THE MOLE Ex: How many moles of NaBr are there in 1.7 x molecules of NaBr? Ex: How many molecules of LiCl are there in 0.5 moles of LiCl? Questions: p. 84 # (odd), 19 and 20

V.3 MULTIPLE CONVERSIONS First: we must know how to find the # of atoms in a given # of molecules Ex: How many atoms are there in 15 molecules of CuSO 4 · 5H 2 O? Question: p. 85 # 21(a,c)

V.3 MULTIPLE CONVERSIONS CONVERSIONCONVERSION FACTOR Moles ↔ # of Particles(Avogadro’s number) Moles ↔ Mass(Molar mass) Moles ↔ Volume STP) (Molar Volume) Molecules ↔ Atoms The MOLE is CENTRAL to all conversions between mass, volume and particles.

THE MOLE FLOWER

V.3 MULTIPLE CONVERSIONS Ex: Find the mass, in grams, of 1.25L of NH 3 (g) at STP: Ex: What is the volume occupied by 45.0g of KOH at STP?

V.3 MULTIPLE CONVERSIONS Ex: What is the mass, in grams, of 175 N (Nitrogen) atoms?

V.3 MULTIPLE CONVERSIONS What to do if density is mentioned in the question: d = m/V If the volume of a liquid or solid is unknown....use V = m/d (you can always find the mass from the moles of substance present) Note: you cannot use 22.4L when calculating a volume of liquid or solid (only gases!)

V.3 MULTIPLE CONVERSIONS Ex: What is the volume occupied by 5.00 mol of ethanol, CH 3 CH 2 OH ( l ) ? (the density of ethanol is g/mL)

V.3 MULTIPLE CONVERSIONS If the number of moles is unknown...use the density and volume to calculate mass m = d x V and then convert mass to moles

V.3 MULTIPLE CONVERSIONS Ex: How many moles of Hg (l) are contained in 56 mL of Hg(l)? (d = 13.6 g/mL)

V.3 MULTIPLE CONVERSIONS Ex: CCl 4 (l) has a density of 1.59 g/mL. How many atoms of C are in 200 mL of CCl 4 ?

V.3 MULTIPLE CONVERSIONS Ex: What is the density of CH 2 F 2 (g) at STP?

V.3 MULTIPLE CONVERSIONS If the molar mass of a gas at STP is unknown....find the density of the gas and then combine the density with volume of 1 mol (22.4L) to find the mass of 1 mole

V.3 MULTIPLE CONVERSIONS Ex: A 3.0 L bulb contains 2.2 g of a gas at STP. What is the molar mass of the gas? Questions: p. 88 # 25-34