Presentation on theme: "Molar mass, percent composition, empirical and molecular formulas"— Presentation transcript:
1 Molar mass, percent composition, empirical and molecular formulas The MoleMolar mass, percent composition, empirical and molecular formulas
2 How many atoms are in here? What about weight? .
3 Atomic Mass A single atom has a very small mass. on the order of grams per atom1 atom has a mass of aboutgBecause this mass is so small, we use a unit called amu to describe the mass of a single atom.1 atom of carbon has a mass of amu1 atom of hydrogen has a mass of amu1 atom of gold has a mass of amu
5 Formula MassFormula Mass – the mass, in amu, of one formula unit of a compound.To calculate the formula mass of a compound;Count and record the number of each atom in the formula.Multiply the number of atoms by the mass of that atom from the periodic table.Add these products together.
6 Definitions: Atom: The smallest particle of an element (ex: He). Molecule: The smallest particle of a compound (ex: H2O)Atomic Weight: The decimal number on the periodic chart (Units: amu)Formula Mass: add up the total atomic weights of the elements in a compound (Units: amu).
9 Formula Mass Calculate the formula mass of Ca Ca = 1(Ca) 40.08 amuUnits = amuClCaCl
10 Formula Mass Calculate the formula mass of CaCl2 CaCl2 = 1(Ca) + 2(Cl) + 2(35.45)=110.98Units = amuCa
11 The MoleThe mass of a single atom is too small a quantity for use in lab.We need to come up with a way to take atomic masses from the periodic table and turn them into a mass that is useable in lab.In 1811, Amedeo Avogadro determined the number of atoms in one mole of a substance.
12 The MoleThe mole establishes a relationship between the atomic mass unit (on periodic table) and the gram (used in lab).Mole (mol) - The amount of a pure substance that contains 6.02 x 1023 particles of that substance.The mole is used to describe a huge amount of any extremely small particle. A mole of gold, a mole of salt and a mole of water each contain 6.02 x 1023 individual units.
13 The Mole One mole of oxygen, one mole of salt and one mole of water. Each sample contains one mole, 6.02 x 1023, particles.
14 Do you think that one mole of gold atoms is going to weigh the same as one mole of aluminum atoms? Why?
15 It doesn’t make much sense to weigh atoms in amu, because the units are so small…… If you express atomic mass (amu) in GRAMS instead of amu (like we did yesterday), than you have one MOLE of atoms.
16 Molar MassA molar mass is calculated the same way that you calculated formula mass (HUP, TWO, THREE, FOUR).The only difference is the unit that will be placed on the final answer because you are calculating the mass of one mole of a substance.If the formula mass of CaCl2 is amu, then the molar mass of CaCl2 is grams.
19 The MoleThere are 6.02 x 1023 carbon atoms in 12 grams, one mole, of carbon.There are 6.02 x gold atoms in 197 grams, one mole, of gold.There are 6.02 x 1023 formula units of calcium chloride in grams, one mole, of CaCl2.
20 So, how do we use the mole in Chemistry? We use the mole to convert between the mass, volume and number of particles in a substance.We can make conversions from:Moles to particles and particles to moles.Moles to grams and grams to moles.Moles to volume of a gas and volume of a gas to moles.Any combination of the above categories!
22 Converting from Moles to Particles. How many particles of Sodium Chloride are there in 2.50 moles of Sodium Chloride (NaCl)?2.50 mol NaCl x x 1023part. = 1.51 x 10241 mole part. NaCl
23 Converting from Moles to Particles How many particles of gold are there in 4.25 moles of Au?4.25 mol Au x x 1023 part.= 2.56x1024 part mole Au
24 Converting from Particles to Moles. How many moles of gold are there in 1.5 x 1024 atoms of gold?1.5 x 1024 atoms Au x 1 mole = moles Au6.02 x 1023 atoms
25 Converting from Particles to Moles How many moles of calcium chloride are there in 3.6 x 1024 particles of CaCl2?3.6 x 1024 units x 1 mole CaCl2 = 6.0 mol CaCl26.02 x 1023 part.
26 Converting from Mass to Moles How many moles of gold are there in 25.0 grams of gold?25.0 g Au x 1 mole Au = mol Aug Au
27 Converting from Mass to Moles How many moles of calcium chloride are there in 75.5 grams of CaCl2?75.5 g CaCl2 x 1 mole CaCl2 = mol CaCl2g CaCl2
28 Converting from Moles to Mass How many grams of Carbon are there in 4.2 moles of Carbon?4.2 mole C x g C = 50. g C1 mole C
29 Converting from Moles to Mass How many grams of water are there in 2.75 moles of water?The molar mass of water is:= g/mol2.75 moles H2O x g H2O = g H2O1 mole H2O
30 Volume of GasesOne mole of gas (6.02 x 1023 particles of gas) occupies a volume of 22.4 liters at standard temperature and pressure.This is true if the gas is monatomic like Helium (He), diatomic like Oxygen (O2) or a compound like Carbon dioxide (CO2).
31 Converting from Volume to Moles of a gas How many moles of oxygen are there in 10.0L of oxygen?10.0L O2 x 1 mole O2 = mole O222.4 L O2
32 Converting from Volume to Moles of a gas How many moles of Carbon dioxide (CO2) are there in 2.50 L of CO2 gas?2.50 L CO2 x 1 mole CO2 = mole CO222.4 L CO2
33 Converting from Moles to Volume of a gas How many liters of Helium are there in 3.6 moles of Helium?3.6 moles He x L He = 81 L He1 mole He
34 Converting from Moles to Volume of a gas What volume will 8.24 moles of CO2 occupy?8.24 mol CO2 x L CO2 = 185 L CO21 mole CO2
35 Multi-Step Conversions If you need to convert from:Mass to particles or particles to mass,Mass to volume or volume to mass,Volume to particles or particles to volume,then you need to perform a 2-step conversion.Notice the mole is not present in the starting or ending quantities. You will ALWAYS use the mole when converting between these units!
36 Multi-Step Conversions How many formula units of calcium chloride are there in 10.0 grams of CaCl2?10.0 g CaCl2 x 1 mole CaCl2 x x 1023 units =g CaCl mole CaCl25.42 x 1022 formula units of CaCl2
37 Multi-Step Conversions What volume would 2.25 grams of hydrogen gas (H2) occupy at standard temperature and pressure?2.25 g H2 x 1 mole H2 x L = L H g H mole
38 Multi-Step Conversions How many atoms of Argon gas are there in a typical light bulb (0.20 liters) at standard temperature and pressure?0.20 L Ar x 1 mole x 6.02 x 1023 atoms = 5.4 x 102122.4 L mole atoms Ar
39 Percent CompositionThe Percent Composition – the percent by mass of each element in a compound.To calculate %Comp:mass of element x 100 = % by massmass of compound
40 Percent CompositionDetermine the percent by mass of each element in calcium chloride (CaCl2).Ca 1 x = x 100 = 36.11% Ca110.98Cl 2 x = x 100 = 63.89% Cl=110.98(molar mass)
41 Percent CompositionCalculate the percent by mass of each element in Calcium sulfate, CaSO4.Ca 1 x = x 100 = % Ca136.15S 1 x = x 100 = % SO 4 x = x 100 = % O=molar mass
42 Percent CompositionYou can determine the %Comp of a substance experimentally.Measure the mass of a sampleDecompose the sample (usually by heating) to separate the component substances.Measure the mass of the substance that remains.Calculate %Comp as before
43 Empirical FormulaEmpirical Formula – the smallest whole number mole ratio of elements in a compound.The empirical formula for a class of molecules is often the same for each sample.Examples: C5H10O5 = CH2OC6H12O6 = CH2OC11H22O11 = CH2O
44 Calculating Empirical Formula Assume you are working with a 100.0g sample so the mass of each element will be the same as the percent of that element (for simplicity).Convert each mass in grams to moles using the molar mass of the element.Find the whole number ratio of these calculated amounts by dividing each mole value by the smallest number of moles calculated.Round to whole numbers and use the ratio to determine the empirical formula.
45 Calculating Empirical Formula A compound was found to contain 29.6% Calcium, 23.7% Sulfur and 46.8% Oxygen. What is the empirical formula for the compound?If you had a 100 gram sample, you would have 29.6g Ca, 23.7g S and 46.8g O.
46 Calculating Empirical Formula Ca 29.6g x 1 mole = mol Ca40.08gS g x 1 mole = mole S32.07gO x 1 mole = mole O16.00g0.739 = = = 3.96Round to the nearest whole numbers, 1 : 1 : 4Therefore the empirical formula is CaSO4
48 Molecular FormulasMolecular Formula – the actual number of atoms of each element in one molecule of a compound.The ratio of molecular mass to empirical mass is the same as the ratio of molecular formula to empirical formula.
49 Calculating Molecular Formula Example:Molecular formula Molecular MassC6H12OEmpirical formula Empirical MassCH2ORatio C6H12O6 = 6 Ratio = 6CH2O
50 Calculating Molecular Formula Ribose has a molar mass of 150g/mol and a chemical composition of 40.0%C, 6.67% hydrogen and 53.3% oxygen. What is the molecular formula for ribose?Begin by calculating the empirical formula for Ribose.
51 Calculating the Molecular Formula for Ribose 40.0g x 1 mole = mole C12.01g6.67g x 1 mole = mole H1.01g53.3g x 1 mole = mole O16.00g3.33 = = = 1Empirical Formula for RiboseCH2O
52 Calculating Molecular Formula Once you know the empirical formula, you can calculate the molecular formula.Covalent bonds can form in many different ratios.Use the empirical formula and the ratio of molar mass to empirical formula mass to determine the molecular formula.
53 Calculating the Molecular Formula for Ribose The molecular mass of Ribose is 150 g/molThe empirical mass of Ribose is 30 g/mol(CH2O = (2 x 1.01) = g/mol)150 = 5 Therefore, the Molecular30 formula is 5 times theEmpirical FormulaC5H10O5
54 The Formula of a Hydrate Hydrate – a compound that has a specific number of water molecules bound to its atoms.Examples:CaCl2 . 2H2OFePO4 . 4H2OMgSO4 . 7H2O
55 Determining the Formula for a Hydrate To determine the formula of a hydrate, the sample must be heated to remove all of the water.The mass of the anhydrous sample is subtracted from the mass of the hydrated sample to determine the mass of water removed.The mass of the anhydrous sample and the mass of the water are used to determine the empirical formula and molecular formula as before.
56 Determining the Molecular Formula of a Hydrate Cobalt (II) chloride is a hydrated salt.If 11.75g of this hydrate is heated, 9.25g of anhydrous cobalt chloride remains. What is the formula for this hydrate?11.75g hydrated salt9.25g anhydrous salt2.50g water
57 Determining the Molecular Formula of a Hydrate 9.25g CoCl2 x 1 mole = mol CoCl2129.83g2.50g H2O x 1 mole = mol H2O18.02g= = 1.951:2 ratio Therefore the formula for the hydrate isCoCl2 . 2H2O