Chapter 6 The mole
Measuring Matter What do you ask for when you buy: 2 shoes 12 eggs 48 doughnuts 500 sheets of paper 1 pair 1 dozen 4 dozen 1 ream
But the number they represent is always constant! Pair, Dozen, Ream These are all ways to batch a group of objects to make them easier to count! The object may change Eggs to Doughnuts But the number they represent is always constant!
What do eggs have to do with Chemistry? How many carbon atoms are in a teaspoon of carbon? 200,666,666,666,666,666,666,667 atoms!!! Can you even pronounce this?
200,666,666,666,666,666,666,667 atoms!!! It would be nice if chemists had a batch like a dozen (but muchhhhhhh bigger) to handle this kind of number!
Meet the Mole! He is the Chemist’s “dozen”!
What is a mole? The chemist’s “dozen” is called the: MOLE (or the unit mol) 1 dozen atoms = 12 atoms 1 mole atoms = 602213670000000000000000 atoms Or 1 mole = 6.02 1023 How many moles are in 1 teaspoon of carbon atoms? .33 moles
Measuring Matter – Moles and Avogadro’s number Mole (mol) – the amount of a substance that contains the same number of particles as the number of atoms in 12 g of carbon-12. Like doughnuts are counted in dozens, the mole is a SI unit for counting the amount of a substance. 1 dozen pencils have the same number of units as 1 dozen doughnuts 1 mole of carbon atoms have the same number of particles as 1 mole of water molecules This does not mean they both weigh the same, only that they have the same number of units or particles.
Measuring Matter – Moles and Avogadro’s number Avogadro’s Number – the number of particles (6.02 1023) in exactly one mole of a pure substance. 1 mole pencils = 6.02 1023 pencils 1 mole water molecules = 6.02 1023 molecules of water
Measuring Matter – Moles and Avogadro’s number If Avogadro’s number is the number of particles in 1 mole, how do you know what kind of particle you have? Remember… Particles can be an atom, molecule, or formula unit?
Particles can be an atom, molecule, or formula unit? What kind of particle? Particles can be an atom, molecule, or formula unit? Atom – one atom Molecule – more than one nonmetal atom Formula unit - compound with a metal or NH4+
Measuring Matter – Moles and Avogadro’s number How do you know what kind of particle you have? Examples KCl N2O2 H2 Na Formula unit molecule molecule atom
Measuring Matter – Moles and Avogadro’s number Avogadro’s number is really an equality! 1 mole C = 6.021023 atoms C What can you use equalities to do? Equalities are conversion factors in Dimensional Analysis problems!
Measuring Matter – Moles and Avogadro’s number How many representative particles are in 3.2 mol of C? 6.021023 atoms C = atoms C 1.91024 1 mol C
Measuring Matter – Moles and Avogadro’s number How many moles of water molecules are in 3.7 1021 molecules of H2O? 1 mol H2O 3.7 1021 molecules H2O = mol H2O .0061 6.021023 Molecules H2O
How do you find this mass? Mass and the Mole Molar mass – the mass, in grams, of one mole of a substance. How do you find this mass? The atomic mass printed on the periodic table has two meanings – it is the average mass of an atom in atomic mass units (u) and the mass of one mole of atoms of a given element in grams.
Mass and the Mole Finding molar mass 1mole H atoms = 1mole C = Atoms – the mass of 1 mole of any atom is the same as the atomic mass in grams. 1mole H atoms = 1.00794 g 1mole C = 12.0107 g 1mol Cu = 63.546 g
Mass and the Mole Molar mass continued 1mole H2O = 18.0153g 2 H = Compounds – the mass of 1 mole of a compound is the sum of the masses of the atoms. 1mole H2O = 18.0153g 2 H = 2(1.00794g) = 2.01588g 1 O = 15.9994g 2.01588g + 15.9994g = 18.0153g
Practice!! Find the molar mass of the following! CCl4 CaO (NH4)2SO4
Mass and the Mole Molar mass is really an equality! For water: 1mole H2O = 18.0153 g What can you use equalities to do? Equalities are conversion factors in Dimensional Analysis problems!
Mass and the Mole How many moles are in 242 g of water? = mol H2O 242 g H2O 1 mol H2O = mol H2O 13.4 18.0153 g H2O
Mass and the Mole What is the mass of 3.77 mol of Au? = g Au 3.77 mol Au 196.96655 g Au = g Au 743 1 mol Au
Multi Step Problems What is the mass of 5.28 x 1028 molecules of water? 18.0153 g = 1.58x106 g 6.02x1023 1 molecules mole
The Last Conversion Factor! What do you do if you are asked to find atoms and your representative particle is NOT an atom?
The Formula Conversion Factor Think about what a formula really tells you! 1 molecule of H2O Doesn’t this mean: 1 molecule H2O = 2 hydrogen atoms And …. In 1 mole of H2O there are 2 moles of Hydrogen The formula is a conversion factor between atoms and compounds!
The Formula Conversion Factor How many hydrogen atoms are in 234 molecules of water? 234 molecules H2O 2 atoms H = atoms H 468 1 molecules H2O
How many Hydrogen atoms are in 54.2g of H2O? 1 2 mol of H = mol 6.02x1023 Atoms 3.62x1024 atoms 1 1 mol of H 18.0153 mol of H2O g
Conversion Review Mass Particles Moles Avogadro’s # Molar mass Formula Atoms
Percent Composition How would you calculate the percent females in this room? All percents are calculated in the same way!
Percent Composition Percent Composition – the percent by mass of each element in a compound. molar mass
Percent Composition What is the percent composition of each element in water? Mass H = Mass O = 2(1.0079 g) = 2.01588 g + 15.9994 g 18.0153 g % H = 11.1898% % O = 88.8101%
Using Percent Composition If a glass of water contains 648 g of water, how many grams of hydrogen would it hold? Remember, water is 11.1898% hydrogen What is 11.1898% of 648 g? 72.5099 g 72.5 g
C3H8O Isopropyl Alcohol
Find the % composition of the elements in rubbing alcohol Molar Mass = C? H? O? 60.0950g 59.9586% 13.4180% 26.6235%
Empirical Formula What does H2O mean? Does it mean 2 atoms of H for every atom of O? YES Does it mean 2 g of H for every 1 g of O? NEVER
Empirical Formula What does H2O mean? Does it mean 2 moles of H for every mole of O? Always! Formulas are not only ratios of atoms, they are also ratios of MOLES
Formulas are ratios of moles!
Circle the empirical formulas! Empirical Formula: simplest whole number ratio of moles of the atoms in a substance. Experimental method that is the first step in finding the formula of a compound. Circle the empirical formulas! H2O NaCl C2H6 NO2 C6H12O6 H2SO4 N2O4
Finding the empirical formula Find the mass of each element in the compound. Usually given If given as %, then change % to g. 25% H and 75% C 25 g H and 75 g C Convert masses to moles. Use molar masses. 25 g H 1 mol H = mol H 25 1.00794 g H
Empirical Formula Find the smallest whole number ratio of moles. a. Write the results of step 2 like a formula. If C = 6.2 mol and H = 25 mol C6.2H25 b. Divide by the smallest mole amount. c. If not all whole numbers, multiply by 2,3, or 4 … CH4 6.2
Empirical Formula Examples for step 3 X3Y4 X3Y2 X.029Y.039 X.009Y.006 X = .029 mol X = .009 mol Y = .039 mol Y = .006 mol X.029Y.039 X.009Y.006 .029 .029 .006 .006 X1Y1.34 X1.5Y1 Multiply by 3 Multiply by 2 X3Y4 X3Y2
Empirical Formula More Examples for step 3 X1Y3 X2Y5 X1.47Y3.68 X1Y2.5 X = 1.47 mol X = 2.4 10-4 mol Y = 3.68 mol Y = 7.3 10-4 mol X1.47Y3.68 1.47 1.47 2.410-4 2.410-4 X1Y2.5 X1Y3.04 Multiply by 2 X1Y3 X2Y5
Empirical Formula - Example A sample of an unknown gas contains 43.2 g of carbon and 115.8 g of oxygen. What is the empirical formula? Find Masses 43.2 g C 115.8 g O Change to moles 43.2 g C 1 mol C = mol C 3.60 12.0 g C 115.8 g O 1 mol O = mol O 7.24 16.0 g O
Empirical Formula - Example Get whole numbers C3.60O7.24 3.60 3.60 C1O2.01 CO2
Empirical Formula Find the empirical Formula of 52.8% Sn, 12.4% Fe, 16.0%C, & 18.8% N.
Molecular Formula Molecular formula - is some whole number multiple of the empirical formula. HO is an empirical formula H2O2 is twice HO (HO)X and X = 2 For C6H12O6, the empirical formula is CH2O and X=6 To convert an empirical formula to a molecular formula you must find X.
Molecular Formula - Example An unknown gas is found to have an empirical formula of NO2 and a molar mass of 92.0 g/mol. What is the molecular formula? Molecular formula = (NO2)X = (NO2)2 = N2O4