Unit 4: Moles, Chemical Composition, Stoichiometry Chapter 7 & 9

Key Concepts Moles Avogadro’s Number Molar Mass Molar Volume Percent Composition Empirical Formula Molecular Formula Stoichiometry Limiting Reactant, Excess Reactant Theoretical Yield, Percent Yield

Moles Mole: SI base unit of measurement for the amount of a substance  It provides a bridge between the atom and the macroscopic amounts of material that we work with in the laboratory.  It allows the chemist to weigh out amounts of two substances, say iron and sulfur, such that equal numbers of atoms of iron and sulfur are obtained.  Dozen: 12 of something Mole: 6.02x1023 of something (particles) Moles and particles are units. We can convert between these units by using this conversion factor.

Avogadro’s Number Carbon with 6P & 6N, atomic mass = 12amu Avogadro massed 12g of carbon-12 and counted the atoms. He counted 6.02 x 1023 atoms and called it 1 mol. Avocado kid vine 1 mol = 6.02 x 1023 particles (atoms/molecules)

Avogadro’s Number Avogadro’s Number: one mole of a substance contains 6.02 x 1023 particles (atoms, molecules) 1 mol 6.02 x 1023 particles 6.02 x 1023 particles 1 mol

“It’s an avocado… Thanks.” My student sent me this over summer… “It’s an avocado… Thanks.”

Avogadro’s Number

Moles, Avogadro’s Number How many moles are in 9.0 x 1021 atoms of carbon? How many molecules are in 2.1 moles of carbon dioxide? 9.0 x 1021 atoms C x 1 mol/6.02 x 1023 atoms = 0.0150 mols C 2.1 mol CO2 x 6.02 x 1023 molecules/1 mol = 1.26 x 1024 molecules CO2

Moles, Avogadro’s Number How many moles are in 4.35 x 1024 molecules of chlorine? How many molecules are in 1.6 moles of HCl? 4.35 x 1024 molecules Cl x 1 mol Cl = 7.23 mol Cl 6.02 x 1023 molecules Cl 1.6 mol HCl X 6.02 x 1023 molecules HCl = 9.63 x 1023 HCl molecules 1 mol HCl

Molar Mass Molar Mass: mass (g) of one mole (mol) of a substance (g/mol) Atomic mass (g) = 1 mol (add up atomic masses from the periodic table) Atomic Mass (g) 1 mol 1 mol Atomic Mass (g) Or

Molar Mass What is the mass of 0.0297 moles of NH3? How many moles are in 3.5 grams of H2O? 0.0297 mol NH3 x 17g/1mol = 0.506 g NH3 3.5g H2O x 1mol/18g = 0.194 mol H2O

Molar Mass What is the mass of 1.946 moles of NaCl? How many moles are in 3.0 g of sodium hydroxide (NaOH)? 1.946 mol NaCl x 58.44 g NaCl = 113.72 g NaCl 1 mol NaCl 3.0 g NaOH x 1 mol NaOH = 0.075 mol NaOH 40 g NaOH

Molar Volume Molar Volume: volume (L) of one mole (mol) of a gas at STP (L/mol) Volume 22.4L = 1 mol at STP (standard temperature & pressure) 22.4L Or 1 mol 1 mol 22.4L Standard Temperature: 0°C Standard Pressure: 1atm

Molar Volume How many moles of O2 occupy 56L at STP? How many liters will 0.25 mol of N2 occupy at STP? 56L O2 x 1 mol O2 = 2.5 mol O2 22.4L O2 0.25 mol N2 x 22.4L N2 = 5.6L N2 1 mol N2

Molar Volume How many moles does 5.6L of Cl2 occupy at STP? How many liters does 2 moles of H2 occupy at STP? 5.6L Cl2 x 1 mol Cl2 = 0.25 mol Cl2 22.4L Cl2 2 mol H2 x 22.4L H2 = 44.8L H2 1 mol H2

Molar Volume Lab Collect the hydrogen gas product in the top of the Eudiometer

6.02 x 1023 particles 1 mol Atomic Mass (g) 22.4L Conversion Factors Avogadro Molar Mass Molar Volume 6.02 x 1023 particles 1 mol Atomic Mass (g) 22.4L STP = standard temperature and pressure standard temp is 0°C (freezing point of water) standard pressure is 1 atm (sea level) *Molar Volume is for gases at STP

Island Hopping Mass (g) Island Particle Volume (L) (atoms/molecules) Volume (L) Convert between mass, volume, and particles.

Converting Between Mass (g), Volume (L), Particles (atoms or molecules) What is the mass of 9L of He? How many liters do 4.1 x 1023 molecules of F2 occupy? How many molecules are there in 5g of carbon dioxide? 9L He x 1mol He x 4.0g He = 1.61g He 22.4L He 1mol He (4.1x1023 atoms F2) x 1mol F2 x 22.4L F2 = (6.02x1023 atoms F2) 1mol F2 5g CO2 x 1mol CO2 x (6.02x1023 molecules CO2) = g CO2 1mol CO2

This is how you get percent composition! Analytical Chemistry Analytical Chemistry: the chemical components of a substance are separated, identified, and quantified. This is how you get percent composition! Ex. coffee is made up of 50%C, 5%H, 16%O, and 29%N. Qualitative Analysis: determines the presence or absence of a compound, but not the mass or concentration (quantity). -uses chemical tests such as acid test for gold, flame test for metals, Kastle-Meyer test for blood. Quantitative Analysis: gravimetric analysis determines the amount of material by weighing the sample before and after some transformation. volumetric analysis determines the concentration of an analyte using titration. Instrumental Analysis: uses instruments, ex: spectroscopy (measures the molecules interaction with electromagnetic radiation), mass spectrometry (measures molecules mass-to-charge ratio using electric and magnetic fields), electrochemical analysis (measures potential (volts) and current (amps) in an electrochemical cell containing the analyte), thermal analysis (calorimetry, measures the interaction of a substance with heat), etc.

Percent Composition Percent Composition: the percentage by mass of each element in a compound % Oxygen = Mass of Oxygen x 100% = 16.00 x 100% = 89% Total Mass 18.02 % Hydrogen = Mass of Hydrogen x 100% = 2.02 x 100% = 11% Total Mass 18.02

Percent Composition Find the percent composition of carbon and hydrogen in methane (CH4) Molar Mass of CH4 = % C = % H4 = Molar Mass of CH4 = 12.011 + (1.001 x 4) = 16.015g % Carbon = mass of C/ mass of CH4 x 100% = 12.011/16.015 x 100% = 75% % Hydrogen = mass H/ mass CH4 x 100% = 4.001/16.015 x 100% = 25%

Empirical Formula Empirical Formula: simplified formula The simplest whole-number ratio of atoms in a compound Ex. AB2C is an empirical formula, but A3B6C3 is not Use percent composition to calculate empirical formula Convert to moles (%  g  mol) Divide by the smallest number of moles

Empirical Formula Ex. Chemical analysis of a substance identified its composition as 60.0% C, 13.4% H, and 26.6% O. Calculate the empirical formula. C = 60.0% = 60g x 1mol/12g = 5mol 5mol/1.7mol = 3mol C H = 13.4% = 13.4g x 1mol/1g = 13.4mol 13.4mol/1.7mol = 8mol H O = 26.6% = 26.6g x 1mol/16g = 1.7mol 1.7mol/1.7mol = 1mol O C3H8O

Molecular Formula Molecular Formula: actual formula The actual number of atoms present, but they’re still in the same ratio as the empirical formula Ex. The empirical formula of Glucose is CH2O, but its molecular formula is C6H12O6 Use empirical formula to calculate molecular formula Calculate the number of times bigger the molecular formula is compared to the empirical formula Experimental Molar Mass = # of times bigger Molecular Molar Mass Bellwork: What is the empirical formula of 43.6% P and 56.4% O? Experiment Periodic Table

Molecular Formula Ex. The empirical formula of a compound is P2O5, and its experimental molar mass is 284g/mol. What is its molecular formula? Molecular Molar Mass = (30.97x2) + (15.999x5) = 141.94 g/mol Experimental Molar Mass = 284 g/mol = 2 times bigger Molecular Molar Mass 141.94 g/mol 2(P2O5) = P4O10

Percent Composition  Empirical Formula  Molecular Formula Ex. The percent composition of a molecule is 25.9% Fe and 74.1% Br with an experimental molar mass of 430g/mol. What is the empirical formula? What is the molecular formula? Fe = 25.9g x 1mol/55.85g = 0.464mol 0.464mol/0.464mol= 1mol Fe Br = 74.1g x 1mol/79.90g = 0.927mol 0.927mol/0.464mol= 2mol Br FeBr2 Experimental Molar Mass = 430g/mol = 2 times bigger Molecular Molar Mass 215.65g/mol Fe2Br4

Percent Composition  Empirical Formula  Molecular Formula Convert to moles (%  g  mol) Divide by the smallest number of moles Molecular Formula: Experimental Molar Mass = # of times bigger Molecular Molar Mass

Fruit Loops = Atoms 2 red fruit loops react with 3 blue fruit loops to make a molecule that contains 2 red and 3 blue fruit loops. Write a balanced equation Make your balanced equation with fruit loops Each group gets a sandwich bag of fruit loops. 2R + 3B  1R2B3

Fruit Loops = Atoms If you had 8 red, how many blues would it react with? How many products could you make? If you had 15 blue, how many reds would it react with? 8R + 12B  4R2B3 10R + 15B  5R2B3

Island Hopping Mass (g) Island Particle Volume (L) (atoms/molecules) Volume (L) Convert between mass, volume, and particles.

Hopping Between Islands Mol Particles (atoms/molecules Volume (L) Mass (g) Island hopping for one kind of atom/molecule. Use Avogadro’s conversion factor, molar mass conversion factor, and molar volume conversion factor to convert between particles, mass, and volume.

Hopping Between Islands Mol Particles (atoms/molecules Volume (L) Mass (g) Mol Particles (atoms/molecules Volume (L) Mass (g) Use the mole ratio to convert between different atoms/molecules.

Hopping Between Islands ATOM/MOLECULE ATOM/MOLECULE Mol Particles (atoms/molecules Volume (L) Mass (g) Mol Particles (atoms/molecules Volume (L) Mass (g) Avogadro Avogadro Molar Mass Mole Ratio Molar Mass Molar Volume Molar Volume

Stoichiometry Stoichiometry: calculating the amount of reactants and products in chemical reactions Mole Ratio: the coefficients in a chemical equation represent its ratio of moles Ex. N2 + 3H2  2NH3 1mol of N2 reacts with 3mol of H2, which yields 2mol of NH3 This balanced equation will always be a 1:3:2 ratio. *The mole ratio for a reaction is always the same!

Stoichiometry N2 + 3H2  2NH3 Ex. What volume of H2 gas would you need to make 8.5g of NH3? 8.5g NH3 X 1mol NH3 x 3mol H2 x 22.4L H2 = 16.8L H2 17g NH3 2mol NH3 1mol H2

Limiting Reactant Limiting Reactant: the reactant that gets used up first; it limits the amount of product that can form Theoretical Yield: produced by limiting reactant Excess Reactant: the reactant that is not completely used up in a reaction Example: boys + girls  couples Less girls = limiting reactant [makes theoretical yield] More boys = excess reactant The limiting reactant (girls) determines how many couples can be made. The excess reactant (boys) is leftover.

Limiting Reactant Convert each reactant to g of product The reactant that makes the least amount of product is the LR, the other reactant is the ER The product made with the LR is the TY

Limiting Reactant Ex. If you combined 56g of iron and 32g of oxygen, how much iron (III) oxide can be made? 4Fe + 3O2  2Fe2O3 Fe: 56g Fe x 1mol Fe x 2mol Fe2O3 x 159.7g Fe2O3 = 80.1g Fe2O3 55.85g Fe 4mol Fe 1mol Fe2O3 O2: 32g O2 x 1mol O2 x 2mol Fe2O3 x 159.7g Fe2O3 = 106g Fe2O3 32g O2 3mol O2 1mol Fe2O3

% Yield = Actual Yield x 100% Percent Yield Percent Yield: tells you how well an experiment went % Yield = Actual Yield x 100% Theoretical Yield Given Calculated Ex. 90% yield means you got 90% of the product you were supposed to get

Percent Yield Ex. You reacted 11.5g of sodium with excess chlorine gas, and it produced 13g of sodium chloride. What is the % yield? Balanced chemical equation: Theoretical yield: % yield: 2Na + Cl2  2NaCl 11.5g Na x 1mol Na x 2mol NaCl x 58.5g NaCl = 29.25g NaCl 23g Na 2mol Na 1mol NaCl % yield: 13g NaCl x 100% = 44.4% 29.25g NaCl