Stoichiometry Chapter 11 & 12

I. Things you should remember From the Moles Unit: Identify particles as atoms, molecules (mc), and formula units (fun) 1 mole = 6.02 x atoms, molecules, or formula units 1 mole atom = mass (in grams) from the periodic table From the Naming & Formulas Unit: How to write a formula given a chemical name From the Chemical Reactions Unit: How to write a chemical equation given words Balancing equations

Example 1: Determine the moles of magnesium there are in 1.23 x atoms of magnesium 1.23 x atoms Mg 1 mole Mg 6.02 x atoms Mg = 2.04mol Mg 1 G: 1.23 x atoms Mg W: moles Mg R: 1 mole Mg = 6.02 x atoms Mg

Example 2: Determine the mass of 3.50 mol of copper mol Cu63.546g Cu 1 mol Cu = 222 g Cu From the p.t  Copper: g Cu = 1 mol Cu 1

Example 3: Calcium and sodium carbonate react together to form calcium carbonate and sodium Na + CO 3 2- Ca+ CaCO 3 Na 2 CO 3 Ca 2+ CO 3 2- Na+

Example 4: Balance Li 2 O(s) + H 2 O(l)  LiOH(aq) Li 2 O(s) + H 2 O(l)  LiOH(aq) 2

Practice 1. Determine the number of water molecules in 11.2 moles of water moles H 2 O 6.02 x molecules H 2 O = 6.74 x moles H 2 O molecules H 2 O 1 G: 11.2 moles water W: mc water R: 1 mole H 2 O = 6.02 x mc H 2 O

Practice 2. Determine the number of moles in 11.9 kg of aluminum kg Al1000g Al 1 kg Al = mol Al g Al From the p.t  Aluminum: g Al = 1 mol Al mol Al 1 G: 11.9 kg Al W: mol Al R: 1 mole Al = g Al 1 kg Al = 1000g Al

Quick Review: Writing Formulas for Ionic Compounds

A reminder of what a chemical formula tells us: EX: aluminum carbonate Al 3+ CO 3 2- Al 2 (CO 3 ) 3

CO 3 2- Al 3+ Al 2 (CO 3 ) 3 CO 3 2- in 1 formula unit of aluminum carbonate: 2 moles aluminum 3 moles carbons 9 moles oxygens 2 atoms aluminum 3 atoms carbons 9 atoms oxygens

Al 2 (CO 3 ) 3 Al o C o o o C o o o C o o 2 moles Al 3 moles CO 3

II. Quantifying Chemical Compounds

We will now do the same thing, but we will have questions that are more detailed about the compounds. First we need to know how to find the molar mass of the whole compound.

I. Molar Mass

1. Definition Remember, molar mass is the mass (in grams) of one mole of an element or compound.

2. Determining Molar Mass The molar mass of the compound is the sum of the molar masses of each atom in the compound. The units for molar mass are grams per mole (g/mol). Grams mole

Ex: Determine the molar mass of sulfur trioxide g/mol or g = 1 mole SO 3 SO 3 + = X 3 ( g)

For the sake of consistency… When calculating molar masses, use all the sig figs presented on your periodic table Let’s show all of our molar masses to the thousands place (you should know it’s not going to make a huge difference) Ex: g/mol But, show our final final stoichiometry answers in 3 sig figs

Ex: Determine the molar mass of calcium nitrate. Ca(NO 3 ) g/mol or g = 1 mol Ca(NO 3 ) 2 =

Ex: Determine the molar mass of calcium nitrate. Ca(NO 3 ) 2 + = X 2+X g/mol or g = 1 mol Ca(NO 3 ) 2

Ex: Determine the molar mass of Iron(III) sulfate. Fe 3+ (SO 4 ) 2- Fe 3+ (SO 4 ) 2- Fe 2 (SO 4 ) 3 First you need the right formula right?

Ex: Determine the molar mass of Iron(III) sulfate. Fe 2 (SO 4 ) 3 +X 2+X g/mol or g = 1 mol Fe 2 (SO 4 ) 3 X 3

Practice: 1. Determine the molar mass of each of the following. a. Li 2 S b. (NH 4 ) 2 CO 3 C. magnesium hydroxide d. copper (II) iodide g/mol Li 2 S g/mol (NH 4 ) 2 CO g/mol Mg(OH) g/mol CuI 2

3. Using Molar Mass in Calculations You would use the molar mass of a compound in dimensional analysis just like you did with elements.

Ex: Determine the mass of 48.6 moles of sodium chloride. Step 1: Write the formula for sodium chloride. Na + Cl - NaCl

Ex: Determine the mass of 48.6 moles of sodium chloride. Step 2: set up a dimensional analysis problem G: W: R: 48.6 moles NaCl Mass (g) NaCl

Ex: Determine the mass of 48.6 moles of sodium chloride. Step 3: Find the molar mass for NaCl because the wanted is MASS Na = 1 X g = g Cl = 1 X g = g g/mol NaCl

Ex: Determine the mass of 48.6 moles of sodium chloride. Step 3: Plug it all in 48.6 mol NaCl58.443g NaCl 1 mol NaCl = 2.84 x 10 3 g NaCl = 2840g NaCl

Tips for working molar mass problems 1. If mass is involved in your problem, determine molar mass of compound. 1 mole X = ____________ g X 2. If you see the terms: atoms, molecules, or formula units, use Avogadro’s number. 1 mol X = 6.02 X ___*___ X * atoms, molecules or formula units

Ex: Determine the number of moles in 582 g of magnesium nitrate Step 1: Write the formula for magnesium nitrate Mg +2 NO 3 - Mg(NO 3 ) 2

Ex: Determine the number of moles in 582 g of potassium nitrate Step 2: set up a dimensional analysis problem G: W: R: 582 g Mg(NO 3 ) 2 Moles Mg(NO 3 ) 2

Ex: Determine the number of moles in 582 g of magnesium nitrate. Step 2: Find the molar mass for Mg(NO 3 ) 2 Mg = 1 X g = g N = 2 X g = g O = 6 X g = g g/mol Mg(NO 3 ) 2

Ex: Determine the number of moles in 582 g of magnesium nitrate. Step 3: Set up a dimensional analysis problem. 582g Mg(NO 3 ) g Mg(NO 3 ) 2 1 mol Mg(NO 3 ) 2 = 3.92 mol Mg(NO 3 ) 2

Reminder: How to type in Scientific Notation into a scientific calculator 6.02 x Keystrokes: 6.02EE or EXP23 (2 nd X -1 )

Ex: Determine the number of molecules in 47.3 g of sulfuric acid. 47.3g H 2 SO 4 1 mol H 2 SO g H 2 SO x molecules H 2 SO 4 1 mol H 2 SO 4 = 2.90 x molecules H 2 SO 4 1 G: W: R: 47.3 g H 2 SO 4 Molecules of H 2 SO 4 1 mol H 2 SO 4 = g H 2 SO 4 1 mol H 2 SO 4 = 6.02 x mc H 2 SO

Practice 1.Determine the number of moles in 27.4 g of TiO 2. 2.Determine the mass of 9.45 mol of dinitrogen trioxide. 3.Determine the mass (in kg) of 5.83 x molecules of HCl. 3. Determine the number of molecules in 782g of N 2 O mol TiO kg HCl 6.19 x molecules N 2 O 3 567g N 2 O 3

II. Percent Composition

The relative amounts of each element in a compound are expressed as percentages. The percent by mass of an element in a compound is the total number of grams of the element divided by the molar mass of the compound multiplied by 100%.

II. Percent Composition Identify all of the atoms in the compound and solve using the equation below to find the abundance of each element. % composition = Mass element Molar Mass X 100 (Mass of Compound)

Ex: Calculate the % composition of propane (C 3 H 8 ). (In other words, what percent of propane is carbon and what percent is hydrogen?) Carbon 3 x = g/mol Hydrogen 8 x = 8.064g/mol Molar mass = g/mol

Ex: Calculate the % composition of propane (C 3 H 8 ) % composition C = g/mol g/mol 100 % composition H = 8.064g/mol g/mol % C 18.3% H

Ex: Calculate the % composition of propane (C 3 H 8 ) These calculations tell us that 81.7% of propane is composed of carbon and 18.3% is made up of hydrogen Note: Percentage is part over whole. Mass of one type of element in a compound / mass of the whole compound

Ex: Determine the percent nitrogen in zinc nitrate. Zn 2+ NO 3 - Zn(NO 3 ) 2 %N = g/mol g/mol To Calculate Molar Mass: Zn = 1 x 65.39g/mol = g/mo N = 2 x g/mol = g/mol O = 6 X g/mol = g/mol Molar mass: g/mol 14.8% of N in zinc nitrate =

EX: Determine the mass of sodium in 450 g of sodium chloride. Na = g/mol Cl = g/mol g/mol %Na = g/mol g/mol 39.3% of Na in sodium chloride = x 450g = 176g Na

Practice 1. Calculate the percent composition of a. calcium chloride b. potassium nitrate 2. Determine the percent oxygen in calcium carbonate 3. Calculate the mass of hydrogen in a. 350 g of C 2 H 6 b g NH 4 Cl 36.1 % Ca, 63.9 % Cl 38.7 % K, 13.9 % N, 47.5 %O 48.0 % O 70.4 g H g H