Chapter 3: Stoichiometry Stoichiometry - The study of quantities of materials consumed and produced in chemical reactions. In order to understand stoichiometry, we must first understand atomic masses

Atomic Masses Elements occur in nature as mixtures of isotopes Carbon = 98.89% 12 C 1.11% 13 C <0.01% 14 C Carbon atomic mass = amu

Try this… Find the average atomic mass of lithium. Naturally occurring lithium is: 7.42% 6 Li (6.015 amu) 92.58% 7 Li (7.016 amu)

How about this one? Fill in the blanks. IsotopeMass (amu)Abundance % % IsotopeMass (amu)Abundance % % %

The mole A unit to count the number of molecules 1 mol = N A = x Dozen = 12 Pair = 2

Samples of 1 mole of Cu, Al, Fe, S, I, and Hg

Molar mass is the mass of 1 mole of in grams eggs shoes marbles atoms 1 mole 12 C atoms = x atoms = g 1 12 C atom = amu 1 mole 12 C atoms = g 12 C 1 mole lithium atoms = g of Li For any element atomic mass (amu) = molar mass (grams)

Try this! Which of the following g samples contains the greatest number of atoms? a) Magnesium b) Zinc c) Silver Mg ~4 mol, Zn ~1.5 mol, Ag ~0.9 mol MAGNESIUM

Rank the following according to number of atoms (greatest to least): a) g of silver b) 62.0 g of zinc c) 21.0 g of magnesium mol Ag mol Zn 0.86 mol Mg ZINC, Silver, Magnesium

Molar mass My expectation is that you do NOT need a refresher in molar mass! So try this!!! What is the molar mass of isopentyl acetate (C 7 H 14 O 2 )? g/mol

If 1 g of C 7 H 14 O 2 is released in a bee sting, how many molecules are released? HINT: – g = 1x10 -6 g – g mol molecules How many atoms of carbon are present?

How many C atoms in bee sting? HINT: each molecule of isopentyl acetate has 7 atoms of carbon

Percent Composition or Mass % There are two ways to describe the composition of a compound. – The number of constituents atoms – The percent of each of its elements (by mass)

isopentyl acetate (C 7 H 14 O 2 ) Lets look at mass percent of an element Find the mass % of C

Try this one. Consider separate gram samples of each of the following: H 2 O, N 2 O, C 3 H 6 O 2, CO 2 Rank them from highest to lowest percent oxygen by mass. H 2 O CO 2 C 3 H 6 O 2 N 2 O

Formulas molecular formula = (empirical formula) n [n = integer] molecular formula = C 6 H 6 = (CH) 6 empirical formula = CH – The lowest ratio of atoms in a molecule (mole ratio)

Working backwards From the percent composition, you can determine the empirical formula.

Determine the empirical formula of a compound that has the following percent composition by mass: K 24.75%, Mn 34.77%, O 40.51% Assume 100 g sample of the compound

n K = , n Mn = , n O = KMnO 4

This one has a twist! A compound is made of only sulfur and nitrogen. It is 69.6% S by mass. Its molar mass is 184 g/mol. What is its empirical and molecular formula? SN & (SN) 4

How to Read Chemical Equations 2 Mg + O 2 2 MgO 2 atoms Mg + 1 molecule O 2 makes 2 formula units MgO 2 moles Mg + 1 mole O 2 makes 2 moles MgO 48.6 grams Mg grams O 2 makes 80.6 g MgO NOT 2 grams Mg + 1 gram O 2 makes 2 g MgO

Things to know about equations They are sentences They describe what happens in a chemical reaction Reactants Products They should be balanced They have the same number of each kind of atoms on both sides

Writing and balancing chemical reactions. There is one question in the FRQ that accounts for a large # of points on the test. This is something you need to be proficient with… KNOW YOUR IONS!!!!!!!

Try this Chromium compounds exhibit a variety of bright colors. When solid ammonium dichromate (NH 4 ) 2 Cr 2 O 7, a vivid orange compound is ignited a spectacular reaction occurs. Assume that the products are chromium (III) oxide, nitrogen gas, and water vapor. Write and balance the equation.

And this one At 1000 o C, ammonia gas, (NH 3 ) reacts with oxygen gas to form gaseous nitric oxide, and water vapor. This reaction is the first step in the commercial production of nitric acid by the Ostwald process. Write and balance the equation for this reaction.

The answers for the last two slides (NH 4 ) 2 Cr 2 O 7 (s) Cr 2 O 3 (s)+N 2 (g)+4H 2 O(g) AND 4NH 3 (g)+5O 2 (g) 4NO(g)+6H 2 O(g)

Stoichiometry How much do you remember? Methanol burns in air according to the equation 2CH 3 OH + 3O 2 2CO 2 + 4H 2 O If 209 g of methanol are used up in the combustion, what mass of water is produced? 235g of H 2 O

A good FRQ for class. Methane (CH 4 ) reacts with the oxygen in the air to produce carbon dioxide and water. Ammonia (NH 3 ) reacts with the oxygen in the air to produce nitrogen monoxide and water. What mass of ammonia would produce the same amount of water as 1.00 g of methane reacting with excess oxygen? 1.4g ammonia

Limiting Reactant Reactant that determines the amount of product formed. The one you run out of first. Makes the least product. Three methods to solve: – Ratio method shown in text – Have versus need – Do 2 stoichiometry problems to find the amount of product. The reactant that produces the LEAST amount is the limiting reagent.

Lets try together! Nitrogen gas can be prepared by passing gaseous ammonia over solid copper(II) oxide at high temperatures. The other products of the reaction are solid copper and water vapor. If a sample containing 18.1g of NH 3 is reacted with 90.4g of CuO, which is the limiting reactant? How many grams of nitrogen gas will be formed? Have versus need method: g NH 3 mol NH 3 mol CuO needed g CuO needed

2NH 3 + 3CuO N 2 +3CuO + 3H 2 O The problem says the we HAVE 90.4g of CuO. Using ALL of the NH 3 given we NEED 127g of CuO; therefore, the CuO is the limiting reactant since we dont have enough of CuO You can do this starting with the CuO as well. Lets double check our work.

2NH 3 + 3CuO N 2 +3CuO + 3H 2 O The problem says the we HAVE 18.1g of NH 3. Using ALL of the CuO given we NEED 12.9g of NH 3 ; therefore, the CuO is the limiting reactant since we have an excess of NH 3. There is an excess of 5.2g of NH 3

2NH 3 + 3CuO N 2 +3CuO + 3H 2 O Do the stoichiometry problem twice. The reactant that gives you the LEAST amount of product is the limiting reactant. The CuO is the limiting reactant. This confirms our results from the have versus need method.

Try one on your own. When 124 grams of aluminum are reacted with 601 grams of iron(III) oxide, aluminum oxide and solid iron are produced. Calculate the mass of aluminum formed. 234g Al 2 O 3 is formed All the Al is consumed and the Fe 2 O 3 remains in excess

Excess Reactant (reagent) The reactant you dont run out of. The amount of stuff you make is the yield. The theoretical yield is the amount you would make if everything went perfect. The actual yield is what you make in the lab.

Percent Yield

Try this Methanol is the simplest alcohol. It is used as a fuel in race cars and is a potential replacement for gasoline. Suppose 68.5kg CO(g) is reacted with 8.6kg H 2 (g). Calculate the theoretical yield of methanol. If 3.57x10 4 g CH 3 OH is actually produced, what is the percent yield?

The answers Start with a balanced equation. CO(g) + H 2 (g) CH 3 OH(l) Theoretical yield = 6.86x10 4 g Percent yield = 52.0%