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1.Write balanced chemical equation 2.Convert quantities of known substances into moles 3.Use coefficients in balanced equation to calculate the number of moles of the sought quantity 4.Convert moles of sought quantity into desired units Mass Changes in Chemical Reactions

How many moles of nitrogen dioxide are produced when you start with 2.01 grams of oxygen gas? 4 NH O 2  4 NO H 2 O Mass to Moles

Given: Unknown: Solution: Balanced Equation: 4NH 3 (g) + 7O 2 (g) → 4NO 2 (g) + 6H 2 O(g) molar mass factor mol ratio.035 mol NO g O 2 x 4 mol NO 2 7 mol O 2 = 1 mol O 2 x g O 2

How many grams of nitrogen dioxide are produced when you start with 8.12 moles of oxygen gas? 4 NH O 2  4 NO H 2 O Moles to Mass

Given: Unknown: Solution: Balanced Equation: mol ratio molar mass factor g NO 2 4 NH O 2  4 NO H 2 O 8.12 mol O 2 x 4 mol NO 2 x 7 mol O 2 = g NO 2 1 mol NO 2

How many grams of nitrogen dioxide are produced when you start with 22.5 grams of oxygen gas? Mass to Mass 4 NH O 2  4 NO H 2 O

Mass to Mass Given: Unknown: Solution: molar mass factor mol ratio molar mass factor = 18.5 g NO g O 2 x 4 mol NO 2 x 7 mol O 2 = 1 mol O 2 X g O g NO 2 1 mol O 2

Limiting Reactants limiting reactant : the reactant that limits the amount of product that can form in a chemical reaction. excess reactant : the substance that is not used up completely in a reaction.

6 green used up 6 red left over Limiting Reagents 3.9

You are at home and ready to make some Betty Crocker Brownies to bring to chemistry class. You know the class is big so you need to make two batches of brownies. You gather all the supplies and this is what you have: 2 Packages of Brownie Mix 1 cup of water 1 cup of vegetable oil 3 eggs Do you have enough supplies to make the brownies? What is limiting you? What is in excess? Ingredients on Brownie Box: 1 Package of Brownie Mix ¼ cup of water ½ cup of vegetable oil 2 eggs

Steps to determining limiting reactant: 1. Balance the equation. 2. Pick a reactant. Convert grams of one reactant into grams of the second reactant. 3. Compare the amount given in the reaction to the amount calculated. a.If the number is smaller, then the second reactant is in excess. b.If the number is larger, then the second reactant is limiting.

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? grams CH 3 OHmoles CH 3 OHmoles H 2 Ograms H 2 O molar mass CH 3 OH coefficients chemical equation molar mass H 2 O 209 g CH 3 OH 1 mol CH 3 OH 32.0 g CH 3 OH x 4 mol H 2 O 2 mol CH 3 OH x 18.0 g H 2 O 1 mol H 2 O x = 235 g H 2 O

Do You Understand Limiting Reagents? In one process, 124 g of Al are reacted with 601 g of Fe 2 O 3 2Al + Fe 2 O 3 Al 2 O 3 + 2Fe Calculate the mass of Al 2 O 3 formed. g Almol Almol Fe 2 O 3 neededg Fe 2 O 3 needed OR g Fe 2 O 3 mol Fe 2 O 3 mol Al neededg Al needed 124 g Al 1 mol Al 27.0 g Al x 1 mol Fe 2 O 3 2 mol Al x 160. g Fe 2 O 3 1 mol Fe 2 O 3 x = 367 g Fe 2 O 3 Start with 124 g Alneed 367 g Fe 2 O 3 Have more Fe 2 O 3 (601 g) so Al is limiting reagent 3.9

Use limiting reagent (Al) to calculate amount of product that can be formed. g Almol Almol Al 2 O 3 g Al 2 O g Al 1 mol Al 27.0 g Al x 1 mol Al 2 O 3 2 mol Al x 102. g Al 2 O 3 1 mol Al 2 O 3 x = 234 g Al 2 O 3 2Al + Fe 2 O 3 Al 2 O 3 + 2Fe 3.9

Theoretical Yield is the amount of product that would result if all the limiting reagent reacted. Actual Yield is the amount of product actually obtained from a reaction. % Yield = Actual Yield Theoretical Yield x 100