CHM101 Lab Stoichiometry Grading Rubric

Transcription

1 Name CHM101 Lab Stoichiometry Grading Rubric Criteria Points possible Points earned Lab Report Q1 (work and units shown) 3 Q2 (work and units shown) 2 Q3 (work and units shown) 3 Q4 (work and units shown) 3 Q5 (work and units shown) 3 Q6 (work and units shown) 3 Q7 (work and units shown) 3 Total 20 Subject to additional penalties at the discretion of the instructor.

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3 Experiment #6. Stoichiometry. Consider a classic recipe for pound cake: 1 pound of eggs, 1 pound of butter, 1 pound of flour, and 1 pound of sugar. (That s why it s called pound cake. ) If you have 4 pounds of butter, how many pounds of sugar, flour, and eggs do you need? You would need 4 pounds each of sugar, flour, and eggs. Now suppose you have 1.00 g H 2. If the chemical reaction follows the balanced chemical equation 2 H 2 (g) + O 2 (g) 2 H 2 O(l) then what mass of oxygen do you need to make water? Curiously, this chemical reaction question is very similar to the pound cake question. Both of them involve relating a quantity of one substance to a quantity of another substance or substances. The relating of one chemical substance to another using a balanced chemical reaction is called stoichiometry. Stoichiometry is a term derived from the Greek words stoicheion (meaning element ) and metron (meaning measure ). Using stoichiometry is a fundamental skill in chemistry; it greatly broadens your ability to predict what will occur and, more importantly, how much is produced. In this exercise, you will explore mass relationships in chemistry using balanced equations and principles of stoichiometry. Let us consider a more complicated example. A recipe for pancakes calls for 2 cups (c) of pancake mix, 1 egg, and 1/2 c of milk. We can write this in the form of a chemical equation: 2 c mix + 1 egg + ½ c milk 1 batch of pancakes If you have 9 c of pancake mix, how much milk do you need? To solve this we will use the recipe above to make a conversion factor for pancake mix and milk. 2"c"mi ½"c"milk" or" ½"c"milk" 2"c"mi Use the version of conversion factor that allows cups of pancake mix to cancel and solve for the amount of milk. 9"c"mi ½"c"milk" 2"c"mi ="""4"¼"c"milk" A balanced chemical equation is nothing more than a recipe for a chemical reaction. The difference is that a balanced chemical equation is written in terms of atoms and molecules, not cups, pounds, and eggs. For example, consider the following chemical equation from above: 2H 2 (g) + O 2 (g) 2H 2 O(l). We can interpret this as, literally, two hydrogen molecules react with one oxygen molecule to make two water molecules. Since we are usually reacting far more than one or two molecules at time, we can also interpret the equation as two moles of hydrogen react with one mole of oxygen to form two moles of water. Some conversion factors based on the reaction stoichiometry are: 2"mol"H 2" 2"mol"H 2" 2"mol"H 2 O" 2"mol"H 2 O" These conversions are known as mole-mole factors and can be used to relate moles of one substance to moles of another. For example, suppose we need to know how many moles of oxygen are needed to react with 2.8 moles of hydrogen. As we did with converting units, we start with our given quantity and use the appropriate conversion factor ensuring that the unit mol H 2 cancels:

4 2.8"mol"H 2" 1"mol"O 2" ="""1.4"mol"O 2" 2"mol"H 2" Mole-mole factors can be combined with conversion factors from molar mass to find mass relationships in chemical reactions. Most stoichiometry problems start with the mass of one molecule and ask you to determine the mass of another substance. As with all stoichiometry problems, it is important that you start with a balanced chemical equation. The general strategy for solving these mass-mass problems is below. For example, let us determine the number of grams of SO 3 that would be produced from the reaction of 45.3 g of O 2 with SO 2 : 2 SO 2 (g) + O 2 (g) 2 SO 3 (g) First, we convert the given amount, 45.3 g of O 2, to moles of O 2 using its molar mass (32.00 g/mol): 45.3"g"O 2" 1"mol"O 2" ="" "g"O 2" Second, we use the balanced chemical reaction to convert from moles of O 2 to moles of SO 3 : 1.4 2"mol"SO 3" =""2.83"mol"SO 3" Finally, we use the molar mass of SO 3 (80.06 g/mol) to convert to the mass of SO 3 : 2.83"mol"SO 3" 80.06"g"SO 3" ="""227"g"SO 3" 1"mol"SO 3" We can also perform all three steps sequentially, writing them on one line as: 45.3"g"O 2" 1"mol"O 2" 32.00"g"O 2" 2"mol"SO 3" 80.06"g"SO 3" 1"mol"SO 3" ="""227"g"SO 3" We get the same answer. Note how the initial and all the intermediate units cancel, leaving grams of SO 3, which is what we are looking for, as our final answer. One additional conversion factor used in combination with stoichiometry problems is based on Avogadro s number (N A ): 6.02 x 10 23, the number of particles in a mole. Avogadro s number does not have a particular unit it takes the unit of the type of particle in the problem, usually atoms or molecules. Below, N A is used as a conversion factor to convert moles of iron to atoms of iron. 7.5 mol Fe x 6.02 x 1023 atoms Fe = 4.5 x x10 24 atoms Fe 1 mol Fe

5 CHM 101: Stoichiometry Name Show all work including units for problems a) How many atoms are in 1 mol Cu? b) How many atoms are in mol Cu? c) How many atoms are in 77.8 g Cu? 2. a) How many moles are x molecules of CuSO 4? b) What is the mass of 3.55 x molecules of CuSO 4? 3. How many molecules are in 1.0 Liter of water? (Hint: start by converting to mass)

6 CHM 101: Stoichiometry Name Show all work including units for problems What mass of HgO is required to produce g of O 2 according to the reaction below? 2 HgO (s) 2 Hg (l) + O 2 (g) 5. How many grams of Ag are needed to react with g of Al(NO 3 ) 3 according to the reaction below? Al(NO 3 ) 3 (s) + 3 Ag (s) Al (s) + 3 AgNO 3 (s) 6. The decomposition reaction of sodium azide (NaN 3 ) is used to inflate air bags with nitrogen gas according to the reaction below. If 95 g of N 2 are needed to fully inflate an airbag, how many grams of sodium azide should be used? 2 NaN 3 (s) 3 N 2 (g) + 2 Na(s) 7. Aspirin (C 9 H 8 O 4 ) is industrially made from salicylic acid (C 7 H 6 O 3 ) according reaction below. How many kilograms of salicylic acid are needed to make 15 kg of aspirin? C 7 H 6 O 3 + C 4 H 6 O 3 à C 9 H 8 O 4 + C 2 H 4 O 2 salicylic acid acetic anhydride aspirin acetic acid