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Honors Chemistry I Wilmington High School Spring, 2015

Chapter 9: StoichiometrySection 9.1: Calculating Quantities in Reactions:

Stoichiometry: The proportional relationship between two or more substances during a chemical reaction

Mole Ratio: A ratio of the coefficients in front of different substances involved in a BALANCED chemical reaction.

Example: Consider the equation 2H2(g) + O2(g) 2H2O(g)

Some examples of mole ratios from this equation: ___________________

___________________

___________________

Mole Road Map: You can use this map to navigate your way through almost all calculations in this chapter.

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Example Set A: Using Mole Ratios

1) Liquid hydrogen peroxide (H2O2) decomposes into liquid water and oxygen gasa. Write a balanced chemical equation for this reaction

b. If 1.34 moles of hydrogen peroxide completely react according to the equation you wrote above, calculate

i. The moles of O2 formed

ii. The moles of liquid water formed

2) Liquid water reacts with carbon dioxide gas to form aqueous carbonic acid (H2CO3). How many moles of carbonic acid form when 2.57 mol of carbon dioxide react with excess water?

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3) Propane (C3H8) gas combusts in the presence of oxygen gas to form carbon dioxide gas and water vapor.a. Write a balanced chemical equation for this reaction

b. How many moles of O2 are necessary to completely burn 4.33 moles of C3H8?

c. How many moles of CO2 form if 4.33 moles of C3H8 are burned?

d. How many moles of H2O are formed if 4.33 moles of C3H8 are burned?

4) Iron (III) oxide reacts with solid aluminum to form solid iron and solid aluminum oxide.a. Write a balanced chemical equation for this reaction

b. If 3.30 moles of iron (III) oxide completely react according to your balanced equation, calculatei. The moles of solid aluminum needed

ii. The moles of solid iron formed

iii. The moles of aluminum oxide formed

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Example Set B: Problems involving mass

Key Concept: USE MOLAR MASSES!!! THIS CAN GET YOU TO THE MOLE RATIO STEP!

1) Ammonia gas (NH3) is formed by reacting nitrogen gas and hydrogen gasa. Write a balanced chemical equation for this reaction

b. What mass of NH3 can be made from 2284 grams of H2 and excess nitrogen gas?

c. What mass of NH3 can be made from 1425 grams of N2 and excess hydrogen gas

2) Use the equation Fe2O3(s) + 2Al(s) 2Fe(s) + Al2O3(s) to answer all questions belowa. How many grams of Al are needed to completely react with 135 grams of Fe2O3?

b. How many grams of Al2O3 can form when 23.6 grams of Al react with excess Fe2O3?

c. How many grams of Fe2O3 react with excess Al to make 475 grams of Fe?

d. How many grams of Fe will form when 97.6 grams of Al2O3 form?

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3) What mass of liquid water is produced if 65.2 grams of calcium carbonate reacts with excess phosphoric acid (H3PO4) to form calcium phosphate, liquid water, and carbon dioxide gas?

4) What mass of oxygen gas forms with 49.89 grams of KClO3 decomposes, forming potassium chloride and oxygen gas as products?

5) What mass of ammonia is formed when 7.50 grams of nitrogen gas reacts with excess hydrogen gas?

6) What mass of sodium is necessary to produce 10.0 grams of hydrogen gas in the reaction 2Na(s) + 2H2O(l) 2NaOH(aq) + H2(g)

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Example Set C: Problems involving volume

Key Concept: FOR VOLUME, YOU MIGHT USE DENSITY (D = m/v) AND/OR MOLAR MASS

1) Use the densities and balanced chemical equation provided to answer the questions that follow:

C5H12(l) C5H8(l) + 2H2(g)

Substance DensityC5H12 0.620 g/mLC5H8 0.681 g/mLH2 0.0899 g/L

a. How many mL of C5H8 can be made from 366 mL of C5H12?

b. How many liters of H2 can form when 4530 mL of C5H8 form?

c. How many milliliters of C5H12 are needed to make 97.3 mL of C5H8?

d. How many milliliters of H2 can be made from 1.98 x 103 mL of C5H12?

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2) When pentane (C5H12) burns in the presence of oxygen, it produces carbon dioxide gas and water vapor. If 85.5 grams of pentane is completely burned, what volume of carbon dioxide is produced? Assume that CO2 cools to room temperature, where its density is 1.997 g/L.

3) Magnesium metal burns in the presence of oxygen to produce magnesium oxide. What mass of magnesium will burn in the presence of 189 mL of oxygen? The density of oxygen gas is 1.429 g/L.

Example Set D: Number of Particles

Key Concept: USE AVOGADRO’S NUMBER!!!!

1) Liquid bromine reacts with fluorine gas to produce liquid bromine pentafluoride.a. Write a balanced chemical equation for the reaction described.

b. How many molecules of BrF5 form when 384 grams of Br2 react with excess F2?

c. How many molecules of Br2 react with 1.11 x 1020 molecules of F2?

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2) Solid sodium reacts with chlorine gas to form solid sodium chloridea. Write a balanced chemical equation for the reaction described.

b. If 2.46 x 1025 chlorine molecules react completely, how many grams of NaCl will form?

c. If you completely react a 25.0 g chunk of sodium with excess chlorine, how many NaCl particles will be formed?

3) How many molecules of carbon dioxide are produced when 79.5 grams of K2CO3 decompose according to the equation K2CO3(s) K2O(s) + CO2(g)?

4) Butane (C4H10) combusts in the presence of oxygen gas.a. Write a balanced chemical equation for this reaction

b. How many water molecules form from the complete combustion of 1.129 x 1024 molecules of C4H10?

c. How many grams of water form if 2.500 x 1024 molecules of butane are burned completely?

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Extra In-Class Practice Problems:

1) Bromine gas reacts with chlorine gas to form BrCl(g)a. Write a balanced equation for this reaction

b. How many moles of BrCl form when 2.74 moles of Cl2 react with excess Br2?

c. How many grams of BrCl form when 239.7 grams of chlorine gas react with excess bromine gas?

d. How many grams of Br2 are needed to react with 4.53 x 1025 molecules of Cl2?

2) Acetylene (C2H2) combusts in the presence of oxygen gasa. Write a balanced chemical equation for this reaction

b. If 15.9 L of acetylene react at STP (standard temperature and pressure), how many moles of CO2 are produced? Note that 1 mol = 22.41 L for any gas at STP!

c. How many milliliters of CO2 (density = 1.977 g/L) can be made when 59.3 mL of O2 (density = 1.429 g/L) react?

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3) Solid lithium hydroxide is used in space exploration to remove exhaled carbon dioxide from the astronauts’ living environment by forming solid lithium carbonate and liquid water. What mass of gaseous carbon dioxide can be absorbed by 1.00 kg of lithium hydroxide?

4) How many grams of water are produced in the combustion of 100.00 grams of glucose (C6H12O6)?

5) Sulfuric acid (H2SO4) is a component of acid rain that forms when SO2, a pollutant, react with oxygen gas and liquid water in the atmosphere. A mid-sized home produces about 25.0 kg of SO2 pollution per year via electricity. Assuming that there is more than enough O2 and H2O available in the atmosphere, what mass of sulfuric acid (in kilograms) is formed by a typical house on a yearly basis?

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6) Another component of acid rain is nitric acid, which forms when nitrogen dioxide (also a pollutant) reacts with oxygen gas and liquid water in the atmosphere. The electricity used by a typical house produces about 16.0 kg of NO2 per year. Assuming that there is sufficient water and oxygen available, what mass of nitric acid (in kg) forms from this amount of pollution?

7) A common laboratory method for preparing small amounts of oxygen gas is the decomposition of solid KClO3, which produces oxygen gas and solid potassium chloride. How many grams of oxygen gas can be prepared from 4.50 grams of potassium chlorate?

8) In photosynthesis, plants convert carbon dioxide and water into glucose and oxygen gas, with solar energy driving the chemical reaction. Suppose you determine that a particular plant consumes 37.8 grams of CO2 during a research study. Assuming that there is more than enough water present to react with all of the carbon dioxide, what mass of glucose (in grams) can the plant synthesize from the carbon dioxide?

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9) Baking soda (NaHCO3) is an active ingredient in antacids (as we already know from our “reaction bag” lab). It reacts with HCl(aq) that is secreted by the stomach, producing aqueous sodium chloride, liquid water, and carbon dioxide gas. Milk of magnesia, which is an aqueous suspension of magnesium hydroxide, can also be used as an antacid. In the case of milk of magnesia, solid magnesium hydroxide reacts with hydrochloric acid to form liquid water and aqueous magnesium chloride. Which is a more effective antacid: 1.00 g of baking soda or 1.00 g of milk of magnesia?

Step 1: Calculate how many moles of HCl that 1.00 g of baking soda can neutralize

Step 2: Calculate how many moles of HCl that 1.00 g of milk of magnesia active ingredient can neutralize.

So, which can neutralize more stomach acid on a gram-per-gram basis?

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Section 9.2: Limiting Reactants and Percentage Yield

When you drive your car, you need a fuel source (octane) and oxygen from the air for the combustion reaction that gets you from place to place. If you run out of gasoline, you can’t go any further once your car stops even though there is plenty of oxygen left in the atmosphere to use. In this situation, octane is the limiting reactant

and atmospheric oxygen is the reactant “in excess.”

From biochemistry, if you don’t get proper amounts of vitamins/minerals, they become the “limiting reactants” in the reactions that go on inside your body. As an example, if you don’t get enough vitamin B-12 on a regular basis, your bone marrow is limited in how many mature red blood cells it can make available, which may eventually lead to anemia.

In firefighting, limiting reactants also play a key role. For a fire to burn, it needs a fuel source, oxygen, and energy. If any one of these resources is limited, the fire cannot continue, even if the other resources are abundantly available. Spraying a fire with water limits the amount of oxygen that can “feed” the fire and it lowers the temperature of the reaction. Fire extinguishers work by lowering the temperature of the fire, but they ALSO lay a “huge blanket” of gases much denser than air (for example, carbon dioxide), which smother the fire and prevent oxygen gas from getting close to the fuel source.

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Limiting reactant: Substance that controls the quantity of product formed during a chemical reaction

Excess reactant: The substance that isn’t completely used up in a reaction

Example Problem Set A: Determining Limiting Reactants and Calculating Theoretical Yield

1) Consider the reaction shown, and answer all of the questions that follow:

PCl3(l) + 3H2O(l) H3PO3(l) + 3HCl(aq)

a. If 225 grams of PCl3 is mixed with 123 g of H2O, which is the limiting reactant?

i. What is the theoretical yield given these quantities of reactants?

b. If 3.00 mol of PCl3 and 3.00 mol of H2O are mixed, determine the limiting reactant.


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Continued from previous page: PCl3(l) + 3H2O(l) H3PO3(l) + 3HCl(aq)

c. If 75.0 g of PCl3 and 50.0 g of H2O are mixed, determine the limiting reactant.


d. If 1.00 mol of PCl3 is mixed with 50.0 g of H2O, determine the limiting reactant.


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2) Zinc citrate – Zn3(C6H5O7)2 – an ingredient in toothpastes, is made by reacting zinc carbonate and citric acid (C6H8O7). The other products are liquid water and carbon dioxide. If there are 6.00 moles of zinc carbonate and 10.0 moles of citric acid available for reaction, determine the limiting reactant and determine the theoretical yield of zinc citrate.

3) Hydrogen sulfide gas is formed when hydrochloric acid reacts with Iron (II) sulfide. FeCl2 is the other product in this reaction. If 130.5 grams of FeS is mixed with 70.4 grams of HCl in solution, determine the limiting reactant and determine the theoretical yield of hydrogen sulfide.

4) Titanium dioxide is used as a white paint pigment. If 3.50 moles of titanium tetrachloride reacts with 4.5 moles of oxygen gas to yield titanium dioxide and chlorine gas. Determine the limiting reactant for this process and determine the theoretical yield.

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Actual Yield, Theoretical Yield, and Percentage Yield:

The actual yield is the measured amount of product from a reaction (e.g., done during a lab activity). The theoretical yield is how much would be measured in a perfect world, based on the exact amounts of reactants used in the experiment (or process).

Formula for Percent Yield:

Example Problem Set B: Calculating Percentage Yield

1) Nitrogen gas and hydrogen gas react to form ammonia. 14.0 grams of nitrogen gas were mixed with 9.0 grams of hydrogen gas, and 16.1 grams of ammonia formed.

a. Write the balanced chemical reaction for this process.

b. Determine the limiting reactant.

c. What is the theoretical yield?

d. What is the percentage yield?

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2) Consider the same equation as written in 1a: __________________________________________a. If 14.0 grams of nitrogen gas react with 3.15 grams of hydrogen gas to give an actual yield of

14.5 grams of ammonia, i. What was the limiting reactant?

ii. What was the theoretical yield?

iii. What was the percent yield?

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3) 16.1 grams of bromine gas are mixed with 8.42 grams of chlorine gas to give an actual yield of 21.1 grams of bromine monochloride:

a. What is the balanced chemical reaction?

b. What is the limiting reactant?

c. What is the percentage yield?

4) Methanol is produced by reacting carbon monoxide with hydrogen gas. If the percentage yield is 92.0%, how many grams of methanol can be made by reacting 5600 grams of CO 1000.0 grams of H2?

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5) Huge quantities of sulfur dioxide are produced from zinc sulfide when it reacts with oxygen gas. The other product is zinc oxide. If the typical yield is 86.78%, what mass of sulfur dioxide should be expected if 4897 g of ZnS is used?

6) Ammonia is synthesized by reacting hydrogen gas and nitrogen gas. If the percent yield is 85.0%, what actual yield is expected if 1.00 kg of N2 react with 225 g of H2?

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Additional Mixed Review:

1) Aqueous solutions of sodium phosphate and barium nitrate are mixed.a. Write the balanced chemical reaction for this double-replacement reaction. Consider the

solubility table from your chapter 8 packet to determine the precipitate formed.

b. Suppose a solution of 3.50 g of sodium phosphate is mixed with a solution containing 6.40 grams of barium nitrate. How many grams of barium phosphate can be formed? Be sure to determine the limiting reactant!

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2) A strip of zinc foil weighing 2.00 g is placed in an aqueous solution containing 2.50 g of silver nitrate. A single-replacement reaction occurs.

a. Write the balanced chemical reaction for this process.

b. What is the limiting reactant?

c. How many grams of silver will form?

d. How many grams of zinc nitrate will form?

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3) Adipic acid (H2C6H8O4) is a material used for the production of nylon. It is made commercially by reacting cyclohexane (C6H12) and oxygen gas:

2C6H12 + 5O2 2H2C6H8O4 + 2H2O

a. Assume that you carry out this reaction starting with 25.0 grams of cyclohexane, and that cyclohexane is the limiting reactant. What is the theoretical yield of adipic acid?

b. If you obtain 33.5 grams of adipic acid from your reaction, what is the percent yield of adipic acid?

4) Imagine that you’re working on ways to improve the process by which iron ore containing Fe2O3 is converted into pure iron. In your tests you carry out the following reaction on a small scale:

Fe2O3(s) + 3CO(g) 2Fe(s) + 3CO2(g)

a. If you start with 150 grams of Fe2O3 as the limiting reactant, what is the theoretical yield of Fe?

b. If the actual yield of Fe in your test was 87.9 grams, what was the percent yield?

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5) Titanium metal can be obtained from its oxide according to the following equation:

TiO2(s) + 2C(s) Ti(s) + 2CO(g)

When 28.6 kg of carbon reacts with 88.2 kg of TiO2, 42.8 kg of Ti is produced. Find the limiting reactant, theoretical yield (in kg), and percent yield.

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