More Stoichiometry Examples
When you write out your answers to problems like these, you'll need to show your calculations using dimensional analysis (unit conversions), as usual. Typically, each problem involves some sort of gram-mole-mole-gram conversion.
  1. Ammonium fluoride reacts with calcium nitrate to produce calcium fluoride, dinitrogen monoxide, and water. If 22.8 g of ammonium fluoride react with 38.2 g of calcium nitrate, how much dinitrogen monoxide could be formed?

A classic limiting reactant problem. Amounts of both reactants are given; you need to figure out how much product can be formed. Answer: 0.466 moles of dinitrogen monoxide

  1. Copper reacts with nitric acid to form copper (II) nitrate, nitrogen dioxide, and water. If 25.0 grams of copper react with an excess of nitric acid to form 7.24 grams of nitrogen dioxide, what is the percent yield of the reaction?

A classic percent yield problem. You need to figure out the theoretical yield of product based on the amounts of reactants given. The actual yield is given. Answer: 9.81%.

  1. Aluminum sulfide reacts with water in a double displacement reaction. If 56.0 g of aluminum sulfide reacts with 48.2 g of water, which is the excess reactant and how much of this reactant would be leftover after the reaction is complete?

Another limiting reactant problem, but this one includes the ancillary question of the amount of leftover reactant. It doesn't ask how much of the product is formed, but you need to figure that out anyway (in moles, at least) to find out which one of the reactants is limiting. Then work backwards from the smaller answer (moles of a product; it doesn't matter which one) to calculate the grams of the excess reactant that would have been consumed. Then subtract that from the available mass of that reactant and you're done. Turns out that Al2S3 is the limiting reactant and would react completely to form 1.117 moles of H2S. Answers: The water is the excess reactant and 7.9 grams of it would be leftover.

  1. Calcium combines with nitrogen to form calcium nitride. If 33.8 g of calcium reacts with 20.4 g of nitrogen gas, which reactant is the limiting reactant? How much calcium nitride is formed? How much of the excess reactant is leftover?

This is a classic limiting reactant problem. You have a given amount of each reactant, and you're asked how much of a product will be formed. You'll need to calculate how much Ca3N2 could be formed from each of the reactants individually, as if they each be completely consumed in the reaction. The final answer is the smaller of your two answers. The limiting reactant is the REACTANT that yields the smaller number of moles of product that could be formed. The excess reactant is the REACTANT that yields the larger amount of product. Answer: Ca is the limiting reactant. 0.422 moles of Ca3N2 would be formed, and 2.7 grams of N2 would remain unreacted.

  1. Dinitrogen tetroxide reacts with dinitrogen tetrahydride to form nitrogen gas and water. How much of each reactant is required to form 100.0 pounds of nitrogen gas?

Start with the 100.0 pounds, convert to grams, then moles of N2. From there, calculate back, using the coeffients in the balanced equation, to find the moles of each reactant required. Answers: 539.6 moles of N2O4 and 1,079 moles of N2H4.

  1. Silver tarnishes in the presence of dihydrogen sulfide (which smells like rotten eggs) and oxygen. The two products of the reaction are silver sulfide and water. How many grams of silver sulfide can be formed from a mixture of 1.1 grams of silver, 0.14 grams of hydrogen sulfide, and 0.080 grams of oxygen?

This is a limiting reactant problem with three reactants. You'll need to calculate the moles of Ag2S that could potentially be formed from each of these reactants. Choose the smallest of your three answers, then multiply by the molar mass to get the mass of Ag2S that could be produced. Answer: 1.0 grams of silver sulfide.

  1. An astronaut excretes about 2500 grams of water a day. Lithium oxide is used in spaceships to absorb this water; the single product of the reaction is lithium hydroxide. How many kilograms of lithium oxide must be carried for a 30-day space voyage for three astronauts?

Start with 30 days. I'm going to assume that this calculation will be for exactly 30 days. If you think it's 30 days, plus or minus 10 days (i.e., one sig fig), then you can round the final answer to one sig fig. I would give full credit for one sig fig, and full credit for two sig figs, if you justified it by saying that you are assuming exactly 30 days. Any more would be unjustified, because the water excretion is known only to two sig figs. Use 2,500 grams/day to get the total grams of water produced in a 30-day period. From there, calculate using standard stoichiometry. Grams H2O to moles H2O to moles Li2O to grams Li2O to kilograms. Answer: 120 kilograms of Li2O

  1. Oxygen masks for producing oxygen in emergency situations contain potassium superoxide (KO2), which reacts with water and carbon dioxide to form potassium hydrogen carbonate and oxygen gas. If a person wearing such a mask exhales 0.85 grams of carbon dioxide every minute, how many moles of potassium superoxide are consumed in 10.0 minutes? How many grams of oxygen are produced in 1.0 hours?

Start with the 10.0 minutes. This is the solid number. The 0.85 g/min is a conversion factor for knowing the grams of CO2 consumed per minute. So multiply those two numbers and you get grams of CO2. From there, it's a simple stoichiometry problem. Grams CO2 to moles CO2 to mole KO2. Answer: 0.19 moles KO2.

The approach for the second part is similar. Answer: 28 grams of O2.

  1. Iron reacts with copper(II) sulfate to form copper metal and iron(II) sulfate. If a solution containing 400. grams of copper sulfate reacts with an excess amount of iron and 151 grams of iron (II) sulfate are obtained, what is the percent yield of the experiment?

The balanced equation for this reaction has all 1's for coefficients. Convert the 400. grams of CuSO4 to moles, then to moles of FeSO4, then to grams. The answer is the theoretical yield. 151 grams is the actual yield. % yield is actual/theoretical (x100). Answer: 39.7%

  1. When 12.82 grams of a mixture of potassium chlorate and sodium chloride are heated, the potassium chlorate decomposes to form potassium chloride and oxygen gas. (This is an easy way to produce oxygen gas in the laboratory.) The sodium chloride does not undergo any reaction. After heating, the mass of residue (potassium chloride and sodium chloride) is 9.45 grams. Assuming that all of the mass lost represents a loss of oxygen gas, and that all of the potassium chlorate decomposes, calculate the percent of potassium chlorate in the mixture.

This one is a little tricky. We have 12.82 grams of the mixture (KClO3 and NaCl). Only the KClO3 and not the NaCl will decompose. The NaCl will just sit there. So what we have left is KCl and NaCl, after the oxygen has escaped. The mass of O2 released is the difference between the original mixture's mass and the final mixture's mass. From this mass of O2, we can calculate the mass of KClO3 that must have been present in the first place. The balanced equation for the reaction does not involve NaCl at all. It is 2 KClO3 reacts to form 2 KCl and 3 O2. Answer: 67.1%
 

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