Stoichiometry - a review

Skills tested by this Quiz

evaluate molecular weight for a given formula
evaluate weight (mass) percentages of elements for a given formula
evaluate amounts (in mass and mole units) produced in a chemical reaction from given conditions
classify reactions by types: combination, combustion, displacement, formation, etc
determine the chemical formula when weight percentages are given and then evaluate the mole percentages of elements in the formula
determine the chemical formula when weight percentages are given and molecular weight is known
determine the amount produced, the actual yield, and other stoichiometry quantities for a given reaction

Stoichiometry Review

Review Purposes

To get an overall view of stoichiometry.
Apply skills learned to perform quantitative chemical analysis.
Apply theories and rules of chemistry to solve problems.
Assess areas of strength and weakness for review purposes.
Improve problem solving strategy and learning efficiency.

Stoichiometry

STOICHIOMETRY is the quantitative relationship of reactants and products. This unit has been divided into the following objects. A brief review is given here so that you can get a birds'-eye or overall view of stoichiometry.

Amounts of substances
Express amounts of substance in mass units of g, kg, tons, and convert them to moles, kilomoles, or millimoles.
Chemical formulas
Represent a substance with a formula that reflects its chemical composition, structure, and bonding; evaluate weight and mole percentages of elements in a substance; and determine chemical formula by elemental analysis.
Reaction features
Define some common features of chemical reactions; classify chemical reactions by common features such as combination, combustion, decomposition, displacement, and redox reactions.
Reaction equations
Express quantitative relationship using chemical reaction equations; evaluate quantities of reactants and products in a chemical reaction; and solve reaction stoichiometry problems.
Excess and limiting reagents
Define excess and limiting reagents; determine excess and limiting reagents in a reaction mixture; and determine quantities produced in a chemical reaction.
Yields
Define theoretical and actual yields due to limiting reagent; apply the concept of limiting reagent to evaluate theoretical yield; convert actual yield to percentage yields.

A Typical Quiz

In these quizzes, we give a set of question that are inter related.

Problem 1

You have weighed x g of CaCl₂ (molar mass 111) for an experiment dealing with chloride ions and you need to find the number of moles of Cl^- ions. Show the conversion from x g CaCl₂ to moles of Cl^- ions.

Hint...
The conversion logic in given below:

          1 mol CaCl₂   2 mol Cl^-      2 x
x g CaCl₂ -----------  ----------- = ------ mol Cl^-
          111 g CaCl₂  1 mol CaCl₂     111

Skill...
Convert amounts of substances in various units.

Problem 2

You have a sample weighing 2.345 g containing equal weight of CaCl₂ (f.wt. 111) and AlCl₃ (f.wt. 133). How many mole of Cl^- ions do you have?

Hint...
Assume you understand the previous Example, then you can give

   2 mol     3 mol     2.345 g
 ( -----  +  ----- ) x ------- = 0.04757 mol Cl^-.
   111 g     133 g       2

Skill...
Develop skills to calculate moles of chloride ions in problems containing unequal weights of CaCl₂ and AlCl₃.

Problem 3

When a 2.00-g mixture of CaCl₂ (111) and RbCl (121) was analyzed, the result reports that the mixture contains a total of 0.0241 mol Cl^- ions. Find weight percentage of RbCl in the sample.

Hint...
Using the method in the previous examples, and assume x g RbCl are present in the mixture, you have

   x     2 (2.00-x)
 ----- + -----------  =  0.0241
  121      111

Solving for x, x = 1.23 g (Practice your solution)

Percentage = 1.23 / 2.00 = 61.4 % RbCl.

Skill...
Making an assumption to make the problem solvable.

Problem 4

When a 2.00-g mixture of CaCl₂ (111) and RbCl (121) was received by an analytical laboratory, the technician dissolved it in a solution and then he used excess amount of AgNO₃ to precipitate the chloride as AgCl. He collected 3.456 g AgCl. What is the percentage of RbCl in the sample?

Hint...
We should convert 3.456 g AgCl into moles of AgCl or Cl^- ions.

             1 mol AgCl
3.456 g AgCl ------------ = 0.0241 mol AgCl or Cl^-
             143.4 g AgCl

The rest of the solution has already been given in Problem 3.

Skill...
Building skills to do useful work.

Problem 5

You are interviewed for a manager's job, and you were asked what you should instruct your technician to do for the following situation. Company X has received a sample containing a mixture of CaCl₂ and BaCl₂. The client of Company X needs to know the percentages of the two components.

Hint...
Instruct your technician to find out if the sample contains any other substance than CaCl₂ and BaCl₂

If it does not contain other substance than Ca²⁺, Ba²⁺, and Cl^- ions, instruct him to precipitate the chloride ions with an excess amount of silver ion.

Instruct the technician how to calculate the moles of chloride present, and how to calculate the percentages.

Skill...
Write instructions for laboratory process.

Example Problems

When properly used, urea, CH₄N₂O, is a good candidate for fertilizer. Unocal is one of the company that produces urea in Kenai, Alaska. It produces 1.1 million tons of urea each year. How many moles is this amount?
Urea is also called carbamide or carbonyldiamide. Since this is a US factory, 1 ton is taken as 983 kg that is equivalent to 18 thousand moles.
Skill -
Converting from mass units to moles and vice versa.
Urea is made by reacting carbon dioxide (CO₂) with anhydrous ammonia (NH₃) under 3,000 psi (pounds per square inch) pressure and at 350°F (177°C or 450 K). CO₂ + 2 NH₃ = CO(NH₂)₂ + H₂O
. 44 . . . . 34 . . = . 60 . . . . . . . . 18 (molar mass) The removal of water that occurs during the reaction is referred to as "dehydration." The resulting molten mixture is further processed into either prills or granules. For each ton of urea made, how much water is removed?
For 1.3 tons of reactants, 1.0 ton of urea is produced. By the way, 3,000 psi is equivalent to 200 atmosphere pressure.
Skill -
Interpret stoichiometry among products and reactants.
The NPK numbers (for example, 25, 3, 7) used in the fertilizer industry refers to the percentage of nitrogen, N, phosphorus, P, and potassium, K. What are the NPK numbers for pure urea?
Based on the formula, urea contains 46.7% N. However, the factory usually gives 46 because most products contain water.
Skill -
Calculate weight and mole percentage from chemical formula.
Conversion of nitrogen into protein by plants is a complicated process. First, urea is hydrolyzed and converted to ammonium carbonate. The hydrolysis of urea is brought about by the unease enzyme, but the reaction may be written as follows: CO(NH₂)₂ + 2 H₂O = (NH₄)₂CO₃
The product is a salt. The solubility of urea is reported to be 1 g in 1 ml of water. Assume that this solution is allowed to hydrolyze by the unease enzyme, which is the limiting reagent, urea or water?
One g of urea is 17 millimoles, which requires 34 millimoles of water to react from the stoichiometry of the reaction equation. One g of water is 56 millimole, that is more than 34 millimoles.
Skill -
Determine excess and limiting reagent.
Alanine CH₃CH(NH₂)(COOH) is one of the aminoacids that are the building blocks of proteins. What is the percentage of nitrogen in this aminoacid?
Skill -
Determine an element percentage from chemical formula.
An old method for protein analysis was to measure the nitrogen content. The amount of protein present was assumed to be 16 times the amount of nitrogen.
Assume that 50 % of the nitrogen in urea is converted to protein by plants. How much protein is produced for one gram of urea?
Percentages of protein in plant material are usually very low.
Skill -
Calculate theoretical yield, actual yield, and percentage yield.
A solid mixture containing Na₂SO₄ and NaCl. What compound would you use to precipitate the Cl^- ion?
Skill -
Apply what you know of substances. AgCl is one of the insoluble few chloride.
A solid mixture containing Na₂SO₄ and NaCl. What compound would you use to precipitate the SO₄^2- ion?
BaCl₂ is a soluble salt, but BaSO₄ is not.
Skill -
Apply what you know of substances.
A 2.21 g mixture containing Na₂SO₄ and NaCl dissolved in water is treated with BaCl₂. The resulting dry BaSO₄ weighs 2.57 g. How many mole(s) of BaSO₄ was collected? (formula wt. BaSO₄, 233.39)
2.57 g / (233.39 g/mol) = ? mol

How many mole(s) of Na₂SO₄ was present in the mixture? (Also 0.0110 mol)
Skill -
Performing unit conversion tasks.
A 2.21 g mixture containing Na₂SO₄ and NaCl dissolved in water is treated with BaCl₂, and analysis shows 0.01101 mol Na₂SO₄ was present. What is the weight of Na₂SO₄? (formula wt. Na₂SO₄, 142.04)
Skill -
Performing unit conversion tasks.
A 2.21 g mixture containing 1.56 g Na₂SO₄ and some unknown impurities is treated with BaCl₂. How many grams of BaSO₄ should form? (formula wt. Na₂SO₄, 142.04, BaSO₄, 233.39)
Conversion calculation:
```
             233.39 g/mol BaSO₄
1.56 g NaSO₄ ------------------- = ? g BaSO₄
             142.04 g/mol Na₂SO₄
```
Skill -

You have acquired all the skills to perform chemical analysis to find out the percentage of certain component in a mixture. For example, consider this problem.
A 2.21 g mixture containing Na₂SO₄ and NaCl dissolved in water is treated with BaCl₂, and 2.57 g of BaSO₄ was obtained. What is the percentage of NaCl in the sample?
Which one of the following samples has the highest percentage of chloride?

pure CaCl₂
pure AlCl₃
equal weight mixture of CaCl₂ and AlCl₃
pure NaCl
pure BaCl₂

Skill -
Interpret chemical formula
The reaction
C₃H₈ + 5 O₂ = 3 CO₂ + 4 H₂O
is best classified as a

formation reaction
decomposition reaction
combustion reaction
exchange reaction
displacement reaction

Skill -
Describe the major features of chemical reactions.
What chemical (give the formula) would you use in order to determine the percentage of CaCl₂ in a solid mixture of CaCl₂ and Ca(NO₃)₂. Assume you can first dissolve the solid in water.
Skill -
Apply the rule: salts of nitrates are usually soluble.
A 1.140 g mixture of NaCl and CaCl₂ dissolved in water is mixed with sufficient solution of AgNO₃ to give 2.868 g of dry AgCl. Calculate the WEIGHT percentage of NaCl? (Na, 23.0; Cl, 35.5; Ca, 40; Ag, 107.9)
Skill -
Technique suggestion:
2.868 g AgCl = 0.0200 mol (Cl^- or AgCl)
Assume the mixture contains x g NaCl, then (1.140-x) g is CaCl₂. Thus, we have:
# mol NaCl + 2 # mol CaCl₂ = # mol Cl^- lead to
```
   x g NaCl         2 (1.140 - x) CaCl₂
--------------  +  --------------------  = 0.0200 mol
58.5 g /mol          111.1 g/mol
```
Solve the equation for x (x = 0.585 g) and calculate the percentage.
A 1.140 g mixture of NaCl and CaCl₂ dissolved in water is mixed with sufficient solution of AgNO₃ to give 2.868 g of dry AgCl. Calculate the MOLE percentage of NaCl? (Na, 23.0; Cl, 35.5; Ca, 40; Ag, 107.9)
Skill -
Determine the weight and mole percentages of mixtures.

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