Introductory Chemistry

Quiz: Writing and using balanced equations;
   moles; stoichiometry.

Cracolice 2/e Ch 7-8-9.

Answer key
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Abbreviations: CF = conversion factor(s); PT = periodic table; SF = significant figure(s).

* Quiz is "closed book" except for PT & calculator.
* If a question requests an explanation, there may be no credit unless you provide an explanation.
* Show clear work for all calculations.
* Use proper SF.
* Show phase for each chemical in your equations.
* Avogadro's number: 6.02 x 1023 (things per mole)

1. Silver nitrate and sodium sulfide solutions are mixed, and there is a black precipitate. Write a balanced equation for the reaction. Show phases. (Silver ion has 1+ charge.)

2. A student is asked to write and balance the equation for a simple double replacement reaction. The student first writes:

Na2SO4 (aq) + Ba(NO3)2 (aq) --> Na2NO3 (aq) + BaSO4 (s)

At that point, the student is stuck. You are more careful; please rescue the student.

3. Calculate the formula mass (molar mass) of sodium tetraborate decahydrate, commonly known as borax. Units?? (The tetraborate ion is B4O72-; deca means 10.)

4. You have 252 g of magnesium sulfate. How many moles is that? Show clear work; with clear units. Use dimensional analysis. Give proper significant figures in your answer.

5. The following balanced equation describes a useful way to make a little chlorine:

2 KMnO4 (aq) + 8 HCl (aq) --> 3 Cl2 (g) + 2 MnO2 (s) + 4 H2O (l) + 2 KCl (aq)

In parts a & b, show clear work. Use dimensional analysis. Report the proper number of SF. Be sure to use enough SF for each molar mass.

a. If 0.21 moles of potassium permanganate react, how many moles of hydrochloric acid are needed?

b. How many grams of chlorine would be produced if 14.2 g of potassium permanganate reacts?

c. How would you be able to separate the desired product (chlorine) from all the other chemicals (left over reactants and other products) in the reaction vessel?

6. You measure the mass of 25 mL of water; it is about 25 g. How many dozens of molecules are in 25 g of water? Use dimensional analysis; show clear work. [Think about... the path from the given (grams) to what is wanted (dozens). It takes several steps. Spend some time trying to find a path. Careful trial and error is often the secret to problems like this; watch the units.]

7. The atom is so small that its size was quite unknown for even decades as the atomic theory took hold. In 1851 the German chemist Justus von Liebig estimated that one microgram (1 μg) of cinnabar -- mercury (II) sulfide -- probably contained millions of atoms. How close was he?

8. This problem deals with energy terms. A major purpose is to introduce you to the energy units you will find on your utility bill (locally, PG&E); it will give you a feel for the magnitude of the various energy units. The problem is also good practice with clear dimensional analysis. Part a is very similar to problems and examples in the textbook (Cracolice Section 9.11), and should be considered "basic"; this part also sets the groundwork for the later parts.

In addition to standard conversion factors, you may need some of the following:
* octane is C8H18; density 0.703 g/mL; heat of combustion = 5.47x103 kJ/mol octane.
* 1 Btu (British thermal unit) = 1055 J; 1 therm = 105 Btu; 1 kwh (kilowatt-hour) = 3.6x106 J.

a. Consider 1 gallon of gasoline. For simplicity, we will use octane for the calculations. (The composition of gasoline varies.) How much energy, in kilojoules, is released by the complete combustion of one gallon of gasoline (octane)?

An interesting issue has come up with this question. This quiz was originally written for my own classes. I assumed that my students can look up standard conversion factors; in fact, I would prefer that they do so, rather than me give them all. However, a person in England wrote me that he did not get my answers for this question; his answers (to this part and therefore to all the following parts) were 20% higher than mine. Why? Because the UK gallon (the "imperial gallon") is 20% larger than the US gallon! So, now that my quiz is on the web for all to use, I think I need to specify that the question is about the US gallon, which is 3.785 liters.

If you find other cases where a question seems to be unclear to the wider audience, because of "cultural assumptions" or such, please let me know.

b. On our local utility bills, gas (i.e., natural gas) usage is given in therms. Convert your answer from part a (the energy released by burning one gallon of gasoline) to therms.

c. On a recent bill, the gas was charged at $0.929 per therm. At that rate for natural gas usage, how much would you be billed for the amount of energy you calculated in part b?

The energy prices given above and in part e below are from April 2005 (PG&E, San Francisco area).

d. On our local utility bills, electricity usage is given in kwh (kilowatt-hours). Convert your answer from part a (the energy released by burning one gallon of gasoline) to kwh.

e. On a recent bill, the electricity was charged at $0.114 per kwh. At that rate for electricity usage, how much would you be billed for the amount of energy you calculated in part d?

To make it easier to compare answers, and to do individual parts, I've shown all answers to 3 SF, and when relevant, used those 3 SF answers for following parts. The data of the problem probably warrant only 2 SF, so I am effectively carrying an extra digit. It is often a good procedure to carry all digits on your calculator, and round only at the end. But the procedure I follow here makes it easier to discuss individual parts.

9. (For fun.) A student went home one day and wanted to impress her family with the usefulness of what she had learned. She asked someone to bring her a glass of a mixture of solid and liquid dihydrogen monoxide. In plain English (i.e., common terms), what did she ask for?


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Last update: August 13, 2011