Chem 11 - Sample Exam #1 Solutions

Sample Exams are designed to give you an idea of the types of questions you can expect.  Exam numbers may not exactly agree with currently assigned material, so you may need to examine other sample exams to see questions that might be similar to the upcoming exam. I have made every effort to show correct answers, but there might be an incorrect answer, so if in doubt, you are probably right.

1. Please give the correct abbreviation for each of the following units of measurement. Then, express its numerical value in Liter, gram, or meter units.  For example: nanogram is abbreviated ng, and 1 ng = 10^-9 g.

• microliter = mL = 10^-6 L
• millimeter = mL = 10^-3 m
• kilogram = kg = 10³ g
• centimeter = cm = 10^-2 m
• deciliter = dL = 10^-1 L
• picogram = pg = 10^-12 g = 10^-15 kg

2. Express 784,000,000 in exponential (scientific) notation. ____7.84 x 10⁸_________________

How many significant figures are there in the number 0.0006042? _____4__________________

3. The degree of agreement among several measurements of the same quantity is called precision and it reflects the reproducibility of a given type of measurement. The agreement of a particular value with the true value is called accuracy.

Substances with constant composition that can be broken down into elements by chemical processes are compounds. The state of matter for an object that has a definite volume but not a definite shape is liquid. A pure substance can be either element or compound.

Give two examples that illustrate the law of multiple proportions: CO and CO₂ & NH₃ and N₂H₄

4. A sample of copper weighing 6.93 g contains how many moles of copper atoms? How many atoms?

Answers: 0.109 mole Cu atoms and 6.57 x 10²² atoms

5. A piece of indium with a mass of 16.6 g is submerged in a graduated cylinder containing 46.3 mL of water. After addition of this metal sample, the water level increases to 48.6 mL. What is the density of indium from these data? If the melting point of indium is 156.2^oC, what will be the physical state of indium at 323^oF?

Answers: Density = 7.2 g/cm³ and Indium would we liquid at 323 ^oF

6. Phosphoric acid can be prepared by reaction of sulfuric acid with "phosphate rock" according to the equation:
Ca₃(PO₄)₂ + 3H₂SO₄ ---> 3CaSO₄ + 2H₃PO₄

What is the molar mass of Ca₃(PO₄)₂ ?   Molar Mass = 310.19 g/mol

How many oxygen atoms are there in 1.55 ng of Ca₃(PO₄)₂ ? Answer: 2.4 x 10¹³ oxygen atoms

Suppose the reaction is carried out starting with 103 g of Ca₃(PO₄)₂ and 75.0 g of H₂SO₄. Which reactant would be the limiting reactant? Answer: H₂SO₄ is limiting reactant

7. A substance contains 35.0% nitrogen, 5.05% hydrogen, and 60.0% of oxygen. What is its empirical formula? How many grams of hydrogen are there in a 185-g sample of the substance?

Answer: Empirical Formula is N₂H₄O₃  and 9.25 g hydrogen

8. Nitroglycerin is C₃H₅(NO₃)₃ (Empirical Formula: C₃H₅N₃O₉). (i) Determine its percent composition (%C, %H, %N, %O). When nitroglycerin is used as an explosive (dynamite), it produces gaseous nitrogen, water, carbon dioxide and oxygen. (ii) Write a fully balanced equation for this reaction (it will take 3-4 passes to fully balance this equation). Then, based on this balanced equation, how many moles of nitrogen would be produced if you started with 100 grams of pure nitroglycerin?

%C = 15.86%

%H = 2.22%

%N =  18.50%

%O = 63.44%

Answer: 4 C₃H₅N₃O₉ ---> 6 N₂ + 10 H₂O + 12 CO₂ + O₂ 

Answer: There are 0.661 mol N₂ of 100 grams of nitroglycerin (Molar Mass: 227/mol)

9. Write the chemical formula for each of the following compounds or ions (ions must show the correct charge).

nitrate ion = NO₃^-

aluminum oxide Al₂O₃

ammonium ion = NH₄⁺ 

copper(II) bromide = CuBr₂ 

aluminum hydroxide = Al(OH)₃ 

hydrosulfuric acid = H₂S (aq) 

sulfurous acid = H₂SO₃ (aq) 

nitric acid = HNO₃ (aq) 

phosphoric acid = H₃PO₄ (aq) 

perchloric acid = HClO₄ (aq) 

potassium chromate = K₂CrO₄ 

chromium(III) carbonate = Cr₂(CO₃)₃ 

dinitrogen pentoxide = N₂O₅ 

copper(I) oxide = Cu₂O 

10. Write the correct name for each of the following compounds.

NH₄NO₃ = ammonium nitrate

NaH = sodium hydride

K₂Cr₂O₇ = potassium dichromate

CCl₄ =  carbon tetrachloride

AgCl = silver chloride

CaS = calcium sulfide 

HNO₂ (aq) = nitrous acid 

N₂O₃ = dinitrogen trioxide

SnI₂ = tin(II) iodide

Al₂(SO₄)₃ = aluminum sulfate 

11. For each of the following, make the indicated conversions (show ALL work):

7.5 miles to kilometers (1 mile = 1.609 km): = 12.0675 km

-40.00 ^oC to ^oF: = -40.00 ^oF

68.00 ^oC to K: = 341.15 K

12. For the following isotopes, give the atomic number (Z), number of protons, electrons, and neutrons present.

Isotope	Z	# protons	#electrons	#neutrons
45Ti	22	22	22	23
64Zn	30	30	30	34
72Ge	32	32	32	40
41K	19	19	19	21

___ T _XX_ F Atoms that have the same number of neutrons but different numbers of protons are called isotopes.

___ T _XX_ F The mass number of a nucleus represents the total number of protons in the nucleus.

Extra Credit (5 points)

Suppose that you lived in 1900 and discovered a new element and wanted to determine its atomic mass. You analyze a mass spectrum and find that there are two isotopes. One of the isotopes has an atomic mass of 20.011 and the other 22.015. The first isotope accounts for 35.00% of the total. What is the average atomic mass of this element as it occurs in nature?

Answer: 21.31 amu for average atomic mass for element X