Leader:
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Instructor:
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Exam Review
Supplemental Instruction
Iowa State University
Grant DeRocher
Chem 167
Houk
02/07/13
1. 1 (12 pts) Fill in the table below with the correct number of protons, neutrons and electrons
# of protons
# of neutrons
# of electrons
36
_25______
11_______
26_______
75
___50____
__25_____
__48_____
CrSn2+
43
K3-
_____19__
____24___
____22___
54
Ar
_______18
______36_
______18_
2. The element Ga has a molar mass of 69.723 g/mole and has only two isotopes. Isotope A has
mass number 68.926 g/mole; Isotope B is .797 g/mole heavier than Isotope A. What are the
mass numbers (g/mole) and abundances of the two isotopes? Show work for partial credit; no
credit for guessing. Keep three sig figs.
Mass
Number
%
Abundance
Isotope A
___68.9256____
___.601_____
Isotope B
___70.9247____
_______.399_
68.9256x+70.9247y=69.723
x+y=1
x=1-y 68.9256(1-y)+70.9247y=69.723
y=.399 x=.601
3. Calculate the molar mass of the following organic compounds. Use the following simple
atomic masses: H = 1.00 C = 12.00 N = 14.00 O = 16.00 g/mole. Do not worry about sig figs.
A. An alkene with three carbon atoms.(Draw picture first)
MM = __42_____ g/mole
6*H= 6.00
3*C=36.00
total=42.00
B. An amide with 7 hydrogen and 3 carbon atoms
MM = ___73____ g/mole
7*1+12*3+1*14+1*16=73
C. Dimethal Ether-ether with 6 Hydrogens
6*1+12*2+1*16= 46
4. Given 115 mL of an aqueous solution of potassium sulfate is analyzed and found to have
250mg of potassium. 75 mL of water are added. What is the molar concentration of potassium
sulfate? (moles per liter) in the second solution after the 75 mL of water is added? Follow sig
figs. Atomic weights: potassium=39.098g/mol and sulfate= 96.061g/mol
K2SO4
2K+
+
SO42-
250mg K X 1 g K
X 1 mol K
X
190mL
1000mg K
39.098g K
1 mol
2 mol
1000 mL = .0165M but only 2 sig figs
1 L
5. Copper reacts with sulfuric acid according to the following equation:
2H2SO4 +Cu
CuSO4 + 2H2O + SO2 (not balanced)
How many grams of sulfur dioxide are created by this reaction if 14.2 g copper reacts with 18.0 g
of sulfuric acid? Use 3 sig figs.
Must find limiting reactant. Put both in terms of moles.
14.2g Cu/63.55g/mol= .2203 mol Cu
18.0g H2SO4/ 98.09g/mol= .1835 mol H2SO4
Start with one and figure out how much you would need of the other to completely react.
Compare this to how much you actually have in order to determine limiting reactant.
.2203 mol Cu * (2 mol H2SO4/1 mol Cu) = .4406 mol H2SO4 required to react completely. We
have .1835 mol which is not enough and so H2SO4 is our limiting reactant. So we need to use the
number of moles of H2SO4 given to find grams of sulfur dioxide.
Now stoichiometry .1835 mol H2SO4*(1 mol SO2/2 mol H2SO4)*(64.07g SO2 /1 mol SO2)
=5.88 g SO2
6. (borrowed from SI Session 6 worksheet. This was a problem #9 we never got to yet.) If 21 g
of H2S is mixed with 38g O2 and it forms 31 g of SO2, what is the percent yield? (Two step
process, think about the things we’ve been working on so far).
2H2S + 3O2
2SO2 + 2H2O
do a limiting reactant first to find out which of the reactants is the limiting reactant and use that
to find the percent yield. So, 21gSO2*1mol
SO2/34.08gSO2*3molO2/2molH2S*32.00gO2/1molO2=29.6 g O2 required. Since we have 38 g
O2 then H2S is our limiting reactant and we can use that to find percent yield.
21g H2S*1molH2S/34.08g H2S*2molSO2/2molH2S*64.06gSO2/1molSO2= 39g SO2
produced.
So 31/39=79% yield
7. The amount of one gas is under the following conditions. What is the gas? No guessing. Must
show work.
Mass= 57.33 g
Volume= 10.0-L
Temperature= 35 degrees C
Pressure= 1.73 atm
R= .08206 L*atm/mol*K
T=35+273.15
N=PV/RT= 17.3/.08206*308.15= .68415 mol
57.33g/.68415 mol=83.80 grams per mole which is Kr gas
C4H12O6 + 4 O2 = 4 CO2 + 6 H2O
Things to know how to do:
-balance equations
-be able to name and identify common ionic and covalent bonds
-use ratios to cancel units to find desired answer
-be able to tell if your answer is reasonable for the problem
-convert grams to moles and vice versa. (molar mass) and how to use it in problems
-know functional groups
-know formula from a reduced picture
-know common cations and anions
-know which ionic compounds are soluble and which are insoluble
-know which acids and bases are strong and weak and what that means about dissociation
-know the difference between molecular and empirical formula and know how to find both.
- know what molar concentration is
-know dilution equation and how to solve dilution problems
- know the basic alkanes
-know how to do stoichiometry problems (using molar ratios from balanced eqns etc)
-know how to find a limiting reactant
-know how to find theoretical and percent yields
-know the ideal gas law and how to manipulate it to solve for what you are trying to find
-know how to find partial pressures and total pressure of gas mixtures
-know the kinetic molecular theory of gasses and when it does not work
-know vander wall’s formula and how to use it