Chem 2 Midterm Review Questions

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FS/08 Chem 2 Midterm Review Questions
1. (Zinc) For the following data set (6.1, 7.2, 6.5, 4.9, 5.3, 5.9)
a. Determine the mean (average).
b. Determine the standard deviation estimate. (Why are we using the standard deviation
estimate and not the standard deviation?)
c. Determine the confidence interval for a single value at 90% and the confidence
interval for the mean at 90% where t = 2.015.
d. How many degrees of freedom does this data set have? How do you determine
degrees of freedom?
2. (Zinc)
Pennies that have been made after 1982 are a composite of zinc and copper. The copper is
plated on top of the zinc. The zinc can be removed from the penny by cutting the coin and creating a
reaction between the zinc and concentrated hydrochloric acid. The copper does not react with
hydrochloric acid.
The penny was measured using Vernier calipers. The diameter was 1.79 cm and the height was
0.13 cm. The penny was weighed and the mass determined to be 2.518 grams. The penny was cut and
placed in excess hydrochloric acid. After the solution stopped bubbling, the penny shell was removed.
The penny shell was weighed and the mass of copper was determined to be 0.071 grams. The density of
3
copper is 8.94 g/cm
.
a. What is the volume of copper (the copper penny shell) in the penny?
b. Calculate the surface area of the copper. The surface area of the copper is equal to the
surface area of the penny. The surface area of a penny can be approximated by using the
2
equation for the surface area of a cylinder: SA
= 2(πr
) + 2πrh.
cylinder
c.
Determine the thickness of the copper coating present in the penny.
d. What is the mass percent of copper in the penny?
e. Since 1982, pennies are composed of 97.5% zinc and 2.5% copper by mass. Calculate the
percent error between the mass percent of copper in the previous answer and the
expected value.
3. (E.F.)
A student analyzes a sample of a material that is known to contain no elements other than
molybdenum (Mo) and sulfur (S). In the student’s experiment, the mass of a sample of molybdenum
sulfide, Mo
S
, is determined to be 0.583 g. The sample is heated in an acid solution and H
S gas is
x
y
2
liberated leaving only molybdenum. The final weight is 0.255 g.
a.
Determine the mass percent of Mo in the sample of molybdenum sulfide, Mo
S
.
x
y
b. Determine the mass of the S in the sample of molybdenum sulfide, Mo
S
.
x
y
c. Determine the mass percent of S in the sample of molybdenum sulfide, Mo
S
.
x
y
d. Determine the empirical formula of the molybdenum sulfide sample based on the values
closest to the values calculated in a & b. The Empirical Formula for this compound is:
Mo
S
, MoS, MoS
, or MoS
2
3
3
4
4. (T.M.) A mixture is known to contain the four compounds in the table. A.) Draw a flow
chart to show the steps that you would use to separate the following compounds. B.) The initial
mass was 5.025g. The resulting masses were benzoic acid = 1.760g, Mg(OH)
= 0.754g, Na
SO
2
2
4
=1.005g, and Zn(OH)
1.256g. Calculate the percent recovery of each component and the total
2
percent recovery.
Cold water
Hot water
3M HCl
3M NaOH
benzoic acid
no
yes
no
yes
Mg(OH)
no
no
yes
no
2
Na
SO
yes
yes
yes
yes
2
4
Zn(OH)
no
no
yes
yes
2

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