Chemistry& 141
Clark College
Homework 4 SOLUTION
Follow the homework guidelines given in the syllabus! Show your work, use units and sig figs, where
appropriate.
1. Assign oxidation states for the specified element in each of the following compounds:
a) Aluminum in aluminum sulfate (Al2(SO4)3) Single atom ions have oxidation states equal to their
charge. Al3+ = +III
b) Chromium in sodium dichromate The correct ionic formula is Na2 Cr2O7. To determine the
oxidation state on Cr, consider the charges on Na and O, and the net charge on the compound,
which is zero:
Na2
Cr2
O7
2 (+1) +
2x +
7 (-2)
=0
solving for x, we find that the oxidation state for each Cr is +VI.
c) Sulfur in Cu2SO3 The sulfite ion has a -2 charge. Using this:
S
O3
x + 3(-2) = = -2
solving for x, we find that the oxidation state for S is +IV.
d) Phosphorus in K3PO4 The oxidation of P is determined in the same fashion as chromium:
K3
P
O4
3 (+1) +
x+
4 (-2)
=0
solving for x, we find that the oxidation state for P is +V
2. For an experiment that you are performing, you need to create 250.0 mL of a 0.357M solution of
sodium sulfate. How much solid sodium sulfate (in mg) should be added to 250.0 mL of water to
create such a solution?
To approach this problem, we need to determine the number of moles present in the solution, using the
concentration and the volume. Once we have # moles, we can use the molecular weight of Na2SO4 to
find g and mg.
mol
x 0.2500 L = 0.0893 mol Na 2 SO 4 needed
L
142.037 g 1000 mg
0.0893 mol Na 2 SO 4 x
x
= 12,700 mg
mol
g
0.357
3. An experiment calls for you to use 200.0 mL of 1.0 M HNO3. All you have available is a bottle of
concentrated nitric acid, which is 15.9 M. How much conc. nitric acid should be used to create the
1.0 M solution?
For dilution problems, we use the expression M1V1 = M2 V2.
M1 V1 = M 2 V2
(1.0 M ) ( 200.0mL ) = (15.9 M ) ( x mL )
x = 13 mL
Homework 4
Fall 2008
Page 1 of 2
Chemistry& 141
Clark College
4. A flask contains 49.8 mL of 0.150 M mercury (I) nitrate. How many milliliters of 0.250 M sodium
chloride are needed to completely precipitate the mercury (I) out of solution? Write a balanced
equation for this reaction and determine the volume needed.
The trick with this one is remembering that mercury (I) is Hg22+!
Hg2(NO3)2 (aq) + 2 NaCl (aq)  Hg2Cl2 (s) + 2 NaNO3 (aq)
49.8 mL x
1L
0.150 mol
2 mol NaCl
L
x
x
x
= 0.0598 L or 59.8 mL
1000 mL
L
1 mol Hg 2 Cl2
0.250 mol
5. You want to analyze a sterling silver spoon for its’ silver content. You do so by taking a piece of the
spoon, weighing 1.175 g, and dissolving it in aqueous nitric acid to convert the silver metal to silver
nitrate. To precipitate out the silver, you add excess sodium chloride to the silver nitrate in the
reaction flask.
a) What is the solid that forms? Write molecular and net ionic equations to represent the two
reactions: the first between silver metal and nitric acid, and the second between silver nitrate and
sodium chloride.
molecular:
AgNO3 (aq) + NaCl (aq)  AgCl (s) + NaNO3 (aq)
net ionic:
Ag+ (aq) + Cl- (aq)  AgCl (s)
b) You isolate 1.449 g of the final solid product. How many grams of silver does it correspond to?
c) What is the mass percent of silver in the spoon?
1 mol Ag
107.868 g
mol
x
x
= 1.091 g Ag
143.321 g 1 mol AgCl
mol
1.091 g Ag
mass percent:
x 100% = 92.8%
1.175 g sample
1.449 g AgCl x
Homework 4
Fall 2008
Page 2 of 2