Chemistry 1000 (Summer 2006)
Problem Set #7: Chapter 3
Answers to Practice Problems
1.
(a)
(b)
(c)
(d)
(e)
(f)
The following list contains examples of ionic compounds better described using condensed
formulae. For each formula unit, list the most likely condensed formula and name that
compound.
e.g. MgO2H2 is unclear. It’s better written as Mg(OH)2 or magnesium hydroxide.
Fe2C3O9
Fe2(CO3)3
iron(III) carbonate
N3H12PO3
(NH4)3PO3
ammonium phosphite
SnH2S2O8
Sn(HSO4)2
tin(II) bisulfate or tin(II) hydrogen sulfate
Ti2S3O9
Ti2(SO3)3
titanium(III) sulfite
PbC4N4
Pb(CN)4
lead(IV) cyanide
N2H4O2
NH4NO2
ammonium nitrite
2.
Lesley is a research assistant who goes scuba diving for sponges which she takes back to the
lab. There, she extracts and purifies molecules which might have medical applications. Her
latest sample contained a particularly interesting drug lead which she is currently calling
Leslamine. When she analyzed Leslamine, she found that it had a molecular mass of 274.37
g/mol and contained 70.05% carbon, 8.08% hydrogen, 10.20% nitrogen and 11.67%
oxygen.
(a)
What is the empirical formula of Leslamine?
In 100 g of Leslamine:
mC = 70.05 g
mH = 8.08 g
mN = 10.20 g
mO = 11.67 g
nC = mC
UC
= 70.05 g C .
12.011 g/mol
nC = 5.832 mol C
nH = mH
UH
= 8.08 g H .
1.0079 g/mol
nH = 8.02 mol H
nN = mN
UN
= 10.20 g N .
14.0067 g/mol
nN = 0.7282 mol N
nO = mO
UO
= 11.67 g O .
15.9994 g/mol
nO = 0.7294 mol O
Therefore,
nC : nH : nN : nO = 5.832 mol C : 8.02 mol H : 0.7282 mol N : 0.7294 mol O
0.7282
0.7282
0.7282
0.7282
nC : nH : nN : nO = 8.009 mol C : 11.0 mol H : 1.000 mol N : 1.002 mol O
Therefore,
the empirical formula is
C8H11NO
(b)
What is the molecular formula of Leslamine?
UC8H11NO = 137.181 g/mol
ULeslamine = 274.37 g/mol = 2.0000
UC8H11NO
137.181 g/mol
Therefore,
the molecular formula is twice the empirical formula
Therefore,
the molecular formula is
C16H22N2O2
3.
Michael needs anhydrous chromium(III) bromide for his next experiment but all he has
available is chromium(III) bromide hexahydrate. Since he has a hot oven available, he
decides that he’ll use that to make some anhydrous chromium(III) bromide by dehydrating
the hydrate. What mass of hydrate must he use in order to obtain 25.00 g of anhydrous
chromium(III) bromide?
CrBr3 · 6H2O
399.799 g/mol
nCrBr3 = mCrBr3
UCrBr3
=
25.00 g
.
291.708 g/mol
nCrBr3 = 0.08570 mol
→
CrBr3
+
6 H2O
291.708 g/mol
nCrBr3.3H2O = nCrBr3 = 0.08570 mol
mCrBr3.3H2O = nCrBr3.3H2O × UCrBr3.3H2O
= (0.08570 mol) × (399.799 g/mol)
mCrBr3.3H2O = 34.26 g
4.
(a)
(b)
(c)
The jar of copper(I) oxide that Jodie has been working with for the past year has started to
turn black, so she suspects that some of the contents have reacted with air and been oxidized
to copper(II) oxide. She submits a sample for elemental analysis, and is told that the sample
contained 82.35% copper and 17.65% oxygen by weight.
Calculate the percent composition for copper(I) oxide and for copper(II) oxide.
copper(I) oxide = Cu2O
%Cu = UCu2 × 100%
%O = UO × 100%
UCu2O
UCu2O
= (2 × 63.546 g/mol) × 100%
= 15.9994 g/mol × 100%
143.091 g/mol
143.091 g/mol
%Cu = 88.819%
%O = 11.1812%
copper(II) oxide = CuO
%Cu = UCu × 100%
%O = UO × 100%
UCuO
UCuO
= 63.546 g/mol × 100%
= 15.9994 g/mol × 100%
79.545 g/mol
79.545 g/mol
%Cu = 79.886%
%O = 20.114%
Was Jodie’s sample copper(I) oxide, copper(II) oxide or a mixture of the two? Why?
It was a mixture. The experimental percent composition fell between the sets
calculated for copper(I) oxide and copper(II) oxide.
Are the contents of the jar likely to be exactly the same as the contents of the sample? Why
or why not?
Probably not. If the copper(II) oxide formed due to reaction with air, there’s likely
more at the top of the jar than the bottom. Unless she thoroughly mixed the contents
of the jar before taking the sample, it’s not necessarily representative of the average
contents of the jar. (It does however confirm her suspicion that the jar contains some
copper(II) oxide.)
5.
(a)
Neil found a jar of nickel(II) acetate sitting open in the back of the storeroom. He knew this
compound formed a hydrate but didn’t know which one. Curious, he carefully weighed out
1.000 g of the hydrate into an evaporating dish, left it in a hot oven until it had completely
dehydrated and recorded the final weight of the sample (0.710 g).
Give the formula for the hydrate.
Ni(CH3CO2)2 · xH2O
nanhydrous = manhydrous
Uanhydrous
=
0.710 g
.
176.782 g/mol
nanhydrous = 0.00402 mol
→
Ni(CH3CO2)2
+
x H2O
176.782 g/mol
18.0152 g/mol
nwater = mwater
Uwater
= (1.000g – 0.710g)
18.0152 g/mol
nwater = 0.0161 mol
x = nwater .
1
nanhydrous
x = 0.0161 mol
0.00402 mol
x = 4.00
Therefore,
(b)
(c)
6.
(a)
(b)
(c)
(d)
(e)
the hydrate is
Ni(CH3CO2)2 · 4H2O
Name the hydrate.
nickel(II) acetate tetrahydrate
How do you think Neil confirmed that the sample had completely dehydrated?
He would have weighed the sample, put it back in the oven for a period of time then
weighed it again. If the two masses were the same, the sample would be completely
dehydrated.
List each group of compounds/ions in order from highest to lowest molar mass.
SO42- > HSO3- > SO32- > SO2
sulfite = SO32sulfur dioxide = SO2
2sulfate = SO4
bisulfite = HSO3ClO4- > Cl2O > ClO3 > ClOperchlorate = ClO4dichlorine monoxide = Cl2O
hypochlorite = ClOchlorine trioxide = ClO3
Al2O3 > Al(OH)3 > (NH4)2O > NH4OH
aluminum oxide = Al2O3
ammonium hydroxide = NH4OH
aluminum hydroxide = Al(OH)3
ammonium oxide = (NH4)2O
Sr(ClO2)2 > SrCl2 > SeCl2 > SCl2
selenium dichloride = SeCl2
strontium chloride = SrCl2
sulfur dichloride = SCl2
strontium chlorite = Sr(ClO2)2
Fe2Se3, FeBr3, FeBr2, FeSe
iron(II) bromide = FeBr2
iron(III) bromide = FeBr3
iron(II) selenide = FeSe
iron(III) selenide = Fe2Se3
7.
(a)
(b)
(c)
(d)
(e)
List each group of compounds/ions in alphabetical order by name.
aluminum phosphide, aluminum sulfide, arsenic triiodide, diarsenic tetrasulfide
Al2S3 = aluminum sulfide
AsI3 = arsenic triiodide
AlP = aluminum phosphide
As2S4 = diarsenic tetrasulfide
beryllium fluoride, diphosphorus pentoxide, lead(IV) chloride, phosphorus tribromide
PbCl4 = lead(IV) chloride
P2O5 = diphosphorus pentoxide
PBr3 = phosphorus tribromide
BeF2 = beryllium fluoride
aluminum chloride, copper(I) chloride, gold(III) chloride, silver chloride
AgCl, = silver chloride (or silver(I) chloride)
AlCl3 = aluminum chloride
AuCl3 = gold(III) chloride
CuCl = copper(I) chloride
ammonium, cyanide, hydroxide, nitrite
CN- = cyanide
NH4+ = ammonium
OH
= hydroxide
NO2- = nitrite
potassium chlorate, potassium chlorite, potassium hypochlorite, potassium perchlorate
KOCl = potassium hypochlorite
KClO3 = potassium chlorate
KClO2 = potassium chlorite
KClO4 = potassium perclorate