Problem Set 11 Solution
1.
How many moles of chlorine are in 0,50 mol CCl4?
Identify the number of significant digits: 2
Given 0.50 mol CCl4
Target: ?mol Cl
Path: mol CCl4 6 mol Cl
Relationship: 1 mol CCl4 = 4 mol Cl
Computation:
Note the number of significant digits - 4 is an integer that has no influence on the number of significant digits and
therefore, the number of significant digits is determined by the given number of moles of carbon tetrachloride.
2.
How many g of oxygen are in 3.1 g Co2 O3 ? Name the compound.
The ionic compound is cobalt(III) oxide because the compound consist of two ions Co3+ and O2The number of significant digits: 2
When computing g of element in a compound it is safe to go to moles first and than to go back to grams. Consequently, we
need to compute the molar mass of cobalt(III) oxide
element
# moles
molar mass
Product
Co
2
117.9 g Co
O
3
48.00 g O
Molar mass of Co2O3
Note that I have observed the number of significant digits: 58.93 has 4 s.d. 6 117.9 g Co has 4 s.d.; when computing the sum I
noted that the last digit shown in the sum must be in the first decimal place 165.9 g Co2 O3.
Given: 3.1 g Co2 O3
Target: ? g O
Path: g Co2O3 6 mol Co2O3 6 mol O 6 g O
Computation:
3.
How many g of sulfur are in 0.75 g Cr2 S3 ? Name the compound.
The ionic compound is chromium(III) sulfide because the compound consist of two ions Cr3+ and S2The number of significant digits: 2
When computing g of element in a compound it is safe to go to moles first and than to go back to grams. Consequently, we
need to compute the molar mass of chromium(III) sulfide
element
# moles
molar mass
Product
Cr
2
104.0 g Cr
S
3
96.18 g S
Molar mass of Cr2S3
Note that I have observed the number of significant digits: 52.00 has 4 s.d. 6 104.0 g Cr has 4 s.d.; when computing the sum I
noted that the last digit shown in the sum must be in the first decimal place 165.9 g Co2 O3.
Given: 0.75 g Cr2 S3
Target: ? g S
Path: g Cr2S3 6 mol Cr2S3 6 mol S 6 g S
Computation:
4.
How many atoms of nitrogen are in 0.5 mol of dinitrogen pentoxide?
The number of significant digits: 1
Given: 0.5 mol N2O5
Target: ?N atoms
Path: mol N2 O5 ! mol N ! N atoms
Computation:
Note that the answer has only one s.d. as it should - why?
5.
Draw Lewis diagrams of these compounds: CBr4; PI3; F2O; SF4; SO2
a.
CBr4;
n = 5 x 8 = 40
v = 4 + 4 x 7 = 32
s=8
b=2
lp = (40-2 x 8)/2 =12
b.
PI3 ;
n = 4 x 8 = 32
v = 5 + 3 x 7 = 26
s=6
b=3
lp = (32 - 2 x 6)/2 = 10
c.
F2 O;
n = 3 x 8 = 24
v = 6 + 2 x 7 = 20
s=4
b=2
lp = (24 - 2 x 4)/2 = 8
d.
Sorry - there was a typo error;
CF4
n = 5 x 8 = 40
v = 4 + 4 x 7 = 32
s=8
b=4
lp = (40 - 2 x 8)/2 = 12
e.
SO2 ;
n = 3 x 8 = 24
v = 3 x 6 = 18
s=6
b=3
lp = (24 - 2 x 6)/2 = 6