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Homework Assignment 3 – OPTIONAL – PRACTICE ONLY – DO NOT TURN IN!!
1.) Calculate the % by mass of each atom present:
a. C3H8
C: 3 x 12.01 = 36.03
H: 8 x 1.01 = 8.08
44.11
%C=
36.03 C
x 100 = 81.75 % C
44.11 total
%H=
8.08 H
x 100 = 18.3 % H
44.11 total
Check: 81.75 + 18.3 = 100.05 = 100.1 close enough!!
b. CH3OH
C: 1 x 12.01 = 12.01
O: 1 x 16.00 = 16.00
H: 4 x 1.01 = 4.04
32.05
%C=
12.01 C
x 100 = 37.47 % C
32.05 total
%O=
16.00 O
x 100 = 49.92 % O
32.05 total
%H=
4.04 H
x 100 = 12.6 % H
32.05 total
Check: 37.47 + 12.6 + 49.92 = 99.99 = 100.0 close enough!!
c. Al2(SO4)3
Al: 2 x 26.98 = 53.96
O: 12 x 16.00 = 192.0
S: 3 x 32.06 = 96.18
342.1
% Al =
%O=
53.96 Al
x 100 = 15.77 % Al
342.1 total
192.0 O
x 100 = 56.12 % O
342.1 total
Check: 15.77 + 28.11 + 56.12 = 100.00!
%H=
96.18 S
x 100 = 28.11 % S
342.1 total
d. K3PO4
K: 3 x 39.10 = 117.3
O: 4 x 16.00 = 64.00
P: 1 x 30.97 = 30.97
212.3
%K=
117.3 K
x 100 = 55.25 % K
212.3 total
%O=
64.00 O
x 100 = 30.15 % O
212.3 total
%P=
30.97 P
x 100 = 14.59 % P
212.3 total
Check: 55.25 + 14.59 + 30.15 = 99.99 – close enough!!
2.) Name or give formulas for the following species (HINT: your list of polyatomic ions might be
helpful!):
a. Potassium permanganate:
b. Ammonium carbonate:
c. Sulfur trioxide:
NCl3
e. Dihydrogen monoxide:
Ca(NO2)2:
g. Cu2Cr2O7:
h. NaC2H3O2:
i.
HCl:
(NH4)2CO3
SO3
d. Nitrogen trichloride:
f.
KMnO4
H2O
calcium nitrite
copper (I) dichromate
sodium acetate
hydrochloric acid
3.) Calculate the empirical formula for Vitamin C which is 40.9% C, 54.5% O, and 4.58% H by mass.
If the molar mass of Vitamin C is 176 grams/mole, what is the molecular formula?
Percentages are the same as grams since the % add up to 100!
40.9 g C x
12.01 grams C
= 3.41 moles C
1 mole C
54.5 g O x
16.00 grams O
= 3.41 mole O
1 mole O
4.58 g H x
1.01 grams H
= 4.54 moles H
1 mole H
Then we divide through by the lowest number of moles to get the empirical formula
3.41 moles C
= 1 mole C
3.41 mole
3.41 moles O
= 1 mole O
3.41 mole
4.54 moles H
= 1.33 moles H
3.41 mole
The moles MUST be whole numbers!! We DO NOT round 1.33 moles down to 1 mole!! We
must multiply all the moles by the SAME factor!
1 mole C x 3 = 3 moles C
1 mole O x 3 = 3 moles O
1.33 moles H x 3 = 4 moles H
Empirical Formula =
C3H4O3
But: is that the molecular formula also?? If so, the mm of this empirical formula will be the
same as the given mm of the vitamin C
Molar mass of C3H4O3 = (12.01 x 3) + (1.01 x 4) + (16.00 x 3) = 88.07 grams/mole
No, the empirical mass is not the same as the given mm of the molecular formula so this is the
empirical formula for vitamin C, but not the molecular formula. We need the whole number
multiple:
Whole number multiple =
176 grams formula
=2
88.07 grams empirical
We must multiply all the subscripts by 2!
Molecular Formula =
C6H8O6
4.) Identify the mass law that each of the following observations demonstrates, AND explain your
reasoning:
a. A sample of ammonium chloride from Arkansas contains the same % by mass as one
from Oregon.
Constant composition: the composition is the same if you
are examining the same compound regardless of where it
came from!
b. A flashbulb contains magnesium and oxygen before use and magnesium oxide after use,
but its mass does not change.
Conservation of mass: matter is not destroyed (or created!)
if a change in composition occurs (e.g. in this chemical
reaction!)
c. Carbon and oxygen form one compound that is 42.9 mass % carbon and another that is
27.3 mass % carbon.
Law of multiple proportions: the same elements C and O
can come together to form a variety of compounds that have
whole number ratios of these atoms (e.g. we have CO and
CO2 as possibilities)
5.) Balance the following chemical reactions – use ONLY whole numbers!!:
a. ___4_____Cr(s) + ____3____O2 (g) → ___4_____Cr2O3 (s)
b. ________SiH4 (g) → ________Si(s) + ____2____H2 (g)
c. ____2____Pb(NO3)2 (s) → ____2____PbO (s) + ___4____NO2 (g) + ________O2 (g)
d. ________ (NH4)2Cr2O7 (s) → ________N2 (g) + ________Cr2O3 (s) + ____4____H2O (g)
e. ________PF3 (g) + ____3___ H2O (l) → ________H3PO3 (aq) + ____3____HF (aq)
f.
________C2H5OH (l) + _____3___O2 (g) → ___2_____CO2 (g) + ____3____H2O (g)
g. ________P4O10 (s) + ____6____H2O (l) → ____4____H3PO4 (aq)