E - 14
MASS
MOLAR
NGSS: HS: PS1-7 Use mathematical representations to support the claim that atoms, and
therefore mass, are conserved during a chemical reaction. CC Energy and Matter. SEP Using
Mathematical and Computational Thinking.
A very prominent piece of “artwork” in any chemistry classroom or laboratory is the Periodic Table
of Elements. To a chemist, this table is not a decoration but an essential tool of the trade. In addition
to ordering all known chemical elements by increasing atomic number, the Periodic Table gives
valuable information about the physical and chemical properties of each element. The first physical
property of elements that we will explore is atomic and/or molecular mass, known to the chemist as
molar mass.
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A quick scan of the Periodic Table shows an irregular shaped arrangement of columns (called
groups or families) and rows (called periods) composed of boxes containing numbers and letters.
Most of the letters are black (indicating elements that are solids at room temperature), but a few are
blue (indicating elements that are liquids at room temperature), or red (indicating elements that are
gases at room temperature). What do these colors tell you about the different elements’ melting and
boiling points? Each individual box provides specific information about an individual element, a
substance composed of only one type of atom.
In each box, each element is represented by a symbol containing one, two, or three letters. The first
letter is always capitalized. Any additional letters are lower case. Some symbols obviously are
derived from the modern name for the element (ex. Li for lithium, Co for Cobalt, or Cl for chlorine).
Other symbols seem not to fit with the element’s modern name. That is because these symbols derive
their letters from foreign names (Na for sodium from the Latin word natrium, Pb for lead from the
Latin word plumbum, and W for tungsten from the German word wolfram.). Many elements which
have symbols from foreign names were discovered in ancient times.
In addition to the symbol, each element’s box also contains several important numbers. One number,
a whole number from 1 - 100+, represents the atomic number of the element. Another number,
larger than the element’s atomic number and usually containing places past the decimal, represents
the atomic mass of the element (In case you forget which is which, remember that your mass is
always greater than your age!). The atomic mass is always given in amu or atomic mass units. The
element boxes on some more complex Periodic Tables may contain many other important properties
of the element besides atomic number and mass (ex. electron configuration, electronegativity, crystal
structure etc.).
Atomic symbols, when placed next to each other, make up the chemical formulas of molecules (ex.
the seven diatomic elements, H2 , N2 , O2 , F2 , Cl2 , Br2 , and I2 , which make a seven on the
Periodic Chart) and/or compounds (ex. H2O). A molecule is a group of two or more atoms joined
by covalent bonds. A compound is a substance containing atoms of two or more different elements
that have been chemically joined. Subscripts in a chemical formula give us the chemical ratio of
elements from which the molecule and/or compound was formed (ex. the molecule Cl 2 contains two
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chlorine atoms/ the compound H2O is two parts hydrogen and one part oxygen.). As we shall explore
more later, the ratios these subscripts represent are mole ratios.
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The molar mass of a compound is defined as the sum of the atomic masses. To calculate the molar
mass of a chemical compound follow the steps below:
1.
2.
3.
4.
5.
6.
7.
Round off all atomic masses to two places past the decimal.
If the “formula” consists of a single symbol with no subscripts, then the atomic mass is the molar
mass (ex. The molar mass of C is 12.01 amu.).
If the formula consists of more than one element symbol, then the molar mass is the sum of all the
atomic masses. (ex. NaCl - 22.99 + 35.45 = 58.44 amu.).
If that formula contains subscripts, the atomic mass of the element before the subscript must be
multiplied by that number of times (ex. C6H12O6 - 6(12.01) + 12(1.01) + 6(16.00) = 180.18 amu.).
If that formula contains subscripts outside parentheses, the atomic masses of the entire group of
elements inside the parenthesis before the subscript must be multiplied by that number of times (ex.
(NH4)2 S - 2(14.01 + 4(1.01)) + 32.07 = 68.17 amu OR 2(14.01) + 8(1.01) + 32.07 = 68.17 amu.).
If a coefficient precedes a chemical formula, the entire molar mass must be multiplied by that
number of times (ex. 2 H2O - 2(2(1.01) +16.00) = 36.04 amu OR 4(1.01) + 2(16.00) = 36.04 amu.).
If a  and a certain number of H2O molecules follows a chemical formula, that number of water
molecules must be added to the molar mass of the anhydrous salt to make the formula for the
hydrate (ex. CuCl2  2H2O - 63.55 + 2(35.45) + 4(1.01) + 2(16.00) = 170.49 amu.).
PRACTICE PROBLEMS:
1. Find the molar mass of the following compounds:
a.
HCl
b.
c.
LiCl
d.
e.
KI
f.
g.
ZnS
h.
i.
MnO
j.
NaOH
NaBr
CsF
CuBr
HgS
2. Find the molar mass of the following molecules and compounds:
a.
H2
b.
Br2
c.
HNO3
d.
H2SO4
e.
Fe2O3
f.
NaHCO3
g.
CaCO3
h.
Li2SO4
i.
MgF2
j.
CoCl2
k.
KMnO4
l.
PbCrO4
3. Find the molar mass of the following compounds:
a.
(NH4)2SO4
b.
c.
Sn(OH)4
d.
e.
Al(NO3)3
f.
g.
Pb(H2C3O2)2
h.
i.
La(ClO3)3
j.
k.
Fe(SCN)3
l.
Ca(OH)2
Cu(NO2)2
Ba(NO3)2
Sr3(PO4)2
Ga2(SO4)3
Ra3(AsO4)2
4. Find the molar mass of the following hydrated compounds:
a.
CaSO3  2H2O
b.
MgSO3  6H2O
c.
NiSO4  6H2O
d.
CuSO4  5H2O
e.
CoCl2  6H2O
f.
BaBr2  2H2O
g.
BaI2  2H2O
h.
NaS2O3  5H2O
i.
Na2B4O7  10H2O
j.
LiSiF6  2H2O
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