Molar Mass
1.
1 mole of any element is equal to the
gram atomic mass (from periodic table)
2.
1 mole of any compound is equal to
its formula mass
3.
1 mole= 6.022 x 1023 molecules or
atoms
4.
Formula mass= add all average
atomic masses of all atoms in the
formula
a.
This is also called molecular
mass or molecular weight.
b.
Ex.- H2SO4
Atomic mass of H= 1.01 u
S= 32.06 u
O= 16.00 u
But there are more than 1 atom of
some elements in the formula H2SO4
so:H2=1.01 x 2=
S=
O4= 16.00 x4=
2.02u
32.06 u
64.00 u
+98.08 u
and in significant figures 98.08 u
5.
molar mass= this is the mass of one
mole of a compound
a. it is calculated the same as formula
mass but instead of u (units) use g
(units).
b. The molar mass of H2SO4 = 98.08g
c. One mole of H2SO4 contains
6.022 x 1023 H2SO4 molecules.
d.Yet if we look at ions there are
2 moles of H+ ions and
1 mole of SO4-2 ions
3 moles of ions total
e. Yet if we look at atoms there are
2 moles of hydrogen
1 mole of sulfur
4 moles of oxygen
7 moles of atoms total
6. Conversions Factors
a. 6.022 x 1023 molecules or 1 mol
1 mol
6.022 x 1023 molecules
b.
moles
x grams
1 mole
=
mass in grams
c.
mass in grams x 1 mole = moles
grams
d. ex. What is the mass in grams of 2.5 moles of
oxygen gas? (O2)
2.5 mol O2
x 32.0 g O2
1 mole O2
= 80 g O2
mass O from periodic table = 16.00 g
O2 will be double
= 32.0 g
7. Percent Composition= the percent by mass
of each element in the compound. All parts
have to total 100%.
Mass of element
Molar mass of compound
Ex. PbCl2
= % of element
molar mass Pb = 207.2g
Molar mass Cl = 35.453g
2 x 35.453g =70.906g Cl2
Molar mass of PbCl2 then equals 278.106g
% Pb = 207.2 g = 0.7451 or 74.51%
278.106g
% Cl2 = 70.906g = 0.2550 or 25.50%
278.106g
In other words, the % composition = part
Total
If the percent composition for 2 mystery
compounds is the same then the compounds
are probably the same. Percent composition
does not change whether you have a small or
large sample.