II. THE MOLE AND BASIC MASS RELATIONSHIPS
A. Atomic Components and Standard Notation
1. Components
p
+1
1.67x10-24g
1 amu
neutron n
0
1.67x10-24g
1 amu
electron e-
-1
9.11x10-28g
0.00055 amu
proton
2. Arrangement
3. Atomic number: the number of protons in the atom.
4. Atomic mass number: the number protons + neutrons.
5. Isotopes: Atoms of the same element with a different number of neutrons.
13 MOLE and BASIC MASS RELATIONSHIPS / Atomic Components
6. Number of electrons in neutral atoms
ALL elements are electrically neutral !
7. Number of electrons in ions
a) Ion: an atom with a net electrical charge.
An ion cannot exist on its own, it is always part of a compound !
b) Positive ions (cations)
Example(1): How many electrons are in a sodium atom, how many electrons are in a sodium ion (Na 1+)?
Example(2): How many electrons are in an aluminum atom, how many electrons are in an aluminum ion (Al 3+)?
c) Negative ions (anions)
Example(3): How many electrons are in a chlorine atom, how many electrons are in a chloride ion (Cl1-)?
Example(4): How many electrons are in a sulfur atom, how many electrons are in a sulfide ion (S 2-)?
14 MOLE and BASIC MASS RELATIONSHIPS / Atomic Weight
B. The Mole and Atomic Weight
1. Mole (mol): 6.02x10+23 things.
Example(1): How many 12C atoms are required to make a sample with a mass of 12g ?
Example(2): How many 24Mg atoms are required to make a sample with a mass of 24g ?
2. Gram atomic weight (GAW): the weight in grams of one mole of atoms.
units: g/mol
(Some books refer to this as the molar mass.)
Example(3): What is the GAW of chlorine?
Example(4): Why isn’t the GAW of chlorine a whole number?
15 MOLE and BASIC MASS RELATIONSHIPS / Atomic Weight
3. Problems
Example(5): What is the mass of 2 moles of C?
Example(6): If 4.0 moles of an element have a mass of 112g what is its GAW?
Example(7): How many moles of Na are 92g?
Example(8): How many atoms of Na are in 92g?
Example(9): How many moles of He are in a sample containing 3x1022 atoms?
Example(10): What is the mass of a single C atom?
16 MOLE and BASIC MASS RELATIONSHIPS / Molecular Weight
C. Gram Molecular Weight / Molar Mass
1. Gram molecular weight (GMW): the weight in grams of one mole of molecules.
units: g/mol
(Also called molar mass, formula weight,
formula mass.)
2. Calculating GMW
Example(1):What is the molar mass of HF?
Example(2): What is the molar mass of C6H12O6?
Example(3): What is the molar mass of elemental oxygen, O2?
3. Problems
Example(4): What is the mass of 2 moles of HF?
17 MOLE and BASIC MASS RELATIONSHIPS / Molecular Weight
Example(5): What is the molar mass of an unknown compound if 3.0 moles have a mass of 180g?
Example(6): How many moles of H2O are in 54g?
Example(7): How many molecules of H2O are in 54g?
Example(8): How many H atoms are in 54g of H2O?
Example(9): What is the mass of a single molecule of C2H6?
18 MOLE and BASIC MASS RELATIONSHIPS / Percent Composition
D. Percent Composition (by mass or weight)
%=
part
x 100%
whole
mass atom
1. %atom =
________________________
x 100%
mass compound
Example (1): What is the % of oxygen in MgO
Example(2): What is the % of each atom in Na3PO4?
Example(3): A compound has a total mass of 4.20 grams. If it is 60.0% carbon, what is the mass of
the carbon in the compound?
19 MOLE and BASIC MASS RELATIONSHIPS / Formulas
E. Empirical and Molecular Formulas
A formula expresses the ratio of atoms in a compound, atom to atom, or mole to mole.
1. Empirical vs. molecular formula
a) Molecular formula: the number of atoms of each type in a molecule,
or the number of moles of each type of atom in a mole of
molecules.
b) Empirical formula: the smallest whole number ratio of atoms in a molecule.
MOLECULAR
EMPIRICAL
N2 O4
C4H10
C6H12O6
H2SO4
?
CH2O
2. Calculation of empirical formulas from mass
Example(1): A compound contains 12g of C and 64g of S, find its empirical formula.
Example(2): A compound contains 0.16 g of S and 0.19 g of F, find its empirical formula.
Example(3): Reduce the following formula to whole numbers: N 0.04O0.06.
20 MOLE and BASIC MASS RELATIONSHIPS / Formulas
Example(4): 1.30 g of S are reacted with chlorine to produce 4.28 g of product, what is the empirical
formula of the product?
3. Calculation of molecular formulas
Example(5): A compound with a molar mass of 92.0g/mol contains 0.28 g of nitrogen and 0.64 g of
oxygen, what is its molecular formula?
4. Formulas from percent composition
Example(6): What is the empirical formula of a compound if it is 26.0% O, 74.0% Na?
Example(7): A compound is 52.0% carbon, 13.0% hydrogen, and the rest is oxygen. If its molar mass
is 138 g/mol what is its molecular formula?