• The mole (or mol) represents a certain number of objects.
• SI def.: the amount of a substance that contains the same
number of entities as there are atoms in 12 g of carbon-12.
• Exactly 12 g of carbon-12 contains 6.022 x 10 23 atoms.
• One mole of H 2O molecules
contains 6.022 x 10 23 molecules.
• 1 mole contains 6.022 x 10 23 entities (Avogadro’s number)
One mole of NaCl contains 6.022 x 10 23 NaCl formula units.
• Use the mole quantity to count formulas by weighing them.
• Mass of a mole of particles = mass of 1 particle x 6.022 x 1023
• The mass of an atom in amu is numerically the same
as the mass of one mole of atoms of the element in grams.
• One atom of sulfur has a mass of 32.07 amu;
one mole of S atoms has a mass of 32.07 g.
Mass of 1 H atom: 1.008 amu x 1.661 x10-24 g/amu = 1.674 x10-24 g
Mass of 1 mole of H atoms:
1.674 x10-24g/H atom x 6.022 x1023H atoms = 1.008 g
PT3
• For compounds, the molecular mass (in amu) is numerically
the same as the mass of one mole of the compound in grams.
• Skill 3-1 Calculate the molecular mass of a compound as the
sum of the atomic masses of its elements.
• Molecular mass H2O = (2 x atomic mass of H) + atomic mass of O
= 2(1.008 amu) + 16.00 amu = 18.02 amu
• So, one mole of water (6.022 x 10 23 molecules) has a mass of 18.02 g.
• Molar mass of NaCl = atomic mass of Na (22.99 amu) +
the atomic mass of Cl (35.45 amu) 22.99 + 35.45 = 58.44 amu
• One mol of NaCl (6.02 x1023 formulas) has a mass of 58.44 g.
Concept 2. The relation between molecular (formula) mass
and molar massTo obtain one mole of copper atoms (6.02 x 10 23 atoms),
weigh out 63.55 g copper.
P5
•The molar mass (M) of a substance is the mass of one mole of
entities (atoms, molecules, or formula units) of the substance.
• Molar mass has units of grams per mole (g/mol).
Concept 3. The relations among amount of substance (in moles),
mass (in grams), and number of chemical entitiesSkill 3-2 Mass - Mole Conversions
6
•Use the molar mass of an element or compound to convert a given
number of moles to mass:
•We can do the reverse with 1/M, and convert any mass in grams to
the number of moles:
•Use Avogadro’s number to convert moles of substance to the number
of entities:Problem: (a) How many grams of silver, Ag, are in 0.0342
mol Ag? (b) How many atoms of Ag are in 0.0342 mol Ag?
7
Plan: (a) To convert moles Ag to grams Ag, use the molar
mass of Ag from the periodic table. (b) To convert moles to
number of atoms, use Avogadro’s number.
Solution: (a) Converting from moles of Ag to mass:
(b) Converting from moles of Ag to number of atoms:
= 2.06 x 1022 atoms Ag
PTProblem: Iron is a most important metal in our society. How
many iron atoms are present in a piece of iron weighing 95.8 g?
8
Plan:
Solution: Converting from mass of Fe to moles:
Converting from moles of Fe to number of atoms:
PT43
•If amounts of two reactants are not in the stoichiometric ratio of
a reaction, one will be in excess; the other is the limiting reactant.
Problem: Rocket fuel, [hydrazine (N 2H 4) and dinitrogen tetraoxide
(N 2O 4)], reacts to form N 2
gas and water vapor. How many grams
of nitrogen gas form from 1.00 x 102
g N2H4
and 2.00 x 102
g N2O4
?
Write the balanced equation:
Convert mass of reactants to mol and find the mol of N2
each forms.
Concept 9 Why one reactant limits the yield of productConcept 10. The causes of lower-than-expected yields
and the distinction between theoretical and actual yields
45
• Ideally 100% of the limiting reactant becomes product.
•This is the theoretical yield, amount of product indicated by the
stoichiometrically equivalent molar ratio in balanced equation.
• The theoretical yield is rarely obtained due to several factors.
•The amount of product actually obtained is the actual yield.
•The percent yield (% yield) is the actual yield expressed as a
percent of the theoretical yield:
