GENERAL CHEMISTRY (CHEM 101)
(Avogadro and moles, periodic table,
isotopes and combustion) QUESTIONS AND
SOLUTIONS 100% VERIFIED VIRGINIA
COMMONWEALTH UNIVERSITY
, 3.091: Introduction to Solid State Chemistry
Maddie Sutula, Fall 2025
Recitation 2
1 Avogadro and moles
When balancing a reaction or determining its yield, it was really important to keep track of the number of
each kind of atom before and after the reaction. Atoms can be difficult to account for in real life due to their
minuscule size. Instead, we usually keep track in moles: a mole consists of Avogadro’s number of atoms (or
molecules), or 6.022 × 1023 atoms or molecules per mole.
Critically, we can convert from moles to a value that is easy to measure in a lab: mass. The molar mass
of a a substance is defined to be the number of grams in one mole of that substance. The molar mass of a
single element is also called the atomic mass; the molar mass of a compound can be obtained by summing
the molar mass of the constituent elements.
Example: How many moles of nickel are in 102 g of nickel? How many moles of H2O are in 50 g of
water? 1 mol Ni
×
102 g Ni = 1.74 mol Ni
58.69 g Ni
1 mol H2O
50 g H2O ×
(15.999 + 2 × 1.0107) g H O = 2.77 mol H2O
2
2 Periodic table
The periodic table is our key to solving problems! It contains a wealth of information that we can use to
understand and calculate the properties of materials. The elements are organized by the number of protons:
the rows are called periods, and the columns are called groups. Atoms are made up of positively charged
protons, neutral neutrons, and negatively charged electrons. Protons and neutrons live at the center of the
atom: together, these are the main source of mass. Electrons orbit around the protons and neutrons (more
on this later). An example Bohr model of carbon is shown here:
We can read off many properties of atoms from the periodic table: An atom has the same number of protons
and electrons. The atomic mass is the sum of the number of protons and neutrons: it is the average mass
of one atom of each element. The units of atomic mass are AMU (atomic mass units, = 1/12 the mass of
a carbon-12 isotope), or equivalently (due to a convenient convention), the number of grams in a mole of a
substance.
1
(Avogadro and moles, periodic table,
isotopes and combustion) QUESTIONS AND
SOLUTIONS 100% VERIFIED VIRGINIA
COMMONWEALTH UNIVERSITY
, 3.091: Introduction to Solid State Chemistry
Maddie Sutula, Fall 2025
Recitation 2
1 Avogadro and moles
When balancing a reaction or determining its yield, it was really important to keep track of the number of
each kind of atom before and after the reaction. Atoms can be difficult to account for in real life due to their
minuscule size. Instead, we usually keep track in moles: a mole consists of Avogadro’s number of atoms (or
molecules), or 6.022 × 1023 atoms or molecules per mole.
Critically, we can convert from moles to a value that is easy to measure in a lab: mass. The molar mass
of a a substance is defined to be the number of grams in one mole of that substance. The molar mass of a
single element is also called the atomic mass; the molar mass of a compound can be obtained by summing
the molar mass of the constituent elements.
Example: How many moles of nickel are in 102 g of nickel? How many moles of H2O are in 50 g of
water? 1 mol Ni
×
102 g Ni = 1.74 mol Ni
58.69 g Ni
1 mol H2O
50 g H2O ×
(15.999 + 2 × 1.0107) g H O = 2.77 mol H2O
2
2 Periodic table
The periodic table is our key to solving problems! It contains a wealth of information that we can use to
understand and calculate the properties of materials. The elements are organized by the number of protons:
the rows are called periods, and the columns are called groups. Atoms are made up of positively charged
protons, neutral neutrons, and negatively charged electrons. Protons and neutrons live at the center of the
atom: together, these are the main source of mass. Electrons orbit around the protons and neutrons (more
on this later). An example Bohr model of carbon is shown here:
We can read off many properties of atoms from the periodic table: An atom has the same number of protons
and electrons. The atomic mass is the sum of the number of protons and neutrons: it is the average mass
of one atom of each element. The units of atomic mass are AMU (atomic mass units, = 1/12 the mass of
a carbon-12 isotope), or equivalently (due to a convenient convention), the number of grams in a mole of a
substance.
1
