Chapter 8
 The
number of particles in a mole is called as
Avogadro’s constant or number. This unit
called the mole, is defined as the number of
atoms in exactly 12 g of carbon-12.
 One mole represents 6.022х10²³ particles.
The particles can be atoms in elements,
molecules in molecular compounds, or ions in
ionic compounds.
 Avogadro’s
constant can be used to convert
an amount in moles to an equivalent number
of atoms. Example 2.66 mol of an element
can be converted to number of atoms by
multiplying the number of moles with
Avogadro’s number.
 Determine
the number of atoms present in
2.5 mol of silicon.
 Convert 3.01х10²³ atoms of silicon to moles
of silicon.
 How many atoms are present in 3.7 mol of
sodium?
 How many moles of silver is 2.888х1015
atoms?
 Sometimes
pure metals like copper may be a
mixture of two isotopes. In any sample of
copper, 69.17% of the atoms are copper-63
and 30.83% of the atoms are copper-65.
 Copper has a relative atomic mass of 63.54
amu. In nature, copper consists of a mixture
of two isotopes,copper-63 and copper-65.The
abundance of copper-63 is 69.17% and that of
copper-65 atom is 30.83%. The relative
atomic mass of copper is a weighted average
of these two isotopes.
 The
mass of a copper-63 atom is 62.94 amu
and that of copper-65 atom is 64.93 amu.
Using this data and the abundance of these
two isotopes, determine the average atomic
mass of native copper. The abundance of
copper-63 is 69.17%. The abundance of
copper-65 is 30.83%.
 Page
279
 1,2,3.
 Molar
mass is the mass in grams that is equal
to the sum of all the atomic masses of the
component atoms of a substance. The molar
mass of an element is numerically equal to
the element’s atomic mass and has a unit of
g/mol
 To
convert amount in moles to mass you
multiply number of moles with the molar
mass of that element.
 To convert mass to amount in moles you
divide the given grams of the element with
the molar mass of the element.
 Determine
the number of moles represented
by 237 g of copper.
 Determine the mass in grams of 3.50 mol of
copper.
 To
find the mass of an atom you divide the
molar mass of an atom with Avogadro’s
number.
 Find the mass of a single silicon atom.
 Page
283
 Q.13,15,16 do all the problems under these.
 Formula
reveal composition: A formula tells
us about the elements that are present in a
compound. Example in KBr compound is
made up of the elements potassium and
bromine.
 Also tells us about the ratio in which these
two elements are present.Potassium bromide
is made up of ions in the ratio of one
potassium cation, K+ for each bromide anion,
Br— .
 The
meaning of a formula does not change
when polyatomic ions are involved.
Potassium nitrate has the formula KNO₃.
Similar to KBr this compound also has a ratio
of 1K+ cation and I NO₃− anion.
 What
is a formula unit of an ionic compound?
 Formula unit is the unit that represents the
simplest ratio of cations to anions. A formula
unit of NaCl consists of one Na+ and one Cl One mole of an ionic compound consists of
6.022х10²³ of these formula units. One mole
of a molecular compound, like H₂O consists
of 6.022х10²³ molecules.
 The molar mass of an ionic compound or
molecular compound is the sum of the
masses of all the atoms in the formula
expressed in grams.
 The
molar mass of water will be calculated
in the following way;
 Page
288
 Q.1 do # a,c, d, j and k
 Q.2. b,d,f and h.
 What
is percentage composition?
 The percentage by mass of each element in a
compound is the percentage composition.
Percentage composition data allows us to
calculate the simplest ratio among the
elements of a compound. A formula that
represents this simplest ratio is called an
empirical formula.
 One
of the substances in a new alkaline
battery is composed of 63.0% manganese and
37.0% oxygen by mass. Determine the
empirical formula of the compound in the
battery?
 Chemical analysis of a clear liquid shows that
it is 60.0% C, 13.4% H, and 26.0% O.Calculate
the empirical formula of this substance.
 Molecular
formulas are multiples of empirical
formulas.
 n(empirical formula)=molecular formula
 Molecular formula tells us about the actual
number of atoms present in a compound

One C-H empirical formula unit
66
Six C-H empirical units bonded
together to form a molecule of
benzene with molecular formula C
 The
empirical formula of a compound is
found to be P₂O5. The molar mass of the
compound is 284 g/mol. Determine the
molecular formula of the compound.
 Determine
the percentage of water by mass
in sodium carbonate decahydrate,
Na2CO3.10H2O
 Page
297
 Q.7 c and d
 Q.8
 a,b,e
 Q.12 all
 Page 305
 Test prep all