2. LEARNING OBJECTIVES
1. Define Stoichiometry;
2. Calculate the number of moles and the
number of particles of a substance in a
given mass of that substance;
3. Calculate the percentage composition.
3. STOICHIOMETRY
opronounce as “stoy-kee-ah-met-tree” or
abbreviate it as “stoyk”.
oThe study of the quantitative relationships or
ratios between two or more substances.
oThe word derives from the Greek word:
stoicheion meaning “element” and metron
meaning “to measure”
4. Measuring Matter
Chemists need a convenient method for
counting accurately the number of
atoms, molecules or formula units in a
sample of a substance. That’s why
chemists created their own counting
unit called the mole.
5. THE MOLE
• commonly abbreviated mol, the SI unit used to
measure the amount of substance.
• it has been established that a mole of anything
contains 6.02213467 x 1023
• 1 mole of an element contains 6.02213467 x
1023 atoms of that element.
6. AMEDEO CARLO AVOGADRO
- was an Italian savant. He is most noted
for his contributions to molecular theory
now known as
Avogadro’s law. In tribute to him, the
elementary entities (atoms, molecules,
ions or other particles) in 1 mole of a
substance, 6.02214179(30) x 1023 is
known as the Avogadro’s number.
( 9 August 1776- 9 July 1856 )
7. THE AVOGADRO’S NUMBER
• The number 6.02213467 x 1023 is called Avogadro’s
in honor to the Italian physicist and chemist Amedeo
Avogadro who, in 1811, determined the volume of
one mole of a gas.
• The Avogadro’s number is usually rounded to three
significant figures- 6.02x 1023
• If you write out Avogadro’s number, it looks like this.
602 213 673 600 000 000 000 000
8. - Is the sum of the masses of the atoms
present in one mole of a substance,
which can be an element or a
compound.
- Expressed as a unit of mass per mole
like g/mol or kg/mol
MOLAR MASS
9. oThe mass in grams of one mole of any pure
substance.
oOne mole of an element has a mass in grams
(g) equal to its atomic weight in atomic mass
unit (amu)
oThe molar mass of any element is numerically
equal to its atomic mass and has the unit g/mol.
MOLAR MASS OF AN
ELEMENT
11. Converting Mass of an Element to
Moles
Use the atomic weight of an element given on the periodic table to
apply a conversion factor to the mass given in the sample problem.
Example:
A roll of copper of wire has a mass of 848g. How many moles of
copper are in the roll?
Moles of Cu = 848g Cu
1 𝑚𝑜𝑙𝑒 𝐶𝑢
63.55𝑔 𝐶𝑢
=13. 34 mol Cu
12. Converting Moles to Mass of an
Element
Example:
Calculate the mass of 0.625 moles of calcium.
Use the molar mass of calcium to apply a conversion factor to
the number of moles given in the problem.
According to the periodic table, the atomic mass of calcium is
40.01 amu, so the molar mass of calcium is 40.01 g.
Mass of Ca = 0.625 mol Ca x
40.01 𝑔 𝐶𝑎
1 𝑚𝑜𝑙 𝐶𝑎
= 25. 01 g Ca
13. Converting Mass to Number of Particles
Example:
Calculate the number of atoms in 4.77 g lead. To find the
number of atoms in the sample, you must first determine how
many moles are in 4.77 g lead.
First,
Moles of Pb= 4.77 g Pb
1𝑚𝑜𝑙 𝑃𝑏
207.2 𝑔 𝑃𝑏
= 0.023 mol Pb
Second,
Atoms of Pb= 0.023 mol Pb
6.02x 10
1 𝑚𝑜𝑙 𝑃𝑏
= 1.38 x 1022 atoms Pb
15. Moles of a Compound
The molar mass of the compound is found by adding
the molar masses of all atoms in a substance.
It is numerically equal to the compound’s formula
(molecular) weight.
Formula weight is the sum of the atomic weights of
the elements the multiplied by the number of atoms of
each element in the formula for the substance. For
substances that exist as the individual molecules, the
formula weight is called the molecular weight.
16. Example: Find the molar mass of ammonia (NH3).
Molar Mass of (NH3) = molar mass of N + 3 (molar
mass of H)
Molar Mass of (NH3) = 14.01g/mol + 3(1.01g/mol) =
17.04 g/mol
Compute for the molar mass of the following:
• MgCl2
• Fe3(PO4)2
• C2H4
17. Converting Mass of a Compound to Moles
Example: A mole of water (H2O) consists of two moles of hydrogen
atoms and one mole of oxygen atoms. How many moles of water are in
10.0 g of water (1.000L at 4.0oC)?
First, you must calculate the molar mass of water.
Molar Mass= 2(molar mass H) + molar of O
= 2(1.01g/mol) + (16.0g/mol)
= 18.02 g/mol
Second, compute for the moles of water using the molar mass
above.
10.0g H2O x
1 𝑚𝑜𝑙 H2
O
18.02 𝑔
= 0.55 mol H2O
18. Percent Composition
The percent of an element in a compound can be
found in the following way.
% by mass of an element = Total mass of component elemet x 100
Molar mass of compound
19. Example: Calculate the percentage composition of
aluminium hydroxide Al(OH)3, a gastric antacid.
Al= 1 x 27.0 g = 27.0 g
O= 3 x 16.0 g = 48.0 g
H= 3 x 1.0g = 3.0 g
78.0g Al(OH)3
Use the total masses as the numerators of the ratio, then divide by the formula
mass.
% Al =
27.0 𝑔
78.0 𝑔
x 100 % =34.6 %
% O =
48.0 𝑔
78.0 𝑔
x 100 % =61. 5 %
% H =
3.0 𝑔
78.0 𝑔
x 100 % = 3.8 %
20. Calculate the percentage composition
of the following:
1. Baking soda or sodium
bicarbonate NaHCO3
2. Ascorbic Acid C6H8O6
21. Empirical Formula
The simplest whole number ratio of atoms of
elements in the compound.
For example, the simplest ratio of atoms of
sodium to atoms of chlorine in sodium
chloride(NaCl) is 1 atom of Na:1 atom and Cl:1
22. Example:
A platinum (II) compound, which is used to
treat tumors, contains 65.0% Pt, 23.6% Cl,
9.35% N, and 2.05% H by mass. Calculate its
empirical formula.
Solution:
Dividing the relative mass of each element by
its atomic mass gives the relative number of
moles of atoms of the element. A tabular
solution follows.
24. Molecular Formula
Molecular formula of a substance is a
whole number multiple of its empirical
formula. The empirical formula of a
compound may or may not be the
same as its molecular formula.
26. Example: Caffeine found in tea and coffee
is a white solid that contains 49.5% carbon,
5.20% hydrogen, 28.8% nitrogen, and
16.5% oxygen by mass. Its molecular mass
is 194.1% g/mole. Find the molecular
formula of caffeine.
27. Solution:
Calculate first the empirical fomula.
Element Mass (g)
Atomic
Mass(g)
Mole
Mole
Ratio
C
H
N
O
49.5
5.20
28.8
16.5
12.0
1.0
14.0
16.0
4.125
5.20
2.06
1.03
4
5
2
1
28. The empirical formula of caffeine is C4H5N2O
To get the molecular formula, solve for the
multiple factor, X.
Molecular/ molar mass of C4H5N2O = 194.1 g/mole.
29. Formula mass of C4H5N2O:
C= 4 moles of C x
12.0𝑔
1 𝑚𝑜𝑙𝑒 𝐶
= 48.0 g
H= 5 moles of H x
1.0𝑔
1𝑚𝑜𝑙𝑒 𝐻
= 5.0g
N= 2 moles of N x
14.0𝑔
1𝑚𝑜𝑙𝑒 𝑁
=28.0g
O= 1 mole of O x
16.0𝑔
1 𝑚𝑜𝑙𝑒 𝑂
= 16.0g
Formula mass of C4H5N2O = 97.0g
30. X= molecular mass
Formula mass
X= 194.1 g/mole
97.0g/mole
X=2
Therefore, the molecular formula is
(C4H5N2O)2 = C8H10N4O2
31. Compute:
Cholesterol, which has a molar
mass of 386g/mol, is composed
of 84.0% C, 11.85% H, and
4.15% O by mass. Find its
molecular formula.
