This is presentation seeks to enhance our further understanding in the field of General Chemistry STEM 12

1. Percent Compositions,
Chemical Reactions
and Equations
CHEMISTRY

2. Percent Compositions of Compounds
• the formula of a compound tells us the numbers of
atoms each element in a unit of the compound.
• we need to to verify the purity of a compound to
calculate the percent of its total mass.
• by comparing the result obtained for our sample, we
could determine its purity.

3. The percent compositions by mass is the percent by
mass of each element in a compound. It is obtained by
dividing the mass of each element in 1 mole by the
molar mass of the compound and multiplying by 100%
percent compositions by mass =
n × molar mass
molar mass of compound
× 100

4. Steps on how
to calculate
the percent
composition
• Find the molar mass of the
compound by adding up the masses
of each atom in the compound
• Calculate the molar mass of the
component in the compound
• Divide the molar mass of the
component by the total molar mass
of the compound

5. Exampl
e
In 1 mole of hydrogen peroxide, there are 2 moles of H
atoms and 2 moles of O atoms. Calculate its percent
composition.
The sum of the percentages is 99.99%.

6. Experimental Determination of
Empirical Formulas
The fact that you can determine the empirical formula
of a compound if you know the percent compositions.
The procedure is as follows. First, chemical analysis tells
the number of grams of each element present in a
given compound. Then, convert the quantities in grams
to number of moles of each element.

7. Example:
When ethanol is burned in an apparatus, carbon dioxide
(CO2) and water (H2O) are given off. Because neither
carbon nor hydrogen was in the inlet gas, you can conclude
that both carbon and hydrogen were present and that
oxygen (O) may also be present. The masses of CO2 and of
H2O produced can be determined by measuring their mass.
Suppose that in one experiment the combustion of 11.5 g of
ethanol produced 22.0 g of CO2 and 13.5 g of H2O.

8. You can calculate the mass of carbon and
hydrogen in the original 11.5 g sample of
ethanol:
Thus, 11.5 g or ethanol contains 6.00 g of
carbon and 1.51 g of hydrogen. The remainder
must be oxygen, whose mass is:

9. mass of 0 = mass of sample - (mass of C + mass of H)
= 11.5 g - (6.00 g + 1.51 g) = 4.0 g

10. Determination
of Molecular
Formula
Molecular formulas give the kind
and number of atoms of each
element present the molecular
compound. It is always the same as
the empirical formula. The
chemical formula will always be
some integer multiple (n) of the
empirical formula (i.e. integer
multiples of the subscripts of the
empirical formula).

11. Steps on how to calculate
for molecular formula
Determine
first the
empirical
formula and
calculate it
Divide the
gram
molecular
mass by the
empirical
formula mass
Multiply each
subscripts to
the empirical
formula by
the number
calculated in
Step 2

12. A sample compound contains 30.36% nitrogen and 69.54%
oxygen by mass determined by mass spectrometer. In
separate experiment, the molar mass of the compound if in
between 90g and 95g.

13. The molecular might be the same as empirical or some
integral multiple of it. So we need to compare the ration of
the molar mass and molar mass of empirical formula to
show integral relationship between the two.

14. Chemical Equations
• Chemical equations are symbolic representations of
chemical reactions in which the reactants and the
products are expressed in terms of their respective
chemical formulae.
• In a chemical equation, reactants are found on the
left side and the products are on the right side.
Therefore, it can be summarized as:
Reactants → Products

15. • The state of matter of each compound or molecule is
indicated in subscript next to the compound by an
abbreviation in parentheses. For example, a compound in
the gas state would be indicated by (g), solid (s), liquid (l),
and aqueous (aq). Aqueous means dissolved in water; it is a
common state of matter for acids, bases, and dissolved
ionic compounds.
Notation for Writing Chemical
Equation

16. • Δ (above or below reaction arrow) Heat required
• hυ (above or below reaction arrow) Light required
• cat or Pt symbol is required formula written above the
arrow is used as a catalyst in the reaction
• Plus signs (+) separate individual reactant and product
formulas, and an arrow (⟶) separates the reactant and
product (left and right) sides of the equation.

17. • To denote stoichiometric relationships, the ‘=’ symbol is
used. In order to describe a state of chemical
equilibrium, the symbol ‘⇌’ is used.
• In order to describe a reaction that occurs in both
forward and backward directions, the symbol ‘⇄’ is used.
• The relative numbers of reactant and product species
stand for by coefficients (numbers placed to the left of
each formula). A coefficient of 1 is typically omitted.

18. • The law of conservation is followed regardless of the type
of chemical reaction that occurs.
• According to the law of conservation of mass, mass is
conserved from reactants to products. Nothing is
destroyed and nothing is created.
• What goes in comes out is a concise statement of the law
of conservation of mass.

19. Steps on how
to write
chemical
equation
• Identify the type of reactant
and product. Write a word
equation.
• Convert the chemical names
into chemical formulas and
write the state symbols.
• Balance the equations.

20. Example:
Solid elemental Phosphorus (P4) reacts with Fluorine gas
to produce gaseous Pentafluoride

21. Chemical Reactions
• Chemical reactions involve a rearrangement of
atoms and ions through breaking and forming of
bonds to create new and different substances
• Since this forming of bonds cannot be directly
observed, there are certain things to look for that
help identify if a chemical reaction has taken place.

22. Types and Evidences that a
Chemical Reaction has Occurred
Formation
of a solid (a
precipitate)
Evolution of
gas (bubble
formation)
Change in
temperature
Here are some evidences that a chemical reaction has
occurred:

23. Most chemical reactions can be
classified into five types:
• Decomposition reaction - a reactant breaks down into two
or more products.
2. Synthesis reaction - two or more reactants form a single
product.
3. Double Displacement - two ionic compounds exchange ions.

24. 4. Single Displacement Reaction - one element replaces
another in a compound.
5. Combustion Reaction - a hydrocarbon reacts with oxygen
to form carbon dioxide and water.