First Year in Dentistry First Semester Al-Azhar University - Gaza Lama El Banna general chemistry

1. Chapter 3
Stoichiometry: Part I: Chemical Formulas
CHEMISTRY
Mortimer 6th edition

2. • Molecules are assemblies of two or more atoms bonded
together.
• Each molecule has a chemical formula.
• The chemical formula indicates
– which atoms are found in the molecule, and
– in what proportion they are found.
• Compounds formed from molecules are molecular
compounds.
• Molecules that contain two atoms of the same element
bonded together are called diatomic molecules.
Molecules and Molecular Compounds

3. Example of Diatomic Molecules
Molecules and Molecular Compounds

4. Molecular and Empirical Formulas
• Molecular formulas
– give the actual numbers and types of atoms in a
molecule.
– Examples: H2O, CO2, CO, CH4, H2O2, O2, O3, and C2H4.
Molecules and Molecular
Compounds

5. •Most molecular substances that we will study in this
class contain only nonmetals.
Molecules and Molecular Compounds

6. • When an atom or molecule loses electrons, it becomes
positively charged.
– For example, when Na loses an electron it becomes Na+.
• Positively charged ions are called cations.
• When an atom or molecule gains electrons, it becomes
negatively charged.
• For example when Cl gains an electron it becomes Cl-.
• Negatively charged ions are called anions.
• An atom or molecule can lose more than one electron.
• When molecules loose electrons, polyatomic ions are formed.
Ions and Ionic Compounds

7. • In general: metal atoms tend to lose electrons to
become cations; nonmetal ions tend to gain electrons
to form anions.
Predicting Ionic Charge
• The number of electrons an atom loses is related to its
position on the periodic table.
Ions and Ionic Compounds

8. Predicting Ionic Charge
Ions and Ionic Compounds

9. Understanding Chemical Equations
Coefficients and subscripts included in the chemical formula have
different effects on the composition.

10. Formula Weights
Formula and Molecular Weights
• Formula weights (FW) is the sum of the atomic weights of
each atom in the chemical formula.
FW (H2SO4) = 2AW(H) + AW(S) + 4AW(O)
= 2(1.0 amu) + (32.0 amu) + 4(16.0)
= 98.0 amu
• If the chemical formula is also its molecular formula then
the weight is called the molecular weight (MW).
MW(C6H12O6) = 6(12.0 amu) + 12(1.0 amu) + 6(16.0 amu)

11. Percentage Composition from Formulas
• Percent composition is the atomic weight for each
element divided by the formula weight of the compound
multiplied by 100:
Formula Weights
   100
CompoundofFW
AWElementofAtomsofNo.
Element% 

12. Molar Mass
• Molar mass: mass in grams of 1 mole of substance (units
g/mol, g.mol-1).
• Experimentally, 1 mole of 12C = 12 g, which can be
written as 12g/mol.

13. • The unit we use to express the quantity of atoms, ions,
and molecules that an object contains is called mole.
• Mole: convenient measure chemical quantities.
• The actual number of atoms, ions, or molecules in 1
mole of something = 6.0221367  1023 (Advogadro’s
number) of that thing.
• Thus,
•1 mole of 12C atoms = 6.02 x 1023 12C atoms
•1 mole of H2O molecules = 6.02 x 1023 molecules
•1 mole of NO3
- ions = 6.02 x 1023 ions
The Mole

14. Visualizing The Mole Concept
Different Units

15. Chapter 4
Chemical Equations

16. • Balanced chemical equation gives number of
molecules that react to form products.
• Interpretation: ratio of number of moles of reactant
required to give the ratio of number of moles of
product.
• These ratios are called stoichiometric ratios.
NB: Stoichiometric ratios are ideal proportions
• Real ratios of reactants and products in the
laboratory need to be measured (in grams and
converted to moles).
Quantitative Information from Balanced
Equations

17. • If the reactants are not present in stoichiometric
amounts, at end of reaction some reactants are still
present (in excess).
• Limiting Reactant: one reactant that is consumed
Limiting Reactants

18. Limiting Reactants

19. Theoretical Yields
• The amount of product predicted from stoichiometry
taking into account limiting reagents is called the
theoretical yield.
• The percent yield relates the actual yield (amount of
material recovered in the laboratory) to the
theoretical yield:
100
yieldlTheoretica
yieldActual
Yield% 
Limiting Reactants