laws of chemical combination + stoichiometry
DESCRIPTION
TRANSCRIPT
Laws of Chemical Combination
Chem1 SY 10-11
-kaaferrer
What makes compounds different?
Law of Constant Composition 1799 Joseph Proust
a chemical compound contains the same elements in exactly the same proportions (ratios) by mass regardless of the size of the sample or source of the compound
For example, water always consists of oxygen and hydrogen atoms, and it is always 89 percent oxygen by mass and 11 percent hydrogen by mass
Does mass change during a chemical reaction?
Law of Conservation of Mass Lavoisier heated a measured amount of
mercury to form the red oxide of mercury. He measured the amount of oxygen removed from the jar and the amount of red oxide formed. When the reaction was reversed, he found the original amounts of mercury and oxygen.
Law of Conservation of Mass 1744 Antoine Lavoisier matter can not be created or destroyed in
ordinary chemical or physical changes. the mass of the reactants (starting materials)
equals the mass of the products
2Mg (s) + O2 (g) → 2MgO (s)
48.6 g 32.0 g 80.6 g
Example
10 grams of CaCO3 on heating gave 4.4g of CO2 and 5.6 of CaO. Show that these observations are in agreement with the law of conservation
Law of Multiple Proportions 1803 John Dalton
States that when two elements combine to form more than one compound, the masses of one element which combine with a fixed mass of the other element are in ratios of small whole numbers
Example: Carbon monoxide (CO): 12 parts by mass of
carbon combines with 16 parts by mass of oxygen.
Carbon dioxide (CO2): 12 parts by mass of carbon combines with 32 parts by mass of oxygen.
Ratio of the masses of oxygen that combines with a fixed mass of carbon (12 parts) 16: 32 or 1: 2
Water has an oxygen-to-hydrogen mass ratio of 7.9:1.
Hydrogen peroxide, another compound consisting of oxygen and hydrogen, has an oxygen-to-hydrogen mass ratio of 15.8:1.
Ratio of the masses of oxygen that combines with a fixed mass of hydrogen is 7.9: 15.8 or 1: 2
Chemical Formula Relationships
How many number of ATOMS? Ca(NO)3
NaNO3
Ba(IO3 )2
Molecular and Formula Weights The sum of all the atomic weights of each
atom in its chemical formula.
Formula mass = ∑ atomic masses in the formula unit
SEATWORK!
Percentage Composition from Formulas Percentage by mass contributed by each
element in the substance.
%element= (# of atoms of that element)(atomic mass of that element)
------------------------------------------------------------------------
formula mass of the compound
Stoichiometry
The quantitative relationships between the substances involved in a chemical reaction, established by the equation for the reaction
Terminologies
ATOM◦ Smallest particle of an element
MOLECULE◦ Smallest unit particle of a pure substance
ION◦ An atom or group of bonded atoms with electrical charge
because of an excess or deficiency of electrons
ELEMENT◦ Pure substance; CANNOT be broken down into 2 or more
pure substances by chemical means
COMPOUND◦ Pure substance; CAN be broken down into 2 or more pure
substances by chemical means
Atomic Mass
Atomic Mass = used to numerically indicate the mass of an atom in its ground state, it is expressed in the non SI unit of u
u = refers to unified atomic mass unit (formerly known as atomic mass unit or amu)
1 amu = 1/12 the mass of carbon-12 atom, therefore the mass of C-12 atom is made EQUAL to 12
amu Carbon-12 atom is an isotope of carbon
1 amu = 1.66 x 10-24 g
Atomic Mass
Mass of e = 1/1800 of mass of p and n so it is negligible making the equation
Atomic mass of 12C = mass of p + mass of n
subatomic particle
charge Mass (amu)
Neutron None 1.0087 ≈ 1
Proton Positive 1.0073 ≈ 1
Electron Negative 5.486 x 10-4 ≈ 0
Atomic Mass vs. Average Atomic Mass
Atomic Mass Average Atomic Mass
For carbon it is 12 u not 12.01 u
*Used to relate the fact that the numerical value assigned to each element in the periodic table reflects the average abundances of the atoms that compose a naturally occurring element
*Related to isotopes
*For carbon it is 12.01 u
*Chemists often will use the term “atomic mass” when they are actually referring to average atomic mass of an atom.
Average Atomic Mass (calculation)
Solve for the Average Atomic Mass of the element Boron
Average atomic mass = ∑ (mass x percent abundance)
Where ∑ means “sum”
Isotope Mass (u) Percent Abundance
11B 11.009305 80.110B 10.012937 19.9
Relative Atomic Mass & Atomic Weight
Have different meaning from atomic mass but synonymous with each other, although of different historical origins
Relative Atomic Mass “Ratio” of the average mass of the atom to the
unified atomic mass; dimensionless
Atomic Weight The name Dalton used in the early 19th century to
numerically describe the weight of atoms relative to each other
The average mass of the molecules of a binary compound (non metal-non metal)
Unit: u
Ex.: The molecular mass of carbon dioxide gas, CO2 is 28.01 u.
Molecular Mass
The average mass of the molecules of an ionic compound (metal-non metal)
Unit: u
Ex.: The formula mass of barium chloride, BaCl2 is 208.2 u.
Formula Mass
The Mole Concept
Used to describe the number of particles (atoms, molecules, etc.) that make up sample of matter
“One mole is the amount of any substance that contains the same number of units as the number of atoms in exactly 12 grams of carbon-12.”
1 mol of any substance = 6.02 x 1023 units of that substance,
where 6.02 x 1023 Avogadro’s Number, N
Molar Mass
Mass of 1 mole of substance SI unit: g/mol Provides a bridge between mass and amount
Conversion
GRAMS MOLESFORMULA
UNITS
Use molar mass
Use N
Atomic mass Molar mass
Unit: g/mol
Unit:amu; u
Atomic level
Macroscopic level
Numerically
equivalent
Comparison of Atomic and Molar Mass
Total massof p+ and no
Mass of 1 mole
Empirical Formula and Molecular Formula
Shows the relative number of atom of each element in the compound
EMPIRICAL FORMULA
A compound is found out to contain 20.0% carbon, 2.2% hydrogen, and 77.8% chlorine. Determine EF of the compound.
Problem 1
Shows the actual number of atom of each element in the compound
MOLECULAR FORMULA
MF = (EF)nwhere n is an integer
therefore,
n = MMF
MEFwhere M is molar mass
Relationship between EF & MF
A compound with the empirical formula C2H5 has a molar mass of 58.12 g/mol. Find the molecular formula of the compound.
Problem 2
The molar mass of a compound that is 54.6% carbon, 9.0% hydrogen and 36.4% oxygen is 88 g/mol. Find MF of the compound.
Problem 3
The Mole
One mole is the amount of any substance that contains the same number of units as the number of atoms in exactly 12 grams of carbon-12.
6.02 x 1023 Avogadro’s Number
MOLAR Mass The mass in grams of one (1) mole of a
substance. g/mol, grams per mole
The molar mass of any substance in grams per mole is always numerically equal to the atomic, formula, or molecular mass of the substance in amu.
moleatom
s gramFormula weight
Avogadro’s
number