STOICHIOMETRY

1
STOICHIOMETRY

• CHEMICAL MEASUREMENTS

Unit 6 /Ch 10
2
Measurements

• Atomic mass The mass of an atom expressed
  relative to the mass assigned to carbon-12.
• Atomic Mass Unit (amu).
• Formula Mass The sum of the atomic masses of
  all the atoms in a compound.
• Examples.

3
The Procedure

• Distribute the subscript outside the ( ).
• List the elements in the compound.
• List the number of atoms for each.
• Look up the atomic mass for each and multiply by
  the number of atoms.
• Include significant figures and scientific
  notation as required.
• Examples.

4
The Mole

• A mole of any element is defined as the number of
  atoms of that element equal to the number of
  atoms in exactly 12.0g of carbon-12.
• The number of atoms in one mole of atoms is
  always the same.
• Avagadros Number (NA) 6.02 X 1023
• The mass in grams of one mole of a substance is
  numerically equal to its atomic mass or formula
  mass in atomic mass units.
• Examples.

5
Whats in a mole?

• Atoms and moles The number of atoms in one mole
  of an element is always 6.02 X 1023.
• Molecules and moles The number of molecules in
  one mole of any molecular compound is always 6.02
  X 1023.
• Diatomic Molecules vs. Elements.
• Formula units and moles The number of formula
  units in one mole of an ionic compound is always
  6.02 X 1023.

6
Molar Mass

• The mass in grams of one mole of a substance is
  called the molar mass of the substance.
• Molar mass depends on the masses of the particles
  that make up the substance (atoms, molecules, or
  formula units).
• The molar mass of a substance can be determined
  from its atomic mass or formula mass.

7
The Mole Concept

• The mole measures both a mass and a number of
  particles.
• It is the central unit for converting the amount
  of a substance from one type of measurement to
  another.
• If you know the mass of a given amount of a
  substance you can calculate the number of moles
  of the substance.

8
The Bridge
mass
particles
Use Avogadros (6.02 E 23)
MOLE
Use molar mass
Use 22.4 L/1 mol
volume
9
Mole Conversions

• Identify known and unknown.
• Select the correct conversion factor.
• Set up an equation to cancel units.
• Solve the problem.
• Sigfigs, scientific notation, units, and
  substance.

10
Mole Conversions

• Mass to moles.
• How many moles of NaCl are in 68.9 g?
• Moles to mass.
• How many grams are in 2.4 moles of LiOH?
• Moles to particles.
• How many atoms are in 1.5 moles of Calcium?
• Particles to moles.
• How many moles are in 3.42 E 24 formula units of
  KBr?

11
Moles and Gases

• Avagadro proposed that at the same temperature
  and pressure, equal volumes of gases contain the
  same number of gas particles.
• One mole of any gas _at_STP has a volume of 22.4L.
• This volume of gas is called the molar volume.
• Used to convert between the volume of gas _at_ of
  STP, the of moles of gas, the particles in
  the gas, and the mass of the gas.
• Practice problems.

12
CompositionThe mass of each element in a
compound compared to the entire mass of the
compound.

• Solve for the molar mass (GFM or GMM).
• Solve for each element using the formula
• grams of element X 100 molar mass
• The sum of the percentages should equal 100.
• Practice problems.

13
Empirical FormulaA formula that gives the
simplest whole number ratio of the atoms of the
elements.

• Assume 100g and convert all signs to grams.
• Convert grams of elements to moles.
• Divide all the elements by the lowest of moles
  to get the ratio.
• Rounding
• 0.01-0.44 round DOWN
• 0.45-0.55 multiply by 2
• 0.56-0.99 round UP
• Write the empirical formula.
• Practice problems.

14
Empirical FormulaSample Problems

• A compound was analyzed and found to contain 13.5
  g Ca, 10.8 g O, and 0.675 g H. What is the
  empirical formula for the compound?
• Determine the empirical formula of a compound
  containing 5.75 g Na, 3.5 g N, and 12.0 g O.

15
Molecular FormulaThe formula that gives the
actual of atoms of each element in a molecular
compound.

• The molecular formula is a multiple of the
  empirical formula.
• Procedure
• n GMM Molecular
  GMM Empirical

16
Molecular FormulaSample Problems

• Ribose is an important sugar that is found in DNA
  and RNA. Ribose has a molar mass of 150 g/mol
  and a chemical composition of 40.0 carbon, 6.67
  hydrogen, and 53.3 oxygen. What is the
  molecular formula for ribose?