Formula Weights mass

1
Formula Weights (mass)

• The sum of the atomic weights of ea atom in a
  chem formula
• Round to 1 decimal point
• H2SO4
• (2 H x 1.0 amu) (1 S x 32.1 amu) (4 O x 16.0
  amu) 98.1 amu
• If the comp is molecular Molecular weight
• Express in grams FM of 1 mole

2
Percent Composition fr Formulas

• Part/Whole x 100 comp
• NaCl
• 23.0g
• 35.5g
• 58.5g
• Na 23.0/58.5 x 100 39.3 Na
• Cl 35.5/58.5 x 100 60.7 Cl

3
The Mole

• Once we know the formula mass for a sub, we can
  determine the of particles it contains
• Formula mass of any sub 6.02 x 10 23 particles
• particles ions, atoms, molecules
• 6.02 x 10 23 Avagadros Number
• 6.02 x 10 23 particles 1 mole of particles
• Just as a dozen 12 ___, a mole 6.02 x 10 23
  particles!

4
Interconverting Masses , Moles Numbers of
Particles

• Grams lt-gt Moles
• Since the FM in grams mole, this is a
  conversion factor
• Grams x Mole moles
  FM grams
• OR Moles x FM grams grams
• moles

5
Interconverting Masses , Moles Numbers of
Particles

• Moles lt-gt of Particles
• 1 mole 6.02 x 10 23 particles
• Mole x 6.02 x 10 23 particles mole
• Mole
• OR particles x mole particles 6.02 x 10
  23 particles

6
Empirical Formulas fr Analyses

• Simplest whole number ratio of elements in a
  compound
• H2O 2H 1O
• Convert given grams to moles.
• Divide by the smallest mole fraction.
• When answer is not a whole number (2.33, 2.25.
  2.5, etc), multiple all answers by smallest
  number to achieve a whole number answer!

7
Molecular Formula fr Empirical Formula

• The actual formula of a compound.
• 1. Must be given the molecular mass!
• 2. Calculate the empirical formula.
• Calculate the mass of the empirical formula.
• Compare the mass of the emp formula to the given
  molecular mass.
• It will be the same or a whole number.
• Molecular formula this whole number multiplied
  by the empirical formula.

8
Calculating Emp Formulas from Compositions

• Treat the comp info as though you have as 100 g
  sample
• 40 C and 60 H
• 40 g C and 60 g H
• Convert grams to moles
• Divide by the smallest mole fraction
• Multiple (if needed) for a whole number

9
Formula of a Hydrate

• A hydrate is a ionic salt with water(s) attached
  to it.
• General formula 1 MNM X H2O
• a shows the attachment of H2O
• Ratio is Always 1 salt to X waters
• MgSO47 H2O, CaCl2 2 H2O

10
Identifying a Chemical Rxn

• 1. 1 or more new subs sre formed
• 2. E taken in or given off
• Exothermic - E released
• Endothermic - E taken in
• 3. Not easily reversible

11
Chemical Equations

• Reactants -gt (change to/yield) Products
• A(s) B(l) -gt C(g) D(aq)
• 2H2 (g) O2 (g) -gt 2 H2O (g)
• S solid l liquid g gas aq aqueous soln
• Set up equation
• Write the formulas of the reactants products
• Balance by changing the coefficients
• Never change the subscripts to balance!

12
Patterns of Chemical

• Single Displacement
• element comp -gt element comp.
• Na HOH -gt H2 NaOH
• Double Displacement
• Comp AB Comp CD -gt Comp AD Comp CB
• Pb(NO3) 2 KI -gt KNO3 PbI

13
Patterns of Chemical Reactivity -cont.

• Combination (synthesis) Rxns
• 2 or more subs combine to form 1 product
• 2Mg O2 -gt 2MgO
• Decomposition Rxns
• 1 reactant produces 2 or more products
• always requires E
• CaCO3 -gt CaO CO2

14
Patterns of Chemical Reactivity-cont.

• Combustion Rxns
• Hydrocarbons combust (burn) w/ O2 in the air
• Form CO2 H2O lots of E!!!
• CH4 2O2 -gt CO2 2H2O E

15
Tips for Balancing Rxns

• 1. Treat a polyatomic ion as a unit if it is not
  changed during a reaction.
• 2. Write water as HOH.
• 3. If an element appears in more than one
  compound on the same side of the equation, leave
  it to last.
• Balance simplest elements first!
• 4. If, in the unbalanced equation, there are an
  even number of atoms of an element on one side
  an odd number on the other, place a coefficient
  of 2 in front of the comp w/ the odd of atoms.

16
Tips for Balancing Rxns -cont

• 5. Combustion of Hydrocarbons
• Multiple of 4 H atoms on each side
• Oxygen is diatomic, so there must be an even on
  both sides
• Must be an even of H2O molecules as products

17
Stoichiometry

• Calculations w/ chemical formulas equations
• Based on the law of conservation of mass

18
Mass- Mass Stoichiometry

• 1. Write the balanced equation.
• 2. Find the moles of the given sub.
• 3. Use the equation to find the ratio of unknown
  moles to given moles.
• 4. Express unknown moles in grams.

19
Limiting Reactants

• Limiting reactant - the reactant th is completely
  consumed in a reaction.
• Determines or limits the amount of product formed
• Excess reactant(s) - the other reactant(s)
• Reactions are often run w/ excess reactants

20
Limiting Reactant Calcs

• 2 reactant quantities are given
• ID the limiting reagent
• Calc the of moles of ea reactant
• Compare their ratio, use balanced equation
• Use the starting quantitiy of LR to determine
  yield

21
Theoretical Yields

• The quantity of product th is calculated to form
  when all of the limiting reactant reacts.
• Actual yield - the amount of product actually
  obtained in a rxn.
• Almost always less than theoretical yield never
  more
• Percent Yield - relates the actual yield to the
  theoreticl (calculated) yield
• Percent yield actual yield x 100
• theoretical yield