Molar Ratios From Empirical Formulas
Empirical formula the smallest whole number ratio of
atoms (or moles of atoms) of each element in a substance
H2O 2 H atoms for every 1 O atom 2 moles of H atoms for every mole of O
atoms
Molar Ratios From Empirical Formulas
The relative number of moles of each element in a substance can be used as a conversion factor called the molar ratio.
Molar ratio = moles element A mole of substance
Molar ratio = moles element Amoles element B
or
Molar Ratios From Empirical Formulas
Fe2O3:
Molar Ratio = 2 moles of Femole Fe2O3
Molar Ratio = 3 moles Omole Fe2O3
Molar Ratio = 2 moles Fe3 moles O
Molar Ratios From Empirical Formulas
Molar ratios can be used to determine the number of moles of a particular element in a given substance.
MolesA
MolesB
Molar Ratio
Molar Ratios From Empirical Formulas
Example: How many moles of Na+ ions are present in 2.5 moles of Na2SO4 ?
Molar Ratios From Empirical Formulas
Remember, once you find the number of moles of a substance present, you can use: Molar mass to find the number of grams Avogadro’s number to find the number of
atoms, ions, or molecules
Moles B
Grams B
Atoms B
Molar
mass
N
Moles A
MolarRatio
Molar Ratios From Empirical Formulas
Example: What is the mass of iron present in 4.00 moles of Fe2O3?
Quantitative Information from Chemical Equations
Coefficients in a balanced equation number of molecules (formula units, etc) number of moles
2 H2 + O2 2 H2O
2 molecules 2 molecules1 molecules2(6.02x1023) molecules 6.02x1023 molecules 2(6.02x1023) molecules
2 moles 1 mole 2 moles
Quantitative Information from Chemical Equations
In industry, aspirin is prepared by:
C7H6O3 + C4H6O3 C9H8O4 + HC2H3O2
Salicylicacid
Aceticanhydride
Acetyl-salicylic
acid
Aceticacid
Chemists want to use the right amount of reactants in order to minimize “left over” reactants that contaminate the product.
Quantitative Information from Chemical Equations
The coefficients in a balanced chemical equation can be used to relate the number of moles of each substance involved in a reaction.
Molar ratios
mol reactant mol reactant mol productmol reactant mol product mol product
3 different molar ratios (and their inverses) can be written:
1 mol N2 1 mol N2 3 mol H2
3 mol H2 2 mol NH3 2 mole NH3
Molar Ratios from Chemical Equations
For the reaction:
N2 + 3 H2 2 NH3
Molar Ratios from Chemical Equations
MOLAR RATIOS = CONVERSION FACTORS
Moles of product that can be formed from a certain number of moles of reactant(s)
Moles of reactants needed to form a certain number of moles of product
Moles of reactant 2 needed to completely react with reactant 1
Quantitative Information from Chemical Equations
Example: If you have 1.0 mole of H2, how many moles of NH3 can you produce?
N2 + 3 H2 2 NH3
Note: Make sure your equation is balanced!
Quantitative Information from Chemical Equations
Example: If you have 1.0 mole of H2, how many moles of N2 will be required to completely react all of the H2?
N2 + 3 H2 2 NH3
Quantitative Information from Chemical Equations
Example: How many moles of N2 are needed to produce 0.50 moles of NH3?
N2 + 3 H2 2 NH3
Quantitative Information from Chemical Equations
Finding # of moles is great BUT
You don’t measure out moles in the lab!
Chemists use a balance to measure the mass of a substance used or produced in a reaction.
How can you determine the mass of reactants or products?
Quantitative Information from Chemical Equations
Use the molar mass to convert from moles to grams The number of grams of a substance
per mole
Quantitative Information from Chemical Equations
Mass (g) Compound A
Moles Compound A
Moles Compound B
Mass (g) Compound B
Molar
mass
Molar
mass
Molar ratio
“The MAP”
Quantitative Information from Chemical Equations
Example: How many grams of water will be produced by the complete combustion of 10.0 g of propane?
Combustion Reactions – Revisited!
You should be able to write a balanced equation for the combustion of an organic compound or a metal.
Organic compounds:
Metals:
CnHm + O2 CO2 + H2O
CxHYOn + O2 CO2 + H2O
Metal + O2 Metal oxide
Not bal.
Quantitative Information from Chemical Equations
Example: Hydrofluoric acid can’t be stored in glass because it attacks the silicates in the glass:
Na2SiO3 (s) + 8 HF (aq) H2SiF6 (aq) + 2 NaF (aq) + 3 H2O
How many grams of HF are needed to dissolve 55.0 g of Na2SiO3?
Quantitative Information from Chemical Equations
Making Bologna Sandwiches
Suppose you were going to make bologna sandwiches:
+ +
2 Bread + 1 Bologna 1 sandwich
Making Bologna Sandwiches
2 Bread + 1 Bologna 1 sandwich
+
We can only make 4 sandwiches because we don’t have enough bologna!
Bologna = limiting reagent or limiting reactant
Limiting Reagent or Limiting Reactants
Similar situations occur in chemical reactions when one of the reactants is used up before the others.
No further reaction can occur
The excess reactant(s) are “leftovers.”
Limiting Reagent or Limiting Reactants
2 H2 (g) + O2 (g) 2 H2O (l)
If we react 10 moles of H2 with 7 moles of O2, not all of the O2 will react because we will run out of H2 first!
Limiting Reagent or Limiting Reactants
10 H2 7 O2 10 H2O + 2 O2
Limiting Reagent or Limiting Reactants
Limiting reagent (limiting reactant): the reactant that is completely
consumed in a reaction
determines or limits the amount of product formed.
Limiting Reagent or Limiting Reactants
Three approaches to identifying the limiting reactant:
Compare the number of moles of each reactant needed with the number of moles of each reactant available
Calculate the number of grams of product that each reactant could formReactant that forms the least amount of product will be the limiting reagent.
OR
Limiting Reagent or Limiting Reactants
Three approaches to identifying the limiting reactant (cont.)
Pick one of the reactants and calculate the grams of the other reactant needed to exactly react with the one you picked
OR
Limiting Reagent or Limiting Reactants
First Method:
1) Convert the mass of each reactant to moles.
2) Pick one of the reactants (it doesn’t matter which one) and calculate the number of moles of the other reagent needed to completely react with the one chosen.
3) Compare the # moles needed vs. # moles available
Limiting Reagent or Limiting Reactants
Second Method:
1) Calculate the number of grams of product that each reactant could form.
2) Limiting reagent = reactant that forms the smallest mass of product.
Limiting Reagent or Limiting Reactants
Third Method:
1. Pick one reactant.
2. Calculate the grams of the other reactant needed.
3. Compare the grams needed with the grams available.
Limiting Reagent or Limiting Reactants
Example: If 10.0 grams of H2 are mixed with 75.0 grams of O2, which reactant is the limiting reagent?
2 H2 (g) + O2 (g) 2 H2O (l)
Limiting Reagent or Limiting Reactants
Method 1
Limiting Reagent or Limiting Reactants
Method 2
Limiting Reagent or Limiting Reactants
Method 3
Limiting Reagent or Limiting Reactants
Once you find the limiting reagent, you can also find the amount of product that can be formed in the reaction. If you used the second method for
identifying the limiting reagent, you’ve already done this!
Moleslimitingreagent
Molesproduct
gramsproduct
Molar
ratio
Molar
mass
Limiting Reagent or Limiting Reactants
Example: How many grams of water are produced by burning 5.00 g of methane in the presence of 5.00 g of oxygen?
CH4 (g) + 2 O2 (g) CO2 (g) + 2 H2O (l)
Example
Since the mass of both reactants is given, you need to check to see if one of them is a limiting reagent.
Example
Yield
Theoretical Yield: the quantity of product that is
calculated to form when all of the limiting reagent reacts
Actual yield: the amount of product actually
obtained in a reaction
Yield
Often, the actual yield is less than the theoretical yield: reactants may not react completely
i.e. the reaction does not go to completion
“by-products” may formunwanted side reactions (competing reactions)
difficulty isolating and purifying the desired product
Yield
Percent Yield: relates the actual yield and the
theoretical yield
% Yield = actual yield x 100%theoretical yield
Yield
Example: In a certain reaction between H2
and CO to form methanol, the theoretical yield is 83.3 g of CH3OH. If the actual yield of the reaction was 81.5 g, what was the % yield?