Chem by s0981Z3

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									Chem. 152                                                                 Dr. Saidane
                                         Stoichiometry


Skills you should have mastered
Conceptual
   1. Explain the significance of the stoichiometric coefficients in a chemical equation.
   2. Explain what is meant by a mole ratio and state how it is used.
Problem-solving
   1. Carry out mole-to-mole, mass-to-mole, and mass-to-mass, mass-to-volume,
        volume-to-volume calculations for any two species involved in a chemical
        reaction.
   2. Calculate the theoretical and percentage yields of the products of a reaction, given
        the mass of starting material.
   3. Identify the limiting reactant of a reaction and calculate the amount of excess
        reactant present, given the initial mass of each reactant.
   4. Use the limiting reactant to calculate the yield of a product.




        How much starting material must be used to obtain a specific amount of product?
How much products will be formed from specific amounts of starting material
(reactants)? Stoichiometry is used to interpret a reaction quantitatively. It involves the
mass relationship between reactants and products in a chemical reaction. All reaction-
stoichiometry calculations start with a balanced chemical equation. This equation gives
the relative number of moles of reactants and products. Solving reaction-stoichiometry
problems requires the use of a mole ratio to convert from moles of a substance A to
moles of a substance B in the chemical reaction. A mole ratio is a conversion factor that
relates the amounts in moles of any two substances involved in a chemical reaction.
There are different types of reaction-stoichiometry problem that can be classified
according to the given and to the unknown.
       Mole-to-Mole Calculations. The known is moles of A and the unknown is
        moles of B: Moles of B = Given moles of A x (moles of B/moles of A)


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        with (moles of B/moles of A), the molar ratio from balanced equation.
       Mass-to-Moles Calculations. The known is the mass of A and the unknown is
        the mole of B.
          a) Convert mass of A to moles of A using the molar mass of A.
          b) Convert moles of A to moles of B using the molar ratio from the chemical equation.
     Mass-to-Mass Calculations. The known is Mass of A and the unknown is the
        mass of B. In this case, follow the following steps:
           a) Convert mass of A to moles of A using the molar mass.
           b) Convert moles of A to moles of B using the molar ratio from the equation.
           c) Convert moles of B to mass of B using the molar mass of B.
       Mass-to-Volume Calculations. The known is the mass of A and the unknown is
        the volume of B. In this case we do the following steps:
           a) Convert the given mass to moles using the molar mass as a unit factor.
           b) Convert the moles of the given to moles of the unknown using the coefficients in
               the balanced equation.
           c) Convert the moles of unknown to liters using the molar volume of a gas as a unit
               factor.
       Volume-to-Volume Calculations. Gay-Lussac discovered that volumes of gases
        under similar conditions combine in small whole number ratios. This is the law
        of combining volumes. The known is the volume of A and the unknown is the
        volume of B. In this case there is one step:
           a) Convert the given volume to the unknown volume using the mole ratio (therefore
               the volume ratio) from the balanced chemical equation.




Limiting Reactant. In the laboratory, reactions are rarely carried out with exactly the
required amount of each of the reactants. In most cases, one of the reactant is present in
excess; that is there is more than the exact amount required to react. The limiting
reactant is the reactant that limits the amounts of the other reactants that can combine
and the amounts of product that can form in a chemical reaction. In order to determine
the limiting reactant, the given amount of either reactant is used to calculate the required




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amount of the other reactant. The calculated amount is then compared with the amount
actually available.


Percent Yield. The theoretical yield of a product is the maximum amount of product
that can be produced from a given amount of reactant. In most chemical reactions, the
measured amount of a product obtained from a reaction is less than the theoretical yield,
and is called the actual yield. The percent yield is the ratio of the actual yield to the
theoretical yield, multiplied by 100.
               Percent yield = (actual yield / theoretical yield) x 100
If a reactant takes part of only one reaction, then the yield of the product would be 100%
because no competing reactions are taking place. However, if a reactant can take part in
more than one reaction at the same time, then the yield of a particular product will be less
than 100% because other products will also form.




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