Chemistry help: Chemical Reactions and Stoichiometry, [Percent Yield]?

Question

1. The element arsenic can be commercially produced when arsenic (III) oxide is heated with carbon in this reaction:
2As2O3 + 3C → 3CO2 + 4As

a) If 8.87 g of As2O3 is used in the reaction and 5.33 g of As is produced, what is the percent yield?

b) In a separate reaction, if 67 g of carbon is used up and 425 g of As is produced, calculate the percent yield of this reaction.

2. Nitrogen dioxide, NO2, can react to form dinitrogen pentoxide, N2O5, by reacting with ozone, O3. The reaction occurs according to this equation:
2NO2 (g) + O3 (g) → N2O5 (s) + O2 (g)

a) Calculate the percent yield for a reaction where 0.38 g of NO2 reacts and 0.36 g N2O5 is produced.

b) What mass of N2O5 will result from the reaction of 6.0 mol of NO2 if there is a 61.1% yield in the reaction?

Answer 1

The basic equation that we will use for these kinds of problems is:
(# moles product/# moles reactant)*(stoichiometric ratio)*100% = percent yield

The stoichiometric ratio comes from the balanced chemical equation, and is the coefficient of the reactant divided by the coefficient of the product.

#1
a) 79.3%
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We are given masses of reactant and product, and are asked to determine percent yield. Percent yield is based on number of moles, not mass. So the first step is to use atomic and molecular weights to determine the # moles of As2O3 and As that we're dealing with.

atomic weight of As: 74.92160 g/mol
atomic weight of O: 15.9994 g/mol
molecular weight of As2O3 = 2*74.92160 g/mol + 3*15.9994 g/mol = 197.8414 g/mol

# moles As = 5.33g / 74.9216 g/mol = 0.071141 mol As
# moles As2O3 = 8.87g / 197.8414 g/mol = 0.044834 mol As2O3

Now we just plug the values into the basic equation and solve:
(0.071141/0.044834)*(2/4)*100% = 79.3%

b) 76.3%
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This is almost exactly like part (a).

atomic weight of C: 12.0107 g/mol
atomic weight of As: 74.92160 g/mol

# moles C = 67 g / 12.0107 g/mol = 5.578 mol C
# moles As = 425 g / 74.92160 g/mol = 5.672 mol As

Plug into basic equation:
(5.672/5.578)*(3/4)*100% = 76.3% yield

You should be able to use the basic equation and the example I've given to solve #2.

Answer 2

Use the Mole Concept.