Chem 400
Section
4.3
Basic
Stoich.
LECTURE PROBLEMS SET #2 Stoichiometry, % Yield, Limited Reagents & Combustion Analysis
Nuss
1. All alkali metals react with water to produce hydrogen gas and the corresponding alkali metal
hydroxide. A typical reaction is that between lithium and water:
The unbalanced equation is Li(s) + H2O(l)  LiOH(aq) + H2(g)
a) How many moles of H2 will be formed by the complete reaction of 6.23 moles of Li with water?
ANS: 3.12 mol H2.
b) How many grams of H2 will be formed by the complete reaction of 80.57g of Li with water?
ANS: 11.70 g H2.
Section
4.4
Limited
Reactant
2. Each of three 125-mL Erlenmeyer flasks contained 100. mL of 1.00 M HCl
amounts of magnesium ribbon were added to each flask as follows:
flask 1 (608. mg); flask 2 (1216. mg); and flask 3 (1824 mg).
(aq).
However, different
Write a balanced chemical equation:_______________________________________________________
For each flask (a) determine which is the limited reagent; (b) how many moles or millimoles of product
would form; and (c) how many moles and grams of excess reagent are left for flask 3?
3. Ethyl alcohol is a very clean burning fuel.
a) Write the chemical equation for the complete combustion of ethyl alcohol (C2H6O) in air.
_____________________________________________________________________________________
b) If 150. mg of the alcohol reacts with 225 mg of oxygen, how many milligrams of product form and
how many milligrams of excess reagent remain? ANS: 206 mg CO2 and 127 mg H20 formed;
42 mg ethanol remains.
4. Rust can be a hydrated mixture of FeO and Fe2O3 with a formula of Fe3O4H2O. What mass of rust
can be produced by mixing 6.0 g of iron, 4.0 g of oxygen gas and 2.0 g of water? Also calculate the
mass of each reactant left "unreacted."
ANS: 9.0g rust produced; 1.7g O2 left over; 1.4g H2O left over
Section
4.5
Percent
Yield
5. A 0.270g sample of "Fool's Gold" (iron pyrite) was heated in the presence of excess oxygen.
The unbalanced equation is FeS2(s) + O2(g)  Fe3O4(s) + sulfur dioxide gas
a) What mass of each product was formed, assuming that all of the iron sulfide reacted?
ANS: 0.288g sulfur dioxide and 0.174g Fe3O4
b) If you performed the above experiment but only obtained 200. mg of sulfur dioxide, what was the
percent yield for the reaction? ANS: 69.4%
c) If you started with 0.40 moles of FeS2 and there is a 69.4% yield, how much sulfur dioxide will
be produced? ANS: 36g
6. Ethane gas, C2H6, was completely combusted in air to give two gaseous products that always result
from combustion.
a) Write the balanced equation
b) If 600. mg of ethane gas reacts with excess oxygen gas to produce 1.30 g of carbon dioxide, what
is the percent yield? ANS: 74.0%
c) If this reaction in a different experiment proceeds in 85% yield, what is the mass in mg of water
produced from the reaction of 48.0 mg of ethane? ANS: 73 mg H2O produced
d) If the reaction again proceeds in 85% yield, what quantity of ethane, in grams, do you need to
start with, in order to actually form 3.00 g of carbon dioxide? ANS: 1.2 g C2H6
7. Phosphorous tribromide can be synthesized via the following reaction:
P4(s) + Br2(l) PBr3 (unbalanced)
Section
4.5
Percent
Yield
cont.
combined
with LR
a) If 20.0g of elemental phosphorous, P4, are combined with 50.0 mL of bromine liquid
(density = 2.93 g/cm3), what is the theoretical yield of the product?
ANS: 165 g PBr3
b) If 96.0 g of product actually form, what is the percent yield?
ANS: 58.0%
c) On another try of the reaction, it proceeded at 85.7% yield. If you start with 50.0 mL of bromine,
what number of grams of the product would you actually get?
ANS: 142g
d) If this reaction proceeds at 85.7% yield and you need to have 100. g of product, how many moles
of phosphorous should you start with, assuming you have excess bromine around?
ANS: 0.126 mole P4
8. A mixture of copper (II) sulfate pentahydrate and potassium sulfate was heated in a thermal
decomposition reaction in order to determine the amount of potassium sulfate in the mixture. The
Section
following data was obtained after carefully performing the experiment. Determine the percent
4.6
potassium sulfate.
Analysis
of
Mass of dry beaker before heating
64.2168 g
mixtures
Mass of beaker and mixture, before heating
65.8341 g
Mass of beaker and mixture after heating
65.2755 g
Show a well labeled calculation including the balanced equation for the reaction. ANS: 4.3% K2SO4
9.
Section 4.6
Formulas by
Combustion
A 428 mg sample of an organic compound that could be either a hydrocarbon or a C,H,O compound
(i.e., it may or may not have oxygen) yielded 0.642g of carbon dioxide and 0.175g of water during a
combustion analysis. What is the empirical formula? Ans:C3H4O3
If the molar mass of the compound is 176.12 g/mol, what is the molecular formula for the compound?
C6H8O6 (ascorbic acid / vitamin C)
10. Suppose you isolate an acid from clover leaves and know that it contains only the elements C, H, and
O. Combusting a 0.513 g sample of the acid in oxygen produces 0.501g CO2 and 0.103g H20. A
second experiment has shown that the molar mass of the compound is 90.04 g/mol. What are the
empirical and molecular formulas of the compound? ANS: H2C2O4