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SECOND QUARTER REVIEWER 3. Use the appropriate molar ratio
from the balanced equation to
TOPIC 1: QUANTITATIVE
calculate the amount (mol) of the
INFORMATION FROM BALANCED
second substance.
EQUATIONS
4. Convert the amount of the second
The coefficients in a chemical equation substance to the desired mass.
represent the relative numbers of
molecules in a reaction. For instance, the
coefficients in the balanced equation
2H2 (g) + O2 (g) → 2H2 O(g)
indicate that two molecules of H2 react
with one molecule of O2 to form two
molecules of H2 O.
We can generalize this observation to all
balanced chemical equations: The
coefficients in a balanced chemical
equation indicate both the relative
numbers of molecules (or formula units) in
the reaction and the relative numbers of
Sample Problem
moles. The figure below shows how this
observation corresponds to the law of Example:
mass conservation.
Ammonia, NH3 , is a leading industrial
chemical used in the production of
agricultural fertilizers and synthetic fibers.
It is produced by the reaction of nitrogen
and hydrogen gases:
3H2 (g) + N2 (g) → 2NH3 (g)
The balanced equation says that 3 moles of
H2 are stoichiometrically equivalent to 1
mole N2 and to 2 moles of NH3 . The ratio of
moles H2 to moles NH3 is 3:2; the ratio of
moles N2 to moles NH3 is 1:2.
The quantities 2 mol H2 , 1 mol O2 , and 2
mol H2 O given by the coefficients are called a. How many moles of NH3 will be
stoichiometrically equivalent quantities. produced if 10.4 moles H2 react
completely with N2 ?
Steps:
2 moles NH3
1. Write a balanced equation for the 10.4 moles H2 x
3 moles H2
reaction.
2. Convert the given mass of the first = 𝟔. 𝟗𝟑 𝐦𝐨𝐥𝐞𝐬 𝐍𝐇𝟑
substance to moles.
, b. How many moles of N2 are needed the conversion factor is “3 moles
to produce 42.4 moles NH3 ? O2 /2 moles Cu2 S”.
1 mole N2 Moles of O2
42.4 moles NH3 x
2 moles NH3 3 moles O2
= 10.0 moles Cu2 S x
2 moles Cu2 S
= 𝟐𝟏. 𝟐 𝐦𝐨𝐥𝐞𝐬 𝐍𝟐
= 𝟏𝟓. 𝟎 𝐦𝐨𝐥𝐞𝐬 𝐎𝟐
c. How many grams will be produced b. How many grams of sulfur dioxide
from 25.7 moles N2 ? are formed when 10.0 mol of
2 moles NH3 17.03 g NH3 copper(I) sulfide is roasted?
25.7 moles N2 x x Plan: We need the grams of product
1 mole N2 1 mole NH3
(SO2 ) that form from the given
= 𝟖𝟕𝟓 𝐠 𝐍𝐇𝟑
moles of reactant (Cu2 S). We first
d. How many grams of NH3 will be find the moles of SO2 using the
produced if 122 g N2 reacts molar ratio from the balanced
completely with H2 ? equation (2 moles SO2 /2 moles
Cu2 S) and then multiply by its
1 mole N2 2 moles NH3 17.03 g NH3
122 g N2 x x x molar mass of SO2 to find grams of
28 g N2 1 mole N2 1 mole NH3
SO2 .
= 𝟏𝟒𝟖 𝐠 𝐍𝐇𝟑
Mass (g) of SO2
Sample Problem 2 moles SO2 64.07 g SO2
= 10.0 moles Cu2 S x x
2 moles Cu2 S 1 mole SO2
In a lifetime, the average American uses = 𝟔𝟒𝟏 𝐠 𝐒𝐎𝟐
1750 lb (794 kg) of copper in coins,
Practice Problems
plumbing, and wiring. Copper is obtained
from sulfide ores, such as chalcocite, or 1. Potassium nitrate decomposes on
copper(I) sulfide, by a multistep process. heating, producing potassium
After an initial grinding, the first step is to oxide and gaseous nitrogen and
“roast” the ore (heat it strongly with oxygen. To produce 56.6 g of
oxygen gas) to form powdered copper(I) oxygen, how many (a) moles of
oxide and gaseous sulfur dioxide. potassium nitrate must be heated?
(b) grams of potassium nitrate must
Balanced Chemical Equation
be heated?
2Cu2 S(s) + 3O2(g) → 2Cu2 O(s) + 2SO2(g)
Ans.: (a) 1.42 moles potassium nitrate; (b)
a. How many moles of oxygen are 143.56 g potassium nitrate
required to roast 10.0 mol of
copper(I) sulfide? 2. Phosphine, PH3 (g) , combusts in
Plan: We are given the moles of oxygen gas to form water vapor
Cu2 S and need to find the moles of and solid tetraphosphorus
O2 . The balanced equation shows decaoxide. How many (a) moles of
that 3 moles of O2 is needed for water vapor will be produced from
every 2 moles of Cu2 S consumed, so the combustion of 36.7 moles
phosphine? (b) grams of
tetraphosphorus decaoxide
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