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Topic 4: The Mole and Stoichiometry
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Topic
The Mole and
Stoichiometry
4.1
The Mole and
Stoichiometry
4.2
Essential Knowledge
Atoms and molecules are too small to count. Mole is the unit used to tell how many particles are in a certain
amount of a substance. A mole is 602,000,000,000,000,000,000,000 particles (atoms or molecules). Expressed
in scientific notation, a mole is 6.02 x 1023 particles.
Scientific notation is used to express very small or very large measurements in powers of ten. It expresses
quantities by using a number between one and ten, which is then multiplied by ten to a power to give the
quantity its proper magnitude.
The sum of the protons and neutrons in an atom is known as the mass number.
Isotopes are atoms of the same element that have different numbers of neutrons. Some isotopes are
radioactive, many are not.
An element’s atomic mass is a weighted average of the masses of all the known isotopes of the element.
The Mole and
Stoichiometry
4.3
The Mole and
Stoichiometry
4.4
The Mole and
Stoichiometry
4.5
The mass of one mole of any element is the same as the element’s atomic mass in grams (called molar
mass).
The molar mass of a compound is the sum of the molar masses of the individual elements that make up the
compound.
The percent by mass of an element in a compound can be determined:
% by mass of element = total mass of element in compound X 100
total mass of the compound
Molar masses from the periodic table can be used to calculate the number of moles in a given mass of an
element or compound. This is because the masses on the periodic table represent the number of grams in one
mole.
Because matter cannot be created or destroyed, the total mass of the products is equal to the total mass
of the reactants in a chemical reaction.
The Mole and
Stoichiometry
Molar masses from the periodic table and mole ratios from the balanced equation can be used to calculate the
mass of a reactant or product.
The molar coefficients from balanced chemical equations are used to predict the masses of reactants or
products.
4.6
The Mole and
Stoichiometry
At STP (standard temperature and pressure, 0o Celsius and 1 atmosphere) the volume of 1 mole of any gas is
22.4 liters.
4.7
The Mole and
Stoichiometry
4.9
The number of moles of a gas (n) can be determined if the pressure (P), temperature (T), and volume (V) of the
gas sample are known, using the constant R according to the following equation:
PV = nRT
An empirical formula shows the smallest whole number ratio of elements in a compound.
Ionic solids are composed of oppositely charged ions arranged in a regular, repeating, crystal lattice structure;
the empirical formula always gives the ratio of positive to negative ions.
Covalent compounds are often in the form of individual molecules; The empirical formula gives the ratio of
atoms in one molecule. Example: The molecular formula for glucose is C6H12O6; the empirical formula is CH2O.
SOL Review: Topic 4
The Mole and
Stoichiometry
One mole is 6.02 x 1023 particles. The molar mass of a compound is equal to the sum of the molar masses of
the atoms in the compound.
4.10
One mole of any gas at STP is 22.4 liters.
Molar masses and mole ratios from balanced equations can be used to calculate the amount of a reactant
or product.
According to the Ideal Gas Law, pressure, volume, number of moles, and temperature (Kelvin) have the
relationship
PV = nRT
Review Questions:
1. The number 6230000000000 is correctly written in scientific notation as :_______
2. The number 0.000000703 is correctly written in scientific notation as :_________
3. Isotopes have the same number of ____________ but different _____________.
4. If an element has two isotopes, X-30 and X-34, and the most abundant isotope is X-30
which occurs 80% whil X-34 occurs 20%.. The average atomic mass would be________.
5. Find the molar mass of each of the following:
a. Br2 ____________
b. CS2 ____________
c. CF4 ____________
d. Ca(OH)2 ________
6. Determine the % carbon in CF4. _________________
7. Determine the number of moles in each of the following:
a. 10.0 grams of neon
b. 20.0 grams of calcium
8. Determine the number of grams in each of the following:
a. 1 mole of hydrogen ___________
b. 3 moles of methane, CH4 __________
9. For the reaction, 2NH3  3H2 + N2,
a. Identify a product. __________
b. Identify the type of reaction. _____________
c. Indicate the number of moles of hydrogen that would be produced from 1 mole of
ammonia. ________
SOL Review: Topic 4
d. Indicate the number of moles of ammonia required to make 5 moles of nitrogen.
_________
e. If you react 10 moles of ammonia how many moles of product would be formed?
____________
f. Show that mass is conserved during this reaction.
10. For the reaction, 2NH3  3H2 + N2,
a. Determine the mass of hydrogen produced from 1.00 moles of ammonia
___________
b. Determine the volume at STP of hydrogen that can be produced from 4.0 moles of
ammonia. _________
c. Determine the mass of nitrogen that can be produced from 0.50 moles of
ammonia. _________
d. Determine the volume of hydrogen gas that can be produced at STP from 0.50
moles of ammonia. ______
Use R=0.0821 L atm/mol K or R= 8.31 L kPa/mol K for the following:
11. How many moles of N2 gas occupy a volume of 25 L at 3 atm and 45 oC?
12. At what temperature, in Celsius, does 3 moles of Cl2 occupy a volume of 14 L at 4 atm?
13. What mass of O2 fills a volume of 17 L when the temperature is 308 K and the pressure is
200 kPa?
Practice Questions
(answers at the bottom)
1. One mole of any substance represents 6.02 x 1023 particles.
a. this is a small number because atoms are large
b. this is a large number because atoms are large
c. this is a large number because atoms are small
d. this is a small number because atoms are small
2. Half of a mole can be written as:
a. 3.01
b. 6.02
c. 3.01 x 1023
d. 6.02 x 1011.5
SOL Review: Topic 4
3. The mass number of an isotope of an element is:
a. protons + neutrons
b. protons + electrons
c. neutrons + electrons
d. the mass from the periodic table
4. Isotopes of the same element have:
a. the same number of protons but different electrons
b. the same number of neutrons but different mass
c. the same number of protons but a different mass
d. the same mass but a different number of protons
5. Which of the following would have the highest % oxygen?
a. H2O
b. CO2 c. NO d. Al2O3
6. 2.0 grams of helium would be:
a. more than a mole b. less than a mole
c. equal to a mole
7. 36 grams of water would be:
a. a. more than a mole b. less than a mole
8. In a normal chemical reaction:
a. the mass must stay the same
c. the energy must sty the same
9. In a chemical equation:
a. the reactants are on the right
c. the products are left
c. equal to a mole
b. the moles must stay the same
d. the substances must stay the same
b. the products are on the right
d. there is not right or left
10. The conditions at STP are:
a. OK and 1 atm
b 273oC and 0atm
c. OoC and 0 atm
d. OoC and 1 atm
11. At STP one mole of a gas occupies:
a. 6.02L b. 11.2 L c. 22.4 L d. 273L
12. Which gas occupies the largest volume at STP?
a. H2 b. H2O
c. CH4 d. C8H18 e. none of these
1. C 2. C 3. A 4. C 5. A 5. C. 7. A 8. A 9. B 10. D 11. C 12. E
SOL Review: Topic 4