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GHW# 5: Louisiana Tech University, Chemistry 100 POGIL (Process Oriented Guided
Inquiry Learning) exercise on Chapter 2. Atoms and Elements: Isotopes, Mole and
Grams of Substances.
Why?
Why do we need the concept of isotopes describing an element? How average atomic masse is calculated
from isotopic masses? In chemistry, why is the concept of mole central to standard measurement of amount of
substance? Why are amount of substances measured in moles when they react according to the number,
stoichiometric coefficient found in chemical equation and why do they react in simple ratios of moles? Could
you measure moles directly measured using a chemical balances which give readings in grams? How is this
problem get boiled down to: converting amount of a substance from grams to mole, and later converting from
moles of products to grams which are actually measured directly. Why is it important that we understand the
concept of mole as the basis for all calculations in chemistry (stoichiometry) and have conceptual understanding
of how Mole is related to Avogadro’s number.
Learning Objectives
The students should be able to understand the concepts and have working knowledge in the following:
Isotopes and average atomic masses
Amount of Substance-The mole
1. Relate masses of elements to the mole, Avogadro's number, and molar mass.
2. Describe the need of a "mole" to do a chemical reaction.
3. Describe the Avogadro's number: 6.022 x 1023.
Molar Mass and Problem Solving
1. Calculate the formula weight given the element.
2. Do gram-mole and mole-gram conversions for elements.
3. Calculate number of moles given mass and the formula.
4. Calculate mass given number of moles and formula.
5. Calculate number of particles (atoms, ions, and molecules) given the number of moles and formula.
6. Identify an element given the mass of an atom in grams.
Success Criteria
Understand the differences between isotopes of an element, be able to calculate average atomic masses, the
concept of moles, converting grams to moles, writing balanced chemical equations, and using stoichiometric
coefficients as conversion factors to grams to mole conversions.
Resources
Chemistry: A Molecular Approach, 2nd Edition-Nivaldo J. Tro - Pearson Prentice Hall.
Prerequisites
High school chemistry: Definition of matter. Scientific method, Dalton’s Atomic theory, Components of atoms:
nucleus (proton and neutrons) and electrons. Atoms, molecules and ions.
New Concepts
Isotopes and Atomic Weights
1.Describe isotopes and the measurement of their masses and percent abundances by a mass
spectrometer.
2. Use a) isotopes b) mass of isotopes in amu c) percent abundances of isotopes to calculate average
atomic masses in amu or relative isotopic composition given mass spectrometer data.
3. Explain the difference between the atomic number and the atomic weight of an element and find
this information for any element from the periodic table
Isotopes and Average Atomic Weights, AAW
a)
Ma x A + Mb x B + ….+ Mn x N
b)
Ma x a + Mb x b + ….+ Mn x N
100
= AAM
= AAM
Ma = mass of isotope a, Mb = mass of isotope b, Mn = mass of isotope n
A = fractional abundance of a, B = fractional abundance of b, N = fractional abundance of nth isotope
a = % abundance of a, b = % abundance of b, n = % abundance of nth isotope
AAM = Average atomic mass (Reported on the Periodic Table)
E. g. Gallium in nature consists of two isotopes, gallium-69, with a mass of 69.926 amu and a fractional
abundance of 0.601; and gallium-71, with a mass of 70.925 amu and a fractional abundance of 0.399.
Calculate the weighted average atomic mass of gallium.
Ans. AAM (Ga) = 69.723 u (amu)
The mole (6.022 x 1023 particles)-Chemists Dozen
Chemists measure chemical in grams as the amount in the reaction. Therefore, we need a conversion
factor to convert grams to atoms or molecules. Mole is the connection or the conversion factor between atoms and
grams. Mole is just a large number 6.022 x 1023 for counting atoms like dozen -12 for counting to make counting
of eggs easier. Since atoms are so small, we need large number of them to make it physically observable and able
to weigh in gram quantities.
Gram Mole and the Avogadro's Number
The gram mole is the grams of any chemical substance using the value atomic mass obtained from the periodic
table. E.g. for carbon gram mole is 12.01 grams of carbon since its atomic mass is 12.01 amu in the periodic
table. if you take atomic mass in grams the number of atoms is simply 6.022 x 10 23 atoms or particles. In other
words gram atomic weight or gram molecular weight contains 6.022 x 1023 atoms, molecules or particles. This
number is called Avogadro's number or mole of particles.
Mole
6.022 x 1023
Gram mole
12.01 g of He
18.02 g of H2O
x.xx g of NO3-
atoms
gram atomic weight
12.01 g of He
---------
Type of particle
molecules
gram molecular weight
----18.02 g of H2O
-----
ions
gram ion weight
--------x.xx g of NO3-
How many moles of iron (Fe) are present in 180.1g of elemental iron?
Atomic mass (A.M) Fe = 55.85 g/mol This is a problem to convert grams to moles.
Conversion factor is 55.85 g Fe = 1 mol
180.1 g Fe
x
1 mol
= 3.225 mol Fe
55.85 g Fe
What is the mass of 7.5 x 105 atoms of Cu in grams?
This problem requires conversion of atoms to grams. 1 mol = 63.55g Cu = gram atomic weight 1 mol = 6.022 x
1023 atoms 6.022 x 1023 Cu atoms = 63.55 grams Cu Therefore,
7.5 x 105 Cu atom x
63.55 g Cu
=7.9 x 10-17g Cu
6.022 x 1023 Cu atoms
An atom weighs 7.47 x 10-23 g. What is the name of the element this atom belongs to? First convert g to amu
and look up in the periodic table and find out the element. Conversion factor: 1g = 6.022 x 1023 amu.
7.47 x 10-23 g x
6.022 x 1023 amu
= 44.98 amu
1g
In the periodic table, element with an atomic mass closer to the value calculated is Sc (Scandium). The element is
Sc.
GHW#5 Name: _________________________ Date submitted:________ Group Name:______
Key Questions (relatively simple to answer using the Focus Information)
Periodic Law
1) What group or family to the following elements belong to?
a) Na
b) Ca
c) Cl
d) He
2) List two elements in each of the following:
a) Main group elements b) Transition elements c) Lanthanides d) Actinides
3) Identify the following elements as either metals, non-metals, metalloids, noble gases, transition
metals, actinides or, and lanthanides.
a) Si
b) Ca
c) Cl
d) Eu
e) Pu
f) Ni
g) Ar
4) Which of the following are main group elements?
a) Si
b) Ca
c) Cl
d) Eu
e) Pu
f) Ni
g) Ar
Key Questions (relatively simple to answer using the Focus Information)
Atomic Structure of elemetns
5) What are isotopes?
6) How many isotopes does the elemental hydrogen, has? Give their symbols. Are they all these isotopes
stable?
7) Naturally occurring carbon had two stable isotopes (carbon-12 and carbon 13) and an unstable isotope,
carbon -14. Write isotopic symbols for
a) carbon-12
b) carbon-13
c) carbon-14:
b) How many protons, neutrons, and electrons are in the carbon-12 isotope?
8) How many electrons, protons, and neutrons a
24
Mg2+ isotope have?
Finding Patterns: The Periodic Law and the Periodic Table
9) Order of elements in the modern periodic table is arranged according to atomic number. In his periodic
table, Mendeleev used atomic weights not atomic numbers.
a) In the periodic table what elements should have been swapped if Mendeleev was to following order of
increasing atomic masses strictly?
b) In the periodic table, Why is atomic masses approximately correlated with atomic number?
c) What is most directly involved in chemical reactivity: nucleus or the outermost electrons?
d) Why do the elements in a group of the periodic table have similar chemical properties?
Atomic Mass: The Average Mass of an Element’s Atoms
10) Silicon exists as a mixture of three isotopes. Determine its average atomic mass based on the following data
to correct significant figure.
Isotope
Mass (amu or u)
Abundance
28
Si
Si
30
Si
29
27.976 9265
28.976 4947
29.973 7702
92.23 %
4.67 %
3.10 %
Molar Mass: Counting Atoms by Weighing Them
11) What is a
a) Mole
b) gram-mole?
12) What is the mass in grams of a Cu atom?
13) An atom weighs 4.66 x 10-23 g. What is the name of the element this atom belongs to?
14) What is the molecular mass (M.M.) of benzene (C6H6) and how many grams are there
in 1 more of benzene?
15) Find the formula weight (F.W.) of NaCl.
16) Determine the number of moles of CO2 (M.M. 44.01 g/mol ) in 454 grams.
17) Determine the mass in grams of 3.60 mol of H2SO4 (M.M 98.08).
18) Calculate the grams present in 3.40 x 10-5 moles 0.100 moles of Na2CO3 (F.W.
105.99).
19) Determine the number of molecules of H2O in 3 moles H2O.
20) How many atoms are present in 17.0 mol of water?
Exercises (Applying concepts)
Of course if we used some other mass unit for the mole such as "pound mole", the
"number" would be different than 6.022 x 1023.
21) Given 5 moles of Sulfuric Acid having a formula of H2SO4 answer the following questions:
a) How many grams of Sulfuric Acid would this be?
b) How many molecules of Sulfuric Acid would that be?
c) How many Hydrogen atoms would be in this amount of Sulfuric Acid?
d) How many Oxygen atoms would there be in this much Sulfuric Acid?
22) Given the chemical reaction: N2 + 3 H2 ---> 2 NH3,
a) Write all more conversion factors in the chemical equation:
b) Determine the moles of H2 that will react with the 4 moles of N2
c) Determine the grams of H2 that will react with the 4 moles of N2