GSCI 163 – The matter of matter
Lecture 8
Activity sheet
Activity one – Exploring the concepts of chemical equation balance, formula weight, moles, Avogadro’s
number.
In this activity you will receive a few cups with different beads inside. These beads will serve as an
illustration of atoms and will help you understand in a concrete way fundamental issues related to how
atoms rearrange in a chemical reaction.
The activity should proceed as follows. You will receive a set of instructions. Then you will focus on
answering the questions that are being proposed in sequence. At each step the instructor may call the
entire class to a group discussion or debate. Please fill in the blank spaces and answer the questions to
the best of your ability. Ask questions and talk to your friends.
Balancing equations
1. Get yourself acquainted with the “atoms” inside the cups on your table
a. Small beads represent hydrogen atoms
b. Medium beads represent carbon atoms
c. Big beads represent oxygen atoms
2. You will be given a number of chemical reactions to balance.
a. First write down the chemical formulas for the chemical reaction in the balance sheet
b. Represent each chemical formula with actual “atoms” fixing them in scotch tape
c. Remember that the number of atoms on both sides of the chemical equation needs to
be equal. Thus add as many compounds as needed to balance the equation
d. Finally, write down the correct balanced equation
Mass of reaction
Let us say we need to make water out of a combination of H2 and O2. Here is the chemical equation of
this reaction:
2H2 + O2  2H2O
Molecular weight
1. First find the masses of the H2 “molecule”. Let us call this the molecular weight of H2. Write
down the “molecular weight.”
2. Find the mass of the O2 molecule. Write down the molecular weight.
3. Find the mass of the H2O molecule. Write down the molecular weight.
4. What percentage of the mass of the molecule of H2O corresponds to the mass of hydrogen?
5. What percentage of the mass of the molecule of H2O corresponds to the mass of Oxygen?
We call these numbers the percentage mass of the formula weight. Try now to find the
percentage masses of hydrogen and oxygen in a real molecule using your periodic table.
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Water H2O Hydrogen peroxide H2O2 –
Glucose C6H12O6 -
Moles
Now we will explore the concept of moles. You should have in front of you cup with “atoms” of
hydrogen (black), carbon (silver) and oxygen (blue).
1. How many molecules of H2 do you have in your cup?
2. What is the total “mass” of H2 molecules in your cup?
3. Can you estimate how many molecules are there in the cup of your neighboring group without
counting them? How? Write the number of molecules.
4. How many molecules of O2 do you have in your cup? What is the total “mass” of O2 molecules in
your cup?
5. Fill in the blanks now. If you react ____________g of H2 with ______________g of O2 available
in your cup, how many grams of H2O will it be produced?
6. Do you have any leftovers from the reaction? If so, which compound and how much?
Suppose I need to perform a much bigger reaction to make lots of water.
1. How many “molecules” of H2 is there in the 1 litter beaker? How many “molecules” of O2 is
there in the 1 litter beaker?
2. Since this is such a big number we need to simplify and say that every 1000 beads we call mile. If
so, how many “miles” are there in the beaker of H2 and O2?
3. If a reaction is made between the H2 and O2, which reactant will be used up? How much of the
other will be left out?
a. Here write the mass left out
b. Write here the number of miles left out
c. Write here the number of elements left out
Application
Here we called “miles” 1000 units of each element. In chemistry we call Moles 6.02x1023 units of the
compound.
Let us practice what we learned.
Consider a reaction of sodium and chloride:
2Na + Cl2  2NaCl
Suppose a reaction of 78.0g of Na with 35.4g of Cl2 produces some NaCl.
1. How many moles of Na are there initially?
2. How many moles of Cl2 are there initially?
3. Will there be any compound leftover after the reaction? Which and how much?
Measurement and precision
In this activity we will do some measurements and discuss the quality and precision of our
measurements.
1. To start the activity, measure the mass of a blue bead. Report your measurement.
2. Is the mass of all blue beads available in the cup equal? List each one of them (make a table)
3. What is the best way to report the mass of the blue beads? How can we report the spread?
4. Write down the concepts discussed in class.
5. What is the mass of a small black bead? How can we measure this number precisely? (how
many significant figures?
6. How can I report the error in the measurement and for the small beads for the entire beaker?
Define:
Average
Statistical error
Systematic error
Standard deviation
Standard deviation of the mean.