Lab: The Mole and Avogadro’s Number
Objectives
Identify and understand the relevance of Avogadro’s number
Calculate the value of Avogadro’s number using laboratory methods
Introduction
The modern practice of chemistry derived
from thousands of years of experimentation
with foods. Processes in fermentation, curing,
preserving, and refining were the very
beginnings of modern chemistry. The modern
day scientific studies of food science and
food chemistry are two of the major areas of
research for chemists and other scientists. In
fact, your introduction to chemistry may have
begun with the study of food chemistry in
your own kitchen when you made hot
chocolate or baked chocolate chip cookies.
Creating chemical reactions and working with
chemicals in the laboratory is very similar to
the cooking process – especially baking. Your
experience with food and cooking will help
you understand important concepts when Figure 1: The Italian scientist Amedeo
working with chemicals. Take, for example, Avogadro (1776 – 1856) made contributions to
the sciences and mathematics. He is best
the process of creating chocolate chip remembered for Avogadro’s law and the
cookies. The cookies themselves are much development of Avogadro’s constant – more
different from the ingredients that you use to commonly known as Avogadro’s number.
create them. The process of mixing and
baking the ingredients to create a cookie is the same process used in a chemical
reaction. Reactants are combined and undergo a change to form a product.
Imagine a situation where you wanted to create the perfect chocolate chip cookie. Each
cookie must contain 10 chocolate chips and you were required to bake three dozen
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cookies. You could easily calculate the number of chocolate chips needed to create the
required cookies, but what quantities of the other ingredients (flour, sugar, eggs, etc.)
would be necessary to create your cookie dough? This type of problem is one faced by
chemists everyday when calculating the quantity of reactants necessary to produce the
desired amounts of products in a chemical reaction. Why is it necessary? Many
chemicals are difficult to obtain and, therefore, are very costly. Being able to accurately
predict the amount of reactants to produce the quantity of products is important to
prevent waste of materials and money. In addition, for safety reasons, certain reactions
must be controlled and conducted using exact quantities of materials.
Counting out chocolate chips is much easier than counting out individual grains of sugar
or flour – imagine if you had to count out individual atoms. In much the same way a
cook measures out the ingredients for the perfect cookie, a chemist has a tool that
provides them the information necessary to measure the quantity of molecules for a
given substance – the mole.
In chemistry, the mole is similar to the dozen – it is a number representing a quantity
that is independent of mass or volume. The mole is derived from the quantity of atoms
in 12 gram of carbon-12:
12 grams of 12C contains 6.02 x 1023 atoms (Avogadro’s number)
12 grams of 12C = 1 mole
1 mole = 6.02 x 1023 molecules or atoms
Review the ingredients for the chocolate
chip cookie recipe in Figure 2 – notice the
quantities listed for chocolate chips, eggs,
and baking soda. Imagine if the quantities
for these ingredients where expressed in
terms of moles and required you to use 1
mole of chocolate chip cookies. How many
moles of eggs and baking soda would you
need to complete your recipe correctly? In
this case, you wouldn’t measure your
ingredients in individual moles, but by their
molar mass.
Figure 2: Chocolate chip cookie recipe displaying
ingredients and quantities in mixed units.
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The molar mass of a substance is dependent
upon the substance’s atomic weight, which can
be found on the periodic table. For instance, an
individual oxygen atom with an atomic weight of
15.9994 is much larger than a hydrogen atom
with an atomic weight of 1.0079. This difference
in weight and mass would be similar to the
difference between an individual egg and an
individual chocolate chip.
You can find the atomic weight for any element
on the standard periodic table. As in the case of
Figure 3: Comparative size of substances
oxygen and hydrogen, atoms with more protons
used in cooking and chemistry. Egg is
and neutrons have a molar mass, (the differences
compared with chocolate chip. Oxygen
atom is compared with hydrogen atom.
in their atomic weights). The relationship between
the mole and the atomic weight provides an easy
way to calculate the atomic mass of a substance. The molar mass is directly related to
the atomic weight; one mole of a substance equals mass, in grams, equivalent to the
atomic weight of the substance. For example, the atomic weight of oxygen is 15.9994;
consequently, the molar mass of oxygen is 15.9994 grams per mole (g/mol).
oxygen
hydrogen
You can now use what you know about the relationship between atomic weight and
molar mass to calculate the mass of 1 mole of H2O, water. The molar mass of oxygen is
15.9994 g/mol and the molar mass of hydrogen is 1.0079 g/mol. Remember there are
two hydrogen atoms in a water molecule.
2 H = 2(1.0079 g/mol)
+ 1 O = 15.9994 g/mol
2 H = 2.0158 g/mol
+ 1 O = 15.9994 g/mol
18.0152 g/mol of H2O
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We can use this information to calculate how many H2O molecules there are in 1 gram
of water. The equation is as follows:
1.00 g H2O x
1 mol H2O
18.0152 g H2O
x
6.02 x 1023 molecules
1 mol H2O
= 3.34 x 1022 molecules H2O
Since your laboratory materials are limited, you will observe the laboratory method. This
method determines the experimental value of Avogadro’s number by floating material,
such as cinnamon or oil, on a surface of water and use dishwashing liquid to create a
measurable molar mass.
Pre-lab Questions
1. How many grams of H2O are necessary to weigh out 1 mole of H2O?
2. How many molecules of water are there one mole of H2O?
3. How many moles of H2O are there in 1.0 g of H2O?
4. How many molecules of H2O are there in 1.0 g of H2O?
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Experiment: Avogadro’s Number
In this laboratory exercise, you will measure Avogadro’s number. Be sure to observe all
of your standard safety protocols when working with chemicals.
Materials
Safety Equipment: Goggles or glasses, gloves, apron
Ground cinnamon
Dishwashing liquid
Dropper or straw
Small glass dish
Ruler
Pure Water
Measuring cups
Measuring spoons
Squirt bottle or small
squirt gun
Procedure:
Part 1
1. Add 2.5 milliliters (mL) (1/2 teaspoon) of dishwashing liquid to an 8.0 ounce (oz)
glass jar or cup.
2. Fill a squirt bottle or squirt gun with pure water and gently rinse (several times)
the dishwashing liquid from the jar to a clean, graduated measuring cup. Do this
several times to transfer all of the dishwashing liquid. Avoid creating suds.
3. Add more distilled water to create 120 mL (1/2 cup) of dishwashing solution.
4. Carefully stir the solution until it is mixed.
5. Use the straw to carefully, drop by drop, create a puddle of water in a small glass
dish or device similar to a petri dish (a small white saucer plate will work well).
Count the number of drops to create a 15 mL (1 tablespoon) puddle. It should be
about 360 drops.
6. Sprinkle a layer of ground cinnamon to cover the water puddle (only include a
light later to cover the surface).
7. Use a clean, dry straw to draw up one or two drops of dishwashing solution.
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8. Drop a single drop on the cinnamon layer and allow it to form a clear circle.
9. Use the ruler to estimate (in centimeters (cm)) the diameter of the circle. If your
diameter is smaller than 3 cm, you need to go back and try again.
10. Record the diameter of the dishwashing solution circle.
11. Clean your laboratory space.
Data and Observations (in the Lab Report)
There are 360 drops in one tablespoon and there are 14.8 mL in one tablespoon.
Calculate the number of drops in 1.0 mL.
The diameter (cm) of the circle formed by the dishwashing solution:
Calculate the surface area of the circle ( d2/4):
Calculate the surface area of the dishwashing circle to molecules per layer (this requires
converting the surface area from meters to nanometers):
cm2
Top Layer
surface area
1 m2
1 x 1018 nm2 1 molecule
. molecules
top layer
=
10,000 cm2 1 m2
0.210 nm2
Calculate the concentration of grams of sodium stearate (dish liquid) per milliliter of
diluted dish solution:
1 g sodium stearate
2.5 mL dish liquid
=
120 mL dish liquid
120 mL diluted solution
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g/mL
Calculate the number of moles of sodium stearate in a single layer. Convert the number
of drops to create the monolayer to mL using the calibrated number of drops per mL.
Multiply the number of grams of sodium stearate per milliliter of solution. Convert to
moles using the molar mass of sodium stearate (296.4 g/mol).
1 drop
(added to dish)
1 mL dish liquid
solution
top layer
drops
g
sodium stearate
1 mL dish liquid
solution
1 mol
296.4 g
(molar mass of
sodium stearate)
=
mol/top layer
Calculate Avogadro’s number by comparing the number of molecules in the top layer
with the number of moles in the top layer.
Avogadro’s number
(experimental)
= molecules / top layer =
moles / top layer
molecules/mole
Post-lab Questions
1. Explain why the number you calculated for Avogadro’s number may match the
actual number of 6.02 x 1023 – be sure to provide a detailed explanation.
2. Calculate the number of moles in 0.457 grams of NaCl, sodium chloride.
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