LAB: Avogadro’s Number
Think of a very large collection of something you can
see.
For example, think about the number of stars you can see overhead on in a very
dark night. We would have to agree that, while there are a lot of stars overhead,
there are many more that we cannot see. It has been estimated that the Milky
Way Galaxy contains about 100 billion stars. The Hubble Space Telescope
recently peered to the edge of the known universe and found evidence of at
least 100 billion other galaxies. So, if each of those galaxies is rather like our own,
there are at least ten-thousand-billion-billion stars in the Universe. That is 1 x 1022 stars!
It would be a little more difficult to estimate the number of grains of sand on the beaches of
the Earth, but at least one web resource calculates the number to be around 1 x 1017. A
different resource calculates the number of sand grains on Earth using smaller grains of sand,
and figures that the number is closer to 1 x 1024. If we take a rough average of these numbers,
the value turns out to be 5 x 1023 grains of sand.
A number even larger than the stars in the Universe or grains of sand on the beaches is
Avogadro's Number, usually represented as 6.02 x 1023. This number represents "the number
of atoms of an element that would be contained in the gram-atomic-mass of that element."
For example, if you had exactly 12.0 grams of C-12, we expect that it would contain exactly
6.02 x 1023 atoms of carbon-12. (By the way, 12.0 grams of carbon would fit quite nicely into
the palm of your hand.) This quantity of atoms is called 1 mole. Let's explore the magnitude
of Avogadro's Number further. ..
Procedure:
1. Obtain a sample of uncooked rice in a clean beaker. (Do not spill the rice into the
sink!)
2. CREATE A DATA TABLE FOR THE FOLLOWING MEASUREMENTS:
2. Determine the amount of time, in seconds, that your lab partner can count out
100 rice grains - one at a time.
3. Determine the length, in centimeters, of 20 rice grains laid end-to-end.
4. Use a dry 10-mL graduated cylinder to determine the volume occupied by
100 rice grains.
5. Determine the mass of 100 grains of rice.
Analysis: Complete the following calculations. Show your work, and use units.
1. How many years would it take to count a mole of rice grains?
2. If the entire world's population helped you to count a mole of rice grains, how
long would it take? Assume the world's population is 7 billion.
LAB: Avogadro’s Number
3. According to one web source, the world's annual production of rice is about 485
billion kilograms, 4.85 x 1011 kg. How many years would it take to grow a mole of
rice grains?
4. Assuming man has cultivated rice for 200,000 years, has there been a mole of rice
grown in that time? EXPLAIN.
BONUS FACT: Who wants to be a mol-lionaire?
If a mole of pennies were divided up and given to every person on earth, each person would
receive around 9.4 X 1013 pennies. (Assume a world population of 6.4 x 109 persons.) If each
individual spent their money at a rate of 1 million dollars each day, it would take over 2500
years to use up the money. Life would not be comfortable, however ... the surface of the earth
would be covered with copper to a depth of 420 meters!