How Many Molecules of Water Are in Lake
Erie?
Concept Development
Problem:
How do you determine the number of molecules of water in Lake Erie?
Materials:
water
balance
eye dropper
beaker
graduated cylinder
Procedure;
Utilizing the equipment listed above and the following information, determine the number of
molecules of water in Lake Erie.
Helpful Information
*Volume = area x depth (cm3)
*1 mL = 1 cm3
* One mole of anything contains 6.02 x 1023 units.
* One mole of water has a mass of 18.02 grams.
* Lake Erie has an average depth of 18 m. (assume that Lake Erie is a rectangular block)
* Lake Erie covers 2.6 x 104 km2 (area)
Hold it a minute! Are you sure that you have all of the information you need? There might be
more data needed from the lab. Think about it.
Summing Up;
1. How many drops of water are in 2 L?
2. How manydrops of waterareinLakeErie?
3. How many molecules of water are in Lake Erie?
4. How many moles of water are in Lake Erie?
5. The recommended daily allowance of water is about 2 L. How many molecules of water should
you drink each day?
Teacher Notes
How Many Molecules of
Water Are in Lake Erie?
Lab Setup
Calculations
Reliability
Interest
Lab time
Process skills
Concept Development
easy
moderate
easy
moderate
excellent
good
excellent
good
-1 class
1 class
Interpreting Data
difficult
difficult
fair
fair
+1 class
Experimenting
Objective:
By completing this activity students will be able to determine
the number of water molecules in Lake Erie. They will utilize
the factor-label method to make difficult calculations
by breaking a problem down into intermediate steps.
Materials:
water
balance
eye dropper
beaker
graduated cylinder
Teaching Strategies;
Students will first need to determine experimentally the number
of drops of water in 1 mL, but let them figure this out on
their own! Drops should be of uniform size. With the use
of a balance, students should then determine that 1 mL of water
has a mass of approximately 1 g. If needed, review the factorlabel method in making calculations.
For simplicity, tell students to assume that Lake Erie
is essentially a rectangular block so they can
successfully calculate the volume.
Sample Data:
1 mL of water = 20 drops
1 mL of water = 1 gram
Summing Up:
1. Number of drops in 2 L:
2 L x 1000 mL x 20 drops
1 L
1 mL
2.
= 40,000 drops
Number of drops of water in Lake Erie:
2. 6 x 104 km2 x 1 , 0 0 0 , 0 0 0 m2 x 10,000 cm2 = 2 . 6 x 1014 cm2 area
1 km2
1 m2
18 m x 100 cm = 1,800 cm (depth)
1m
vol = area x depth
2.6 x 1014 cm2 x 1,800 cm = 4.68 x 1017 cm3
4.68 X 1017 cm3 x 1 mL = 4.68 X 1017 mL
1 cm3
4.68 x 1017 mL x 20 drops = 9.36 x 1018 drops of water
1 mL
3. Molecules of water in Lake Erie:
4.68 x 1017 mL x 1 g x 1 mole x 6.02 x 1023 molecules
1 mL
18.02 g
1 mole
1.56 x 1040 molecules
4. Moles of water in Lake Erie:
____1 mole_______ x 1.56 x 1040 molecules =
6.02 x 1023 molecules
2.59 X 1016 moles
5. Molecules of water needed to meet daily requirement:
2 L x 1000 mL x 1 g x 1 mole x 6.02 x 1023 molecules =
1 L
1 mL 18.02 g
1 mole
6.68 x 1025 molecules
Adapted txom an id«« d«aorlb*d in Th« 8ci«no« T««oh«r.