LAB: Measuring Molecule Size

CHEMISTRY LAB
MEASURING THE SIZE
OF A
MOLECULE

INTRODUCTION:

Because atoms and molecules are so small, it is difficult to determine their sizes. Special instruments are normally needed for such measurements. Here, however, a simple experiment using water, soap, and chalk dust will be conducted to give an approximate measure of the size of a molecule.

MATERIALS:

liquid soap or detergent
chalk dust from eraser
large, flat pan (pie pan)
medicine dropper
metric ruler
water

PROCEDURE:

1. Use a medicine dropper to hold a small sample of liquid soap or detergent. Collect a drop of soap at the tip of the dropper. REMEMBER: In science, a dropper is a transfer pipette. We always hold it vertically, never at an angle.

2. Measure the diameter of the drop, in millimeters, with a ruler and record this measurement in the DATA TABLE. Also record the radius of the drop. (The radius is half the diameter.)

3. Add water to the pan until it is nearly filled. Sprinkle a light coating of chalk dust over the surface of the water. Cover the surface as evenly as you can.

4. Add a single drop of soap or detergent to the center of the pan. Note what happens to the chalk dust.

5. Use the boundary of the chalk dust to estimate the diameter of the soap drop as it is spread out in a circular layer on the surface of the water. Record the diameter and radius of the soap layer on the DATA TABLE.

DATA TABLE

Form of Soap
Diameter
(mm) Radius
(mm) Volume
(mm³) Height
(mm)

Drop (sphere)

XXXXX

Layer (cylinder)

DATA TABLE
Form of Soap	Diameter (mm)	Radius (mm)	Volume (mm³)	Height (mm)
Drop (sphere)				XXXXX
Layer (cylinder)

CALCULATIONS:

6. The drop will be close to the shape of a sphere as it leaves the dropper. Calculate the volume of the drop. (The volume of a sphere of radius r is ⁴/₃ r³). Record this volume in the DATA TABLE.

7. Although the soap layer appears to be a flat circle, it actually has some depth. Consequently, the soap layer has the shape of a cylinder. The height of the soap layer must be at least equal to the thickness of one soap molecule. The value of the height (or approximate thickness of the soap molecule) can be calculated from the formula for the volume of a cylinder: V = r²h. This equation can be rearranged to solve directly for the height h = ^V/ r². Note that the volume of the soap is the same whether it is in the form of a sphere or a cylinder. Substitute your value for the volume of the soap drop and your value for radius of the soap layer into the rearranged equation, and solve for the height of the soap layer. Record the height value in the DATA TABLE. This is the approximate diameter of a soap molecule.

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