My textbook told me that the oil slick was 41.8 cm in radius. But I know that a molecule isn't that big, let alone twice as big for the diameter. But, on the homework question they gave me other info too. They said that the oil droplet is 9.00*10^-7 kg, and that the density is 918 kg/m cubed. I have no idea where to start. Someone please help me.:(

1. You know the weight (kg)of the droplet
2. You know the density (kg/m^3)of the droplet.

If you divide the weight by the density, this is what you will get:

\frac {kg}{kg/m^3} = m^3

That's simply a volume -- Assume that it's spherical.

You have the volume of the oil blob. Do you know the formula for the volume of a sphere? Set the volume of the droplet equal to the formula for the volume of the sphere. Solve for the radius. Double it to get the diameter of the blob.

I guess that your book didn't phrase the question well. The 'molecule' should be replaced by 'droplet', or 'blob', or any similar wording.

Some other thing is wrong too, I keep getting 0.62 mm as the radius (assuming the oil is spherical) :(