d Size of a molecule We select an oil molecule because it has a density less than the density of water Oil floats on the surface and does not dissolve in the water If the water has a large enough surface area we assumed that thin oil will spread out in a layer one molecule thick called a monomolecular layer and not form little hills of molecules If we know the original volume of oil and the surface area that it forms then we can calculate the thickness of a monomolecular layer dividing the volume by the area Use a tray with area  30 cm2 so as not to restrict the oil film Sprinkle the surface of the water with a very fine light powder eg talc powder When you put oil on the water it pushes the powder aside so you can easily see the area covered by the oil Pour thin petroleum distillate oil into a burette Find the volume of fifty drops by running oil from the burette drop by drop and counting the drops Allow one more drop to fall on a piece of plastic Touch the oil drop with the point of a glass rod and then touch the prepared water surface The oil spreads out Measure the approximate area over which it spreads Estimate what fraction of oil was removed by the glass point by using the glass point to remove successive fractions from the drop until it has been used up The volume of oil put on the water can be calculated and an estimate made of the thickness of the oil layer about 106 mm This is an approximate dimension of a single molecule of the oil