Density is a fundamental physical characteristic of any sample
It is considerably more important than other physical properties such
size or shape (incidental physical properties that may be of no help in
in that the numerical value of density for a pure substance at a
temperature is a constant. The density may be readily
reproducibly determined in the laboratory if the mass and volume of a
can be determined. Density (d = m / V) may be calculated by dividing
mass by the volume.
Today, we will be comparing two methods for determining the density
of an unknown liquid: pycnometer, and syringe. Like all of
the other procedures you have been practicing, there are tradeoffs in
accuracy, analysis time, and amount of material needed.
A pycnometer a precision piece of glassware that consists of two portions: a bottle and a stopper. The bottom portion is a small bottle with a volume which may be accurately determined from the mass of water it holds at a particular temperature. The stopper is a capillary tube with a ground glass (frosted) bottom that fits snuggly into the ground glass (frosted) neck of the bottle. For example, if the pycnometer holds 5.02 g of water, then its volume must be about 5.02 mL because 1 g of water occupies about 1 mL (remember that the density of water is very nearly 1.00 g/mL at the temperature of the laboratory). Since the volume of the container may be accurately determined using the density of water, the entire experiment involves only the determination of the weights of the empty dry pycnometer, the pycnometer full of unknown liquid, and, finally, the pycnometer full of water, IN THAT ORDER.
(Posted 9/2/03 by C.R. Snelling
( Updated 8/28/09 by J. Neilan)