Measuring the Density of a Liquid

Introduction:

Density is a fundamental physical characteristic of any sample of matter. It is considerably more important than other physical properties such as size or shape (incidental physical properties that may be of no help in identification) in that the numerical value of density for a pure substance at a particular temperature is a constant. The density may be readily and reproducibly determined in the laboratory if the mass and volume of a sample can be determined. Density (d = m / V) may be calculated by dividing the 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.

Pycnometer Procedure:

In this portion of the experiment, you will be using  a Gay-Lussac specific gravity bottle, also known as a pycnometer (pick-nah'-mih-ter):


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.

  1. Obtain a dry pycnometer from the lab instructor DO NOT WASH THE PYCNOMETER. It must be dry for the accurate determination of its mass. It is important that the stoppers are not exchanged from bottle to bottle, since each stopper is ground to fit one bottle and no other.
  2. Weigh the clean dry bottle to the nearest 0.001 g.
  3. Obtain approximately 15 mL of the unknown liquid.
  4. Pour the liquid into the specific gravity bottle until the bottle is completely full.
  5. Insert the stopper so the ground glass (frosted) end is in the bottle.
  6. Carefully dry the outside of the bottle with a tissue and weigh.
  7. Pour the unknown liquid down the sink and rinse the bottle several times with distilled water. (The unknown liquids are aqueous based and can be poured down the sink.)
  8. Fill the bottle with distilled water, dry the outside and weigh.
  9. Determine the temperature of the distilled water.
  10. Calculate the volume of the pycnometer using the literature value for the density of water at the temperature of the water.
  11. Calculate the specific gravity and density of the unknown liquid.
Syringe Procedure:

In this portion of the experiment, you will be using a syringe to determine the density of an unknown liquid. 
  1. Determine the mass of a clean, dry, empty 1 mL syringe.
  2. Withdraw 0.50 mL of the liquid to be measured into the syringe and then determine the mass of the syringe containing the liquid.
  3. The difference between the mass of the empty syringe and the syringe containing liquid is the mass of the liquid.
  4. To determine the density, you need this mass of the liquid divided by the volume of the liquid which you read on the syringe.

(Posted 9/2/03 by C.R. Snelling
( Updated 8/28/09 by J. Neilan)