Determination of the density of an unknown liquid

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Determination of the density of an unknown liquid


To determine the density of a liquid using pycnometer method.


In order to classify and identify materials of a wide variety, scientists use numbers called physical constants (e.g. density, melting point, boiling point, index of refraction) which are characteristic of the material in question. These constants do not vary with the amount or shape of the material, and are therefore useful in positively identifying unknown materials.  Standard reference works have been complied containing lists of data for a wide variety of substances. The chemist makes use of this in determining the identity of an unknown substance, by measuring the appropriate physical constants in the laboratory, consulting the scientific literature, and then comparing the measured physical constants with the values for known materials. This experiment illustrates several approaches to the measurement of the density of liquids and solids.

Density is a measure of the “compactness” of matter within a substance and is defined by the equation:

Density = mass /volume

The standard metric units in use for mass and volume respectively are grams and milliters or cubic centimeters.  Thus, density has the unit grams/milliter (g/ml) or grams/cubic centimenters (g/cc).  

Density determination by pycnometer is a very precise method. It uses a working liquid with well-known density, such as water. We will use distilled water, for which temperature dependent values of density rH2O are shown in Table 1. The pycnometer is a glass flask with a close-fitting ground glass stopper with a capillary hole through it. This fine hole releases a spare liquid after closing a top-filled pycnometer and allows for obtaining a given volume of measured and/or working liquid with a high accuracy.

  • Let M to be mass of empty pycnometer
  • Let Mw to be mass of pycnometer with distilled water inside
  • Let d be the density of distilled water at temperature T
  • Let V be the volume of distilled water
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The value of (

) is the ,ass of water contained in the pycnometer at the certain temperature. Thus, if the density of water, d at t°C, the volume V of the pycnometer is then ,

      Vw =


If the mass of the unknown liquid is determined at t°C, the density of the liquid is the ratio of this mass to its volume as given by equation 1 as below,

dL =

                       dL =

….(Eq 2)

Materials & Apparatus:

Distilled water        

Unknown liquid


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