Determination of the density of an unknown liquid

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 =

……(Eq1)

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

Pycnometer

Weighing balance

Procedure:

1. A dry, clean and empty pycnometer was weighed by using weighing balance. The weight of the pycnometer used was recorded.
2. The pycnometer was filled up with distilled water and air bubbles were allowed to rise to the top before inserting the stopper.
3. The filled pycnometer was left at room temperature°C for 5 minutes.
4. The neck of pycnometer was picked up by using two layers of papers between the fingers to avoid expansion of the pycnometer due to heat of the hand.
5. The pycnometer was dried and weighed again with distilled water.
6. The volume of the pycnometer, V was found and recorded.
7. The steps 1 – 6 were repeated by using an unknown liquid.

Results & Calculations:

        Mass of empty pycnometer = 25705.6mg

Table 2.2: Mass of pycnometer in different condition  

Since the experiment was done at temperature of 26 °C, the density of distilled water, d = 0.996783 g cm-3.

1) Determination of volume of distilled water, V

Volume of distilled water, Vw =

 

                                                         =

 

   = 51.06 cm3

The volume of distilled water, Vw is equal to volume of pycnometer, V.

Hence, V = 51.06 cm3.

2) Density of unknown liquid, dL

 Density of unknown liquid, dL =

 

                                                           =

 

                                                           = 0.7898 g cm-3

 By using the reference table of the following:-

Table 2.4: Reference table for identification of unknown liquid

According to the calculation, the density of unknown liquid, dL = 0.7898 g cm-3

The density of the unknown is the nearest to the density of ethanol. Therefore, it can be concluded that the unknown liquid is ethanol.

Discussion:

The volume of an object increases with increasing temperature, because of the matter’s volumetric thermal expansion. The density of an object depends on its temperature, with higher temperature resulting in lower density. Exception is water in temperature range 0-4 °C, for which the density increases with increasing temperature. The density of a gas further depends on the pressure as well. Nevertheless, this effect is negligible in a case of liquid and/or solid matter. A material’s density is defined as its mass per unit volume. It is, essentially, a measurement of how tightly matter is crammed together.. One of the most common uses of density is in how different materials interact when mixed together. The change in density can also be useful in analyzing some situations, such as whenever a chemical conversion is taking place and energy is being released.

Density determination by pycnometer is a very precise method. It uses a working liquid with well-known density, such as water. 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.

Although a pycnometer is used to determine density or specific gravity, it measures volume which is a balance is used to determine mass. Relative density is the ratio of the mass in air of a given volume of a material at a stated temperature to the mass in air of an equal volume of distilled water at the same temperature.

Density is often used as a point of identification in the determination of an unknown substance such as in this experiment. The density of the unknown might be used to distinguish the unknown from other suspects. Density can also be used to determine the concentration of solutions in certain instances. When a solute is dissolved in a solvent, the density of the solution will be different from that of the pure solvent in itself. Several techniques are used for the determination of density. The method used will depend on the type of the sample and on the level of precision desired for the measurement. In general, a density determination will involve the determination of the mass of the sample with a balance, but the method used to determine the volume of the sample will differ from situation to situation. Several methods of volume determination are also explored. For solid sample, diff methods may be needed for the determination of the volume, depending on whether or not the solid is regularly shaped. If a solid has a regular shape, the volume of the solid may be determined by geometry. If the solid does not have a regular shape, it is possible to determine it by Archimedes principle.

Questions:

Briefly describe the technique of using a pycnometer in determining the density of a solid sample. It the type of pycnometer used in this experiment suitable for density determination of a solid sample?

Answers:

The technique was described as follows:-

1. The mass of an irregular solid can be determined by weighing method.
2. When the solid is placed into a pycnometer filled with a liquid of known density such as distilled water, the volume of the liquid which will overflow is equal to the volume of the solid.
3. The mass of the liquid which will overflow is determined as the difference between the sum of the mass of the pycnometer filled with liquid plus the mass of the solid and the mass of the pycnometer filled with liquid after the solid has been placed inside.
4. The volume occupied by this mass is determined from the known density of the liquid. It is necessary that the solid in insoluble in the liquid used.
5. The density of the solid is determined from these measurements of mass and volume (BPearson, 2009).

Since density is defined as the ratio of the mass of a body to its volume, its experimental determination requires the measurement of these two quantities for the selected piece of material. The type of pycnometer used in this experiment is suitable for the density determination of a solid sample as it gives the appropriate information required to determine the density of a solid sample as in the following:-

The volume overflowed from the pycnometer = mass of solid in the pycnometer

Precaution steps:

1. Make sure weight the mass of pycnometer by using same electronic balance for all steps.
2. Dry any excess water at surface of pycnometer before weigh.

Conclusions:

The unknown liquid is ethanol by density 0.7898 g cm-3.

References:

1. C.J. Ksanda and H. E. Merwin, Improved Technique In Micropycnometric Density determination, retrieved 29 January 2010 from
2. BPearson, Lesson-Plan - Density of Liquids and Solids – Pycnometer Method, retrieved 29 January 2010 from http://blog.cencophysics.com/2009/07/density-liquids-solids-pycnometer-method