EXPERIMENT #45: DENSITY OF AN UNKNOWN LIQUID

Name: Marie Dunn Lab Partner: Neera Nuff

Objective: To determine the identity of an unknown liquid by measuring it’s

denisty density.

Prelab questions: The density of methanol is 0.791 g/mL. What is the mass of a

125 mL sample of methanol?

Safety Precautions and Chemical Handling:

Proposed Procedure:

1. Obtain sample of unknown liquid. Note color, odor, etc.

2. Record mass of clean, dry 10.00 mL volumetric flask with stopper.

3. Fill volumetric flask exactly to the mark with unknown liquid and stopper

the flask.

4. Measure mass of file filled flask with stopper.

5. Empty and dry fal flask.

6. Repeat steps 2-5 with additional sample of unknown.

7. Calculate density of unknown and compare to densities of known substances.

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Procedure Notes and Data Tables:

Unknown # 123 Flask A Flask B Flask B repeated

Mass empty flask +

stopper

11.4198 g

11.2571 g 11.3921 g

Mass flask+

stopper+ liquid

19.5915 g 17.3892

19.2471 g

19.6124

Mass Liquid 8.1717 g 7.9900 g 8.000 g

Density liquid 0.8172 g/mL 0.7990 g/mL 0.8000 g/mL

Average Density of Liquid 0.8054 g/mL

Probable Identity of unknown # 123: methanol

Notes and observations:

1. Unknown was a Colorless tar transparent liquid. It had a slightly sweet odor.

Liquid did not cling to sides of container when swirled.

2. Chem wipes were used to dry the flask after washing. Small drops of water

were left in the flask after using the chem wipes.

3. Three trials were run intead instead of two.

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Expt # 45- Density of Unknown liquid.

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forgot to stopper flask after

adding liquid

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Calculations:

Sample Density calculation (Trial “A”)

Mass unknown liquid = (Mass flask+ stopper+ liquid)- (Mass empty flask + stopper)

19.5915 g-11.4198 g =8.1717 g

Density = mass/volume = 8.1717 g/10.00 mL = 8.172 g/mL

Average Density = (0.8172 g/mL+0.7990 g/mL+0.8000 g/mL) = 0.8054 g/mL

3 trials

% error = [(density unknown-density methanol)/density methanol] X 100%

= [(0.8054 g/mL)-(0.7914 g/mL)/0.7914 g/mL] X 100%= 1.8%

Discussion

The probable identity of unknown # 123 was determined to be methanol from its

experimental density of 0.8054 g/mL. This compares to the literature value of 0.7914

g/mL (CRC Handbook, 66th ed., p. C-351). The experimental density differs from the

literature value by 1.8%. Methanol is colorless, slightly less viscous than water and has a

slightly sweet odor. Observations of unknown # 123 are consistent with the physical

properties of methanol.

The precision of the determined density was limited by the ability of the experimenter to

correctly fill the volumetric flask, the accuracy of the flask itself, and the accuracy of the

balance. The balance is accurate to 0.0001 g and the volumetric flask is accurate to 0.02 mL.

Sources of experimental error include:

Having droplets of liquid in the neck of the flask

Handling the flask with bare hands mass of fingerprints

Water droplets remaining in the flask prior to filling with unknown

Densities of liquid vary slightly with temperature. If the temperature in the lab was

not 25°C, the true values of the density of the unknown liquid may have differed

from the literature values for 25°C.

Expt # 45- Density of Unknown liquid.

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This lab could be improved by improving the method of drying the volumetric flask before filling it

with the unknown liquid. The flask could be dried by small amount of acetone could be used to rinse

the flasks after washing. Any acetone remaining in the flask could be removed by evaporation. Mass

errors due to fingerprints on the flask could be eliminated if the experimenter wore gloves or

otherwise avoided touching the flask with bare hands. Finally, the experimenter should remove

droplets clinging to the neck or outside of the flask by using a KimWipe.

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Expt # 45- Density of Unknown liquid.

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