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Uncertainty

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Density of an Unknown Liquid

Objective
To determine the identity of an unknown liquid by measuring its density.

Reference
"Dartmouth College, Chemistry 5 Lab Manual", pp. 25 - 37.

Prelab Procedure Outline
1. Obtain sample of unknown liquid. Note physical properties such as odor, viscosity, and color.

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

3. Carefully transfer unknown liquid to volumetric flask. Fill to mark exactly. Stopper flask.

4. Measure mass of filled flask.

5. Empty and dry flask.

6. Repeat steps 2 - 5 for additional liquid sample.

or Prelab Procedure Flowchart

Obtain unknown sample in beaker


note odor, color, viscosity
Weigh clean, dry 10 mL volumetric flask and stopper
record
Transfer liquid to flask
fill to mark, use dropper, stopper flask
Weigh filled flask
record
Empty, dry flask. Repeat for additional liquids.

Sample Calculations:

If flask is 10.0 g and flask + liquid is 15.0 g

Mass liquid = 15.0 g - 10.0 g = 5.0 g

Density liquid = 5.0 g / 10.00 mL = 0.50 g/mL

Prelab Analysis Outline
1. Subtract mass of empty flask from mass of filled flask to calculate mass of liquid.

mLiquid = mflask & liquid - mflask

2. Divide liquid mass by liquid volume to calculate density.

D=mliquid/10.00 mL

3. Compare to known densities to indentify liquid.

or Flowchart

Calculate Mass Liquid


mLiquid =
mflask & liquid - mflask
Calculate Density
D=mLiquid/10.00 mL
Compare to known density values

Procedure, Data and Results
Empty Volumetric Flask
Washed two 10.0 mL volumetric flasks and dried with KimWipes, inside and out.
Labeled A and B with pencil.
Weighed flasks A and B on analytical balance
Flask A 11.2571 g Flask B 11.3921 g

Filled flasks to mark with unknown liquid 1.
Weighed on analytical balance.

Emptied and dried flasks A and B.
Weighed empty flasks.
Filled to mark with unknown liquid 2.
Weighed on analytical balance.

Unknown liquid 1

Flask A

Flask B
mass empty flask 11.2571 g 11.3921 g
mass flask & liquid 19.2571 g 19.6124 g
mass liquid 8.0000 g 8.2203 g
Density liquid 0.8000 g/mL 0.8220 g/mL

Average density liquid: 0.8110 g/mL
Probable Identity: methanol

Unknown liquid 2

Flask A

Flask B
mass empty flask 11.2575 g 11.3918 g
mass flask & liquid 21.3308 g 21.5086 g
mass liquid 10.0733 g 10.1168 g
Density liquid 1.0073 g/mL 1.0117 g/mL

Average density liquid 2: 1.010 g/mL
Probable Identity: water

The first liquid unknown was determined to be methanol from its experimental density of 0.8110 g/mL. This compares to the literature value of 0.7914 g/mL (CRC Handbook, 66th ed., p. C-351). Methanol is colorless, slightly less viscous than water, and has a slightly sweet odor.

The second liquid was determined to be water from its experimental density of 1.010 g/mL. The literature value for the density of water at 25 °C is 0.997 g/mL (CRC Handbook, 66th ed., p. F-10). The water sample looked and smelled like water.

This method of determining densities was accurate to within 2% for methanol and to within 1% for water. Limitations on the precision of the determination were 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 out of 19 g (0.0005%) and the flask is accurate to 0.02 mL out of 10 mL (0.2%). Since both of these values are less than the discrepancy between the experimental and literature density values in this experiment, they were not the limiting factors in the precision of the results. Possible error by the experimenter that would account for the experimental precision being less than expected are not filling the flask to the mark properly or having droplets of liquid in the neck of the flask or on the outside of the flask. Handling the flask with bare hands could have introduced an error in the mass by adding fingerprints. Finally, the densities of liquids vary slightly with temperature and this could account for the discrepancy between the experimental results and literature values. If the temperature in the lab was not 25 °C, the true values of the densities of the liquids may have differed from the literature values for 25 °C.

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