Chemistry 11 Lab #6

Determining a pure substance from it properties

You will be given one of a number of unknown substances.  By analyzing the substance's density, solubility in water and boiling point and melting point you will be able to determine which substance it is from the tableof possible substances below.

Substance
 Density (kg/m3)
 Melting point (°C)
 Boiling point  (°C)
(at Cbba's altitude)
 Solubility
(in water)
Acetone
 790
 –95 46
 vs
Cyclohexane
 780
 6.6
 71
 nos
Ethyl Acetate
 900
 –84
 67
 ss
1-propanol
 800
 –126
 87
 vs
2-propanol
 709
 –90
 72
 vs
Water
 1000
 0.0
 90
 vs

Start by getting an labelled unknown pure substance from me.  Note the label of your unknown.


Analyzing Density

You have done this a few times now so the method should be pretty straightforward!
  1. Use the 10 mL graduated cylinders.  Start by massing it with the electronic balance and note the mass with error.
  2. Add 10 mL of of your sample to the cylinder.  Use the eye dropper to add the last amount slowly to get as close to 10 mL as you can.  Note the error on the volume, and if you can't get a reading 10 mL note the reading you actually have.
  3. Mass the cylinder with unknown and note the mass.
  4. Calculate the density with error.  Save the liquid for other parts of the lab.

Analyzing Melting Point
  1. Make a slushie ice bath with about a teaspoon of salt (this lowers the melting point of water so you get a colder slushie) about 3 cm deep in a 150 or 250 mL beaker and rest it on a ring stand with wire gauze to support it.
  2. Add about 10 mL (use the sample from the section above) of the sample to a large test tube and clamp the test tube in place in the ice bath.
  3. Place a thermometer in the sample liquid and take the temperature of the sample every 30 seconds.
  4. Note the temperature when (and if) ice crystals form in the sample and continue taking temperature measurements 2 to 3 minutes beyond this point.  Also note if no ice crystals form by the time the sample reaches a degree or two below 0°C.
  5. Save the sample in the test tube for the next section, and keep the apparatus as it is, but put the ice mixture aside for another group to use.

Analyzing Boiling Point
  1. Fill another 150 or 250 mL beaker about 3 or 4 cm deep with tap water and add about a teaspoon of salt (it raises the boiling point).  Place the test tube in it as above.
  2. Add a boiling chip to the test tube and use a bunsen burner to heat the water.  (Keep the flame away from the mouth of the test tube as your sample unknown may be flammable.)
  3. Take temperature measurements every 30 seconds and note the temperature when the sample first begins to boil.
  4. Do not move the hot beaker or other equipment until it has cooled.  The test tube and clamp can be raised out of the water to cool more quickly.
  5. Save the sample in the test tube for the next part

Analyzing solubility
  1. Use the eye dropper to add 20 drops of water into a small test tube .
  2. Add your unknown sample 2 to 3 drops at a time to the same test tube and shake the mixture around without spilling it.  KEEP TRACK OF HOW MANY DROPS OF UNKNOWN YOU HAVE ADDED.
  3. After shaking wait 30 seconds or so and note if the unknown appears to have dissolved into the water, or if it appears to be separate.  If it has all dissolved repeat step 2.  If it does not appear to have dissolved, stop and note the total number of drops of unknown added.  Do not add more than 20 drops in total.
  4. We call the sample nos (no observable solubility) if you stop after the first 2 to 3 drops.  We call it vs (very soluble) if you can add all 20 drops and the mixture reamins separated.  We call is ss (slightly soluble) if you can add more than 2 to 3 drops but less than all 20 and get the liquids to remain mixed. into layers. Note the total number of drops you added.
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