Illustrated Guide to Home Chemistry Experiments

150 DIY Science: Illustrated Guide to Home Chemistry Experiments

It’s important to know the actual boiling point of water under
ambient pressure, because we’ll use that boiling point as a
baseline reference datum to determine the boiling point elevation
(ΔTb) of the various molal solutions and from those data the
molar masses of the solutes. To determine the boiling point of
water, take the following steps:

1. If you have not already done so, put on your splash
   goggles, gloves, and protective clothing.


2. Fill the 250 mL beaker about halfway with water, place it
   on the heat source, and heat the water until it comes to a
   full boil.


3. Immerse the thermometer in the beaker, making sure
   that it does not contact the beaker itself (suspend it with
   a stand and clamp, or find some other way), allow the
   thermometer to stabilize, and record the temperature
   reading in Table 8-1. (With most thermometers, you can
   interpolate a reading to 0.5°C or closer.)


4. Empty and dry the beaker.

PRTII:A REREp pA moLAL SoLUTIoNS
of SodIUm CHLoRIdE ANd SUCRoSE
To test the effect of molality and dissociation on boiling point,
we need to prepare solutions of ionic and molecular (covalent)
compounds of known molality. I chose to use sodium chloride
and sucrose because both of these chemicals are inexpensive,
readily available, and extremely soluble in water. Sodium
chloride is ionic. In solution, sodium chloride dissociates into
sodium ions (Na+) and chloride ions (Cl-), and should therefore
have a van’t Hoff factor of 2. Sucrose is molecular, and should
therefore have a van’t Hoff factor of 1.

Coincidentally, the solubility of both sodium chloride and
sucrose in water at room temperature is just over 6 mol/L. We’ll
therefore prepare 6 molal, 3 molal, and 1.5 molal solutions of
both of these compounds.

1. If you have not already done so, put on your splash
   goggles, gloves, and protective clothing.


2. Label a foam cup “6 molal sodium chloride”, place it on
   the balance, and tare the balance to read 0.00 g.


3. Add hot tap water to the foam cup until the balance
   reads as closely as possible to 100.00 g. As you
   approach 100.00 g, use a pipette or dropper to add
   water dropwise.


4. Remove the cup from the balance, substitute a weighing
   paper, and transfer 35.07 g of sodium chloride to the
   weighing paper. This amount (0.6 moles) of sodium
   chloride added to 100.00 g of water is sufficient to make
   up a 6.0 molal solution of sodium chloride.


5. Transfer the 35.07 g of sodium chloride to the foam
   cup. Stir it periodically as you are making up the other
   solutions until all of the sodium chloride dissolves.
   6. Repeat steps 2 through 5 in two additional labeled foam
   cups to make up sodium chloride solutions of 3.0 molal
   (100.00 g of water plus 17.53 g of sodium chloride)
   and 1.5 molal (100.00 g of water plus 8.77 g of sodium
   chloride).
   7. Repeat steps 2 through 6 in three additional labeled
   foam cups to make up sucrose solutions of 6.0 molal
   (100.00 g of water plus 205.38 g of sucrose), 3.0 molal
   (100.00 g of water plus 102.69 g of sucrose) and 1.5
   molal (100.00 g of water plus 51.34 g of sucrose).

PRTIII: A dETERNEmI THE BoILING poINTS

of SodIUm CHLoRIdE, SUCRoSE ANd SodIUm

CARBoNATE SoLUTIoNS

In this part of the lab, we determine the boiling points of the

sodium chloride and sucrose solutions.

1. If you have not already done so, put on your splash
   goggles, gloves, and protective clothing.


2. Transfer the 6 molal sodium chloride solution to the 250
   mL beaker. Place the beaker on the heat source and apply
   gentle heat with constant stirring until the contents of the
   beaker come to a full boil.


3. Immerse the thermometer in the beaker, making sure it
   does not contact the beaker itself, allow the thermometer
   to stabilize, and record the temperature reading in Table
   8-1.


4. Remove the beaker from the heat and place it aside to
   cool. Transfer the solution from the beaker back into its
   labeled foam cup. Rinse the beaker thoroughly and dry it.


5. Repeat steps 2 through 4 for the 3.0 molal and 1.5 molal
   sodium chloride solutions and the 6.0 molal, 3.0 molal,
   and 1.5 molal sucrose solutions.


6. Using the observed boiling point elevations for each of
   the solutions and the van’t Hoff factors for each solute,
   calculate the formula weights of sodium chloride and
   sucrose, and enter those values in Table 8-1.