Chemistry 11 Lab #9
Solutions
The lab will be set up with a number of work stations. You can do these
in any order you choose. It is crucial to get reading errors for each
measurement.
Station #1 Solubility of salt (NaCl) in water
- Mix about 200 mL of liquid water and an equal amount of ice together
and allow it to sit for a few minutes in a 500 mL beaker. (What temperature
should the water/ice mixture be after sitting?)
- Grind a small handful of rock salt to a fine powder with mortar and
pestle, or use some salt ground by a previous group. Accurately weigh about
30 g of salt (NaCl) and note mass. Then do the same to about 10 g of salt
- Accurately measure 100 mL of the liquid water with a graduated cylinder
and transfer to a 250 mL beaker. Place the beaker in the ice bath that
should still be in the 500 mL beaker.
- Add the 30 g of salt and mix until dissolved. Proceed to add a little
less than half the 10 g sample of salt and mix to dissolve. Tne slowly
add salt a few grains at a time all the while mixing to dissolve, until
no more salt can dissolve.
- Calculate the total amount of salt dissolvable in 100 g of water at
this temperature. Compare to the value in your text.
- Add the rest of the salt to the water and note what happens. Then put
the beaker on the hot plate and see what happens as the solution heats
up. Place the beaker in the ice bath and allow to cool. Note what happens.
Station #2 Crystallization of sodium thiosulfate
- Put 10 to 20 mL of sodium thiosulfate crystals and a few drops of water
in a 150 mL beaker. (Note: the sodium thiosulfate crystals actually have
water trapped in them and so you don't even really need water to dissolve
them. They dissolve in their own water of crystallization.) Stir and note
what happens.
- Place the beaker on the hot plate and heat until all the crystals have
dissolved.
- Allow the solution to cool in a water bath inside a 250 mL beaker for
at least 5 minutes. Note what happens.
- Add a crystal of sodium thiosulfate and note what happens. Provide
a possible explanation. (Hint: a supersaturated solution is one in
which there is more solute dissolved than at the saturation point.
Precipitation, or crystallization, does cannot occur unless there is somewhere
for the crystals of precipitate to form. A similar event occurs in
making popcorn. The water inside the kernel is well above the normal
boiling point of water at atmospheric pressure when it is heated in the oil.
Think of your phase diagram graph. When the kernel finally
ruptures the water is still a liquid but is now well above its boiling point,
so it rapidly turns to a gas. The rapid expansion "pops" the kernel.)
Station #3 Melting Point and Effect of Adding Sodium Chloride
- Sprinkle salt on an ice cube and note what happens.
- Take crushed ice and add salt to it
in a 250 mL beaker and allow it to sit a few minutes to reach thermal equilibrium.
Remember from a prevu=ious lab: the coexistence of the two phases in
equilibrium ensures that the temperature must be the melting/freezing point of (in this case) saltwater.
Find the temperature.
Station #4 Boiling Point and Effect of Adding Sodium Chloride
- Bring about 250 mL of saltwater in a 500 mL beaker to a boil with a bunsen
burner, ring and stand, and wire gauze. Note the temeprature once the
water is at a full boil.