All Chemistry ClassesSolubility LabName ______

Purpose: To gather laboratory data in order to construct a solubility curve.

Procedure:

  1. Assemble the apparatus as shown. The bulb of the thermometer should be placed about 2 cm from the bottom of the test tube.
  2. Weigh 6 to 7 grams (the exact weight must be known – record it in your data chart now!) of Potassium Nitrate and pour the salt into the dry test tube (Note – you only add salt to the tube one time in this experiment).
  3. Add 4.0 mL of water to the test tube. Heat the test tube in the water bath until the salt dissolves. You should stir the solution to help it dissolve, but no amount of stirring will help until you get the tube to at least 60 Co for the first trial.
  4. Once the salt has dissolved, remove the test tube and clamp from the ring stand. Place the test tube in a beaker of tap water (a cold water bath) and stir constantly. Record the temperature when the crystals first appear. If you miss finding the right temperature, you can always put the tube back in the hot water, get it to dissolve, and then try again.
  5. Add water to the test tube (2.0 mL) and then repeat the experiment (that is, stir and reheat until the salt dissolves, then put in the cold water bath – don’t add more salt, just more water). Then add water in the following amounts and repeat the experiment: 2 mL, 4 mL, 4 mL, and 4 mL. For the last few observations, you may have to add ice to your cold-water bath in order to get the temperature low enough for the crystals to form.
  6. Calculate the concentration for each trial using the formula:

Concentration in g/100 mL = grams of salt x100

volume of water

Data Chart: Chart 1

Grams of salt (same value each time) / Volume of water mL / Concentration
(answer 3 sig figs) / Temperature when crystals appear Co
4.0
6.0
8.0
12.0
16.0
20.0

Lab Requirements:

  1. On a piece of graph paper, construct a graph of solubility (concentration in g/100 mL) vs. temperature. Plot points, and then draw the best curve line that you can. Draw these points as dots, with circles around them. Complete all of this in pencil. The values that you are plotting here for concentrations are what we will later consider to be the actual values.
  1. On the same piece of graph paper, plot the following points in the chart below, and then draw the best curve line that you can. Draw these points as dots with squares around them. Complete all of this in pen or marker. The values that are plotting here for concentrations are what we will later consider to be theoretical values.

Chart 2 – Theoretical concentrations used to construct a graph

Concentration (g/100mL) / Temperature Co
204 / 90
166 / 80
135 / 70
110 / 60
85 / 50
64 / 40
47 / 30
32 / 20
21 / 10
  1. Complete the following chart. Notice that you will be using your temperatures to find the theoretical values for concentrations. The concentrations from the chart in #2 are to be used to make the graph only – do not use those values in the chart below. You need to use your temperatures to find the corresponding theoretical concentrations.

Chart 3 – a comparison of actual concentrations to theoretical concentrations

Temperature (your temperatures from Chart 1) / Actual Concentration
(from Chart 1) / Theoretical Concentration
(determine from the graph)
  1. Perform 6 % error calculations on the back of your graph paper. Calculate the % error in concentration at each temperature recorded in Chart 3.
  1. Staple the graph paper to this paper. Make sure that the front of this paper faces up and the graph faces up as well.