The Solubility of Potassium Nitrate

Aim:

The aim of this experiment is to find out by how much the solubility of potassium nitrate into distilled water increases when the solution is heated, and if yes, by how much.

Hypothesis:

According to data on the internet, 3.75 × 10­¹ moles of potassium nitrate dissolve in 100g of water. I believe this information may be correct. I also believe that as the solute is absorbing outside heat, the energy is increased causing it to dissolve both faster, with more potassium nitrate.

Apparatus:

Bunsen burner

25g Potassium nitrate

Distilled water

Boiling tube

Clamp

Thermometer

Method:

1) Weigh out 25g Potassium Nitrate into a boiling tube.

2) Add 10ml water

3) Heat solution over bunsen until potassium nitrate dissolves, moving the tube in and out of the flame so it doesn’t boil over.

4) Allow to cool and record temperature at which crystals appear

5) Add 5ml of water and repeat steps 3) and 4)

6) Repeat step 5) until you have 30ml of water in the boiling tube

7) Transform data so it is expressed as a mass of potassium nitrate per 100g of water.

8) Graph

Results:

Water (ml)Temp at which crystals appear ( C)Potassium Nitrate (g) per 100g of water

10101250

1599166.6

2097125

2595100

309383.3

Mass of potassium nitrate dissolved at 40 : 64g

Mass of potassium nitrate dissolved at 80 : 169g

Mass obtained as crystals when solution cooled from 80 to 25 : 134g or 79.3%

Two graphs: one from the internet, one made according to our results.

Conclusion:

From the results we can see that the solubility of potassium nitrate is increased as the temperature is increased. The relationship is nonlinear, which means it doesn’t form a straight line when plotted on a graph. Now, there are three main factors, which affect solubility (besides pressure).

Temperature will affect solubility. If the solution process absorbs energy then the solubility will be increased as the temperature is increased. If the solution process releases energy then the solubility will decreased with increasing temperature.

Molecular size will affect the solubility. The larger the molecule or the higher its molecular weight the less soluble the substance will be. Larger molecules are more difficult to surround with solvent molecules in order to solvate the substance. In the case of organic compounds the amount of carbon “branching” will increase the solubility since more branching will reduce the size (or volume) of the molecule and make it easier to solvate the molecules with solvent.

The polarity of the solute and solvent molecules will affect the solubility. Generally polar solute molecules will dissolve in polar solvents and non-polar solute molecules will dissolve in non-polar solvents. The polar solute molecules have a positive and a negative end to the molecule. If the solvent molecule is also polar, then positive ends of solvent molecules will attract negative ends of solute molecules. This is a type of intermolecular force known as dipole-dipole interaction. All molecules also have a type of intermolecular force much weaker than the other forces called London Dispersion forces where the positive nuclei of the atoms of the solute molecule will attract the negative electrons of the atoms of a solvent molecule. This gives the non-polar solvent a chance to solvate the solute molecules. I know all of this information is irrelevant, except for the bit about temperature, but I decided to include it anyway to give you (the reader) a wider perspective on the processes.

In the case of potassium nitrate heat is absorbed to make the solution process more effective, thus making the solvate more soluble. This means that the higher the temperature, the more potassium nitrate can dissolve in distilled water. This proves my hypothesis to be correct.

Errors:

There are quite a few possible margins for error. The biggest one has to be human error. It is up to the observer’s own judgment to decide when the crystals are forming, and the human eye can easily be deceived. For example, a person very eager to see crystals will probably see them before they actually appear, and take the temperature then. Human error can be reduced by having the same person take all the readings and observations. Another problem could be any water evaporating from the boiling tube, which could result in inaccurate readings. Unfortunately there is no way of eliminating this error that I can think of. One should simply state that the results are approximate.