CHEM 106 General Chemistry II

 

EXERCISE 1. Solubility

The mechanism and the extent to which a solute dissolves in a solvent depends upon the nature of each substance and the temperature of the system. Molecular solids (e.g. sugars) and ionic solids (salts) both dissolve in water. However, they both dissolve in different ways. The intermolecular forces holding sugar molecules together are relatively weak. When a sugar is placed in water these bonds are broken and individual molecules are released into solution. It takes energy to break bonds between molecules and it also takes energy to break the hydrogen bonds in water. These hydrogen bonds have to be disrupted in order to insert a sugar molecule into the substance. The energy needed for this is produced by the formation of bonds between slightly polar sugar molecules, (sucrose: C12H22O11) and polar water molecules. This process works so well between sugar and water that up to 800g of sugar can dissolve in 1 liter of water.
The positive and negative ions in ionic solids (or salts) are held together by the strong force of attraction between particles with opposite charges. When a salt dissolves in water the ions are released and become associated with the polar solvent molecules. Salts dissociate with their ions when they dissolve in water.

e.g. H2O + NaCl (s) → Na+ (aq) + Cl - (aq)

There are several factors that will affect solubility between different compounds
Temperature - If the solution process absorbs energy then the solubility will be
increased if there is a temperature increase. If the solution releases energy (exothermic, i.e. between sugar and water) then solubility will decrease.
Molecular Size - If the size or weight of the individual molecules is large then solubility will be low because larger molecules are difficult to surround with solvent molecules.
Polarity - Generally only polar solute molecules will dissolve in polar solvents and only non-polar solute molecules will dissolve in non-polar solvents. Polar solute molecules have partial positive and partial negative ends. If a polar solute molecule is placed in a polar solvent then the positive ends of solvent molecules will attract the negative ends of solute molecules. This type of intermolecular force is known as dipole-dipole interaction. The type of intermolecular force in present in non-polar molecules is called London Dispersion forces. Here the positive nuclei of the atoms of the solute molecules will attract the
negative electrons of the atoms of a solvent molecule.

In these exercises, you will be making saturated solutions. The saturation point of a solution is reached when no additional solute can be added to or contained by the solvent. When the solvent temperature is decreased the ability of the solvent to contain the solute is decreased. When saturation temperature is just exceeded, the solute will begin to precipitate. We will accept that the temperature where precipitation is observed is a good approximation of the saturation temperature.


Materials:

       3- 100 ml Erlenmeyer flasks

       3- large test tubes

       graduated cylinder or burette

       thermometer

       Hot plate or ring stand, ceramic mat, Bunsen burner

       2-400 ml beakers

       Boiling Chips

       Ice

       Foil, or cork or parafilm

Method:

Add tap water to the 400 ml beaker to 2/3 the volume, and set on the hot plate or ring stand and bring to a boil. You will use this to heat your sample in the test tube.

Add ice to the second 400ml beaker. You will use this to cool your sample in the test tube.

Solubility of Salts
Weigh out 9g of sodium chloride and record its mass to the nearest tenth of a gram. Place this in a large test tube and add 20 ml of distilled water with a graduated cylinder or burette. The burette is better for measuring small amounts of water because it is more accurate. Now heat the test tube gently until all the crystals have gone into solution (should be around 80C). If at 80C all the crystals have not gone into solution, add 1ml of water, and stir until all the salt is dissolved at 80C. Repeat as necessary until all the salt is dissolved but be careful to record the total volume of water added to the sample.
Allow the solution to cool while stirring with a thermometer. When crystals start to appear record the temperature.
Now add another 2ml of water into the flask. Reheat until all crystals have again gone into solution. Remove from heat and allow the solution to cool while stirring with thermometer. When crystals start to appear record the temperature.
Add another 2ml of water and repeat the same method, recording the new temperature at which crystals appear. Repeat until you have at least 3 different saturation temperatures at different total volumes.

Salt 1

 

V (ml)

 

 

 

 

 

Mass (g)

 

T (oC)

 

 

 

 

 


Solubility of Organic Solids:

To a test tube add 0.3 g of Naphthalene weighed to the nearest hundredth of a gram. Go to the hood and with a medicine dropper add cyclohexene until the naphthalene goes into solution. Now add the same number of drops to a graduated cylinder, and read the volume required to bring the naphthalene into solution. Record the mass of naphthalene, the volume of cyclohexene solvent and the temperature of the room.

Solubility of Sucrose:

This is a real experiment for you to design. Design an experiment to determine the solubility of sucrose in water. Make a solubility curve for a range of temperatures. Keep track of materials, methods and procedures used. You must report exactly what was done. Once a determination is made, look up the solubility of sucrose and compare your results. Show all calculations and report your conclusions.

Calculations:

Solubility is commonly reported as the grams of solute that can dissolve in 100 grams of solvent. From your data make a table of solubility in standard units for each recorded temperature.

Using Excel, make a plot of solubility vs. temperature for each salt. Plot the points and find the best fitting curve that describes the data. Be sure your plot includes the R-squared value and equation for best fit. (Do not force through zero)

Determine from your data the solubility of Naphthalene in cyclohexene. Report your answer in standard units and show all work.

 

Additional Work:

For each salt above determine the following:

       Solubility, S, is sometimes reported as the concentration in molarity, M, at the point of saturation. Determine the molarity of each saturated solution at room temperature.

       Determine the molality of each saturated salt solution at room temperature.

       Determine the mole fraction for each saturated salt solution at room temperature.

       Determine the percent by mass for each saturated salt solution at room temperature.