Colligative Properties of Solutions – Hello Gramedia friends, do you know about colligative properties of solutions? Previously, you studied material on the concept of molarity in tenth grade. On this occasion, let’s learn chemistry about colligative properties of solutions!
Inventor Biography
Marie Francois Raoult (1830-1901) scientist who concluded about the saturated vapor pressure of a solution.
Marie Francois Raoult was a French chemist who conducted research on the behavior or colligative properties of solutions. Marie Francois Raoult is a scientist who concluded about the saturation vapor pressure of a solution. He was born in Fournes, in the département of Nord on 10 May 1830. He became a candidate for répétiteur at the Lycée of Reims in 1853.
After holding several intermediate positions, he was appointed to a professorship of chemistry at the Sens lycée in 1862. He there prepared a thesis on electromotive force which earned him a doctorate in Paris the following year. In 1867 Raoult got an assignment in a chemistry class at Grenoble, three years later he managed to occupy the chair of chemistry until his death in 1901.
Raoult’s earliest research was physical in character, largely concerned with the voltaic cell phenomenon; then there was a period when more purely chemical questions got his attention. François-Marie Raoult died on April 1, 1901 in Grenoble.
Raoult is famous for his experiments with solutions. His first paper on the lowering of the freezing point of liquids due to the presence of solutes in them was published in 1878. This was followed by investigations and studies with various solvents, such as benzene and acetic acid, which led him to believe that there was a simple relationship between molecular weight a substance and the freezing point of the solvent.
If one mole of a substance is dissolved in 100 moles of solvent, the temperature of the solution will decrease. Another relation obtained from his experiments is the decrease in the vapor pressure of the solvent, caused by the presence of a substance dissolved in it, which is proportional to the molecular weight of the substance being dissolved. These are used to determine the molecular weight of a substance, and are also used to support the hypothesis of the decomposition of electrolytes in solution.
Definition of Colligative Properties of Solutions
The colligative property of a solution is a property of a solution that does not depend on the type of solute, but only on the concentration of the solute particles. There are two types of colligative properties of solutions, namely colligative properties of electrolyte solutions and colligative properties of non-electrolyte solutions.
Molarity, Molality, and Mole Fraction
In a solution, there are several properties of a substance that are determined only by the number of solute particles. Because the colligative properties of a solution are determined by the number of solute particles, it is necessary to know the concentration of the solution.
1. Molarity (M)
Molarity is the number of moles of solute dissolved in 1 liter of solution.
Note:
M = molarity.
Mr = molar mass of solute (g/mol).
V = volume of solution.
2. Molality (m)
Molality (molalan) is the number of moles of solute in 1 kg (1000 grams) of solvent. Molality is defined by the following equation.
Note:
m = molality (mol/kg).
M r = molar mass of solute (g/mol).
mass = mass of solute (g).
P = mass of solvent (g).
3. Mole fraction
Colligative Properties of Nonelectrolyte Solutions
In a pure solvent system the boiling point, freezing point, vapor pressure and osmotic pressure will only be affected by the solvent molecules themselves. However, in a solution system consisting of a solvent and a solute, the presence of a solute in a solvent will cause a certain change in the four properties of the solvent.
Volatile solutes cause the saturated vapor pressure of the solution to be greater than the saturated vapor pressure of the solvent, while non-volatile solutes tend to lower the saturated vapor pressure of the solution. The change in vapor pressure will also have an effect on the boiling point and freezing point of the solution so that the colligative properties of the solution occur.
Based on this analysis, the colligative properties of solutions consist of four properties, including:
- Vapor pressure drop (∆P).
- Boiling point elevation (∆Tb).
- Freezing point depression (∆Tf).
- Osmotic pressure (π).
Although colligative properties involve solutions, they do not depend on interactions between solvent and solute molecules, but depend on the amount of solute dissolved in a solution. The colligative properties consist of vapor pressure lowering, boiling point elevation, freezing point depression and osmotic pressure.
1. Vapor Pressure Drop
Evaporation is an event that occurs when the liquid particles leave the group. The weaker the intermolecular forces of attraction of a liquid, the easier it is for the liquid to evaporate. The easier the liquid evaporates, the greater the saturated vapor pressure.
The amount of vapor formed above the surface of a liquid is called the vapor pressure. When the liquid particles leave their group to become vapor, at the same time the vapor will return to being a liquid. The pressure that arises when there is an equilibrium between the number of liquid particles to become vapor and the amount of vapor to become liquid is called saturated vapor pressure.
The liquid molecules leaving the surface cause the vapor pressure of the liquid to form. The easier the liquid molecules turn into vapor, the higher the vapor pressure of the liquid. If the pressure of the liquid is dissolved by a solute that does not evaporate, then the particles of this solute will reduce the evaporation of the liquid molecules.
The Dead Sea is an example of the lowering of the vapor pressure of a solvent by a non-volatile solute. This very high salt water is located in a very hot and dry desert area, and is not connected to the open sea, so the concentration of dissolved substances is even higher. The vapor pressure drop equation can be written as follows.
Information:
- P 0 = vapor pressure of pure liquid.
- P = vapor pressure of the solution.
In 1878, Marie Francois Raoult, a French chemist, conducted an experiment on the saturated vapor pressure of a solution, so that she concluded that the saturated vapor pressure of a solution is equal to the mole fraction of the solvent multiplied by the saturated vapor pressure of the pure solvent. This conclusion is known as Raoult’s Law and is formulated as follows.
Information:
- P = saturated vapor pressure of the solution
- P 0 = saturated vapor pressure of pure solvent
- X p = mole fraction of solvent
- X t = mole fraction of solute
2. Boiling Point Elevation
The boiling point of a liquid is the constant temperature at which the liquid boils. At this temperature, the vapor pressure of the liquid equals the pressure of the surrounding air. This causes evaporation in all parts of the liquid. The boiling point of liquids is measured at a pressure of 1 atmosphere.
Based on the research results, it turns out that the boiling point of the solution is always higher than the boiling point of the pure solvent. This is due to the presence of solute particles in a solution blocking the evaporation of solvent particles. Therefore, evaporation of solvent particles requires more energy.
Information:
- Tb = boiling point elevation ( o C).
- kb = molal boiling point elevation constant ( o C kg/mol).
- m = molality of solution (mol/kg).
- Mr = relative molecular mass.
- P = total mass of substance (kg).
Table of Boiling Point Elevation (Kb) of Several Solvents
| Solvent | Boiling point | Default (Kb) |
|---|---|---|
| Acetone | 56,2 | 1.71 |
| Benzene | 80,1 | 02.53 |
| Camphor | 204.0 | 05.61 |
| Carbon tetrachloride | 76.5 | 04.95 |
| cyclohexane | 80.7 | 02.79 |
| Naphthalene | 217,7 | 05.80 |
| Phenol | 182 | 03.04 |
| Water | 100.0 | 00.52 |
3. Freezing Point Depression
The freezing point of a solution is the temperature at which the vapor pressure of the liquid equals the vapor pressure of the solid or the point at which water begins to freeze. The normal freezing point of a substance is the temperature when the substance melts or freezes at a pressure of 1 atm (normal conditions). External pressure has little effect on the freezing point. At a pressure of 760 mm Hg, water freezes at 0 C.
If a solute is added to a pure solvent to form a solution, the freezing point of the pure solvent will decrease. This happens because it is difficult for the solvent molecules to turn into the liquid phase because the solute particles block the movement of the solvent particles. For example, the normal freezing point of water is 0°C. However, in the presence of solutes at 0 oC the water has not frozen. So the difference between the freezing point of the solvent (Tfo) and the freezing point of the solution (Tf) is called freezing point depression (ΔTf).
The presence of a solute in a solution will cause the freezing point of the solution to be lower than the freezing point of the solvent. The equation can be written as follows.
Information:
- Tf = freezing point depression ( o C).
- kf = freezing point change constant ( o C kg/mol).
- m = molality of solution (mol/kg).
- Mr = relative molecular mass.
- P = total mass of substance (kg).
Table of Freezing Point Depression (Kf) of Several Solvents
| Solvent | Freezing point | Rated (Kf) |
|---|---|---|
| Acetone | -95.35 | 2.40 |
| Benzene | 5.45 | 5,12 |
| Camphor | 179.8 | 39,7 |
| Carbon tetrachloride | -23 | 29,8 |
| cyclohexane | 6,5 | 20,1 |
| Naphthalene | 80.5 | 6,94 |
| Phenol | 43 | 7,27 |
| Water | 0 | 1.86 |
4. Osmotic pressure
Van’t Hoff.
Osmotic pressure is the force required to balance the force of solute through a semipermeable membrane into solution. A semipermeable membrane is a membrane through which solvent molecules can pass and solutes cannot. According to Van’t Hoff , the osmotic pressure of a solution is formulated as follows.
Information:
- = osmotic pressure.
- M = molarity of the solution.
- R = gas constant (0.082).
- T = absolute temperature.