Using English For Report

Title:Determination of the molar mass based on the freezing point drop
Day/Date:Friday,17th march 2017
Purpose: 1) To be able set the point of freezing pure liquid and freezing point of solution in the solvent
          2) To be able to determine molar mass and unknown compounds based        on the decrease of freezing point
Theoretical basis:
The colligative nature of the solution is the nature of the solution which does not depend on the kind of solute but solely determined by the amount of solute (Syukri,2010:210).
When a solvent is added with a small amount of solute,there will be a solution which decreases the saturated vapor pressure,increase in the the boiling point, decrease the freezing point, osmosis pressure, and others(Elis,2011:201).
The addition of non-volatile solutes into pure liquid causes a drop in the vapor pressure of the liquid. This results in  a decrease in the freezing point of the solution and an increase in the the boiling point of the solution when compared to the pure liquid. How much change occurs depends only on the amount of the dissolved substance and on the dissociation level of the solute. Changes in freezing and boiling points are not related to the chemical identity of the substance in question. Decrease in vapor pressure,boiling point rise, and decrease of freezing point are known as colligative properties(Handayana,2010:298).
There are five koligative properties of the solution:
1. Decrease in saturated vapor pressure,from Roult’s law
The solvent vapor pressure is not zero and changes according to the composition of the solution at a certain temperature. Roult’s law states that the vapor pressure of the solvent above a solution (PA) is equal to the product of vapor pressure because x1 =1-x2. For a solution consisting of two components,Roult’s law is written as:
ΔP1 = P1-P10=x1P01-P10=-x2P¬¬10
So,the change in solvent vapor pressure id directly proportional to the mole fraction of the solute(Oxtoby,2010:309).
2. Osmosis Pressure
The number of particles in the solution is determined by the concentration of the solution and the nature of the solution itself. The number of particles in the non-electrolyte solution is not equal to the number of particles in the electrolyte solution even though the concentrations are the same(Oxtoby,2010:309).
3. Increase of boiling point
The normal boiling point of pure liquid or solution is the temperature at which the vapor pressure reaches 1 atm. Since the solute decreases the vapor pressure, the temperature of the solution should be increased to boil(Oxtoby,2010:310).
4. Decrease of freezing point
If the solvent water and air pressure 1 atm, then the freezing point of the solution is expressed as:
Tf =(0-ΔTf)0C
(Oxtoby,2010:311)
5. Osmotic pressure
This pressure is a pressure applied to a solution which can stop the transfer of solvent molecules into the solution through a semipermeable membrane (osmosis process)(Oxtoby,2010:312).
The concentration of the substance is the number of moles pervolume. Molality is the mass ration and t is independent of temperature. Molality and molarity are almost equal (Bird,2010:257).

Tools and materials:
Tools to be used:
1000 ml beaker
50 ml beaker
Large reaction tube
Thermometer
Spray bottle
Stir bar
Stopwatch
Analytical balance
Materials to be used:
Benzenal (C6H6)
P-xylene
Ice cube
Aquades
Salt

Work steps:

Results:

A. Fozen solvent determination of pure solvent

1. The finally buret reading : -

2. The firstly buret reading : -

3. The volume of p-xylene used : 25 mL

Discussion:

In this experiment,we conducted two types of that,there are:

A.  Frozen solvent determination of pure solvent

In this experiment, we used a p-xylene solvent because this solution was a benzene used as a solvent. The use of salt in this experiment is for as a stabilizer of ice temperature because the salt can inhibit the process of melting ice. Salt used should not be too much because it will affect the process of decrease in freezing and the results will be less accurate. However,if the salt used is too small then the frost drop does not reach an accurte temperature. We use a thermometer to measure the temperature of the solvent when the solvent is frozen but keep the thermometer from touching the bottom of the test tube as it will affect the stability of the freezing point temperature of the tested solution.

From the graph,it can be seen that there is a decrease in temperature every 15 seconds. Although our experiments failed because this solution did not freeze but there was a decrease of temperature from 100 C until -40 C. This indicates that there is an effect if ice used to decrease this freezing point even though it does not freeze. When we see the decrease of temperature in every 15 seconds,it can be seen that the longer time the slower it goes down or change.

B.  Determination of molar masses of unknown compound

For this experiment,we did not do it because this experiment can only be done if the previous experiment successed. But the previous experiment was unsuccessful so we could not determine the molar mass of a compound we didi not know yet. According to the literature,if we have obtained the determination of freezing point of pure solvent,then we can look for the molar mass of unknown compound by the formula:

ΔTf = Tf (pelarut) – Tf (larutan)

Then

ΔTf = Kf . m

This calculation can be done by drawing a line that is at the initial and final temperature when the temperature obtained to the point where the solution or solvent p-xylene freeze.

Conclusion:

For these experiment, we can concluded that:

1.The freezing point of pure liquid and freezing point of solution in the corresponding solvent can be determined by recording the temperature each time continuously, resulting in a constant expressed by:

ΔTf = Kf x m

that : ΔTf = Decrease in freezing point

         Kf = Molar freeze point constant

         M = Molality

2. Determining the molar mass of an unknown compound based on the decrease of freezing point through experiment is by inserting a compound in other compound or other solution. And measured the temperature of any given period of time. These can expressed by:

ΔTf = Kf x m ;n = M x V

Obtained molar: M

Literature:

Syukri,s.2010. Kimia Dasar 2.Bandung:ITB

Handayana,Pudjaamaka.2010.Kimia Fisika Universitas Edisi Ke-6 Jilid 1. Jakarta:Erlangga

Elis.2011.Diktat Kuliah:Kimia Dasar I(Kimia Anorganik).Bandung:ITB

Oxtoby,dkk.2010.Prinsip-prinsip Kimia Modern edisi ke-4 Jilid 1.Jakarta:Erlangga

Bird,Tony.2010.Kimia Fisika Untuk Universitas.Jakarta:PT Gramedia