Dissolution of a substance in a solvent suppresses the freezing point of the solution formed. The freezing depression depends on the amount of the material dissolved in the solvent according to the following equation:
Where,
ΔT = freezing point depression
Kf = freezing point molar constant of solvent
M = molal concentration = mass of material / molecular weight kg solvent
= No. of moles of solute / kg solvent
Apparatus and Materials:
Naphthalene, substance X, test tubes, beaker (500 cm3), thermometer, glass rod,
stopwatch, retort stand, stopwatch.
Procedures:
Part 1: Determination of the Freezing Point of Naphthalene
- A clean and dry test tube was weight.
- 5 g of Naphthalene was added to the test tube and was weight again.
- The naphthalene was melted by using a water bath.
- When all naphthalene was melted, the test tube was transferred from the water bath and was dried and clamped on a retort stand.
- The liquid naphthalene was stirred continuously so that no temperature gradient develops.
- The temperature was recorded every 15 seconds for a few minutes and then every 30 seconds for another 10-15 minutes.
- A graph of temperature versus time was plotted.
- The melting and cooling steps were repeated.
Part 2: Determination of the Freezing Point of Naphthalene
- 0.5-0.75g of substance X provided was weighed accurately.
- Substance X was added to naphthalene in a test tube which was used in Part 1.
- Melting and cooling steps were carried out.
- The melting and cooling steps were repeated by using different masses of substance X by adding 0.15-0.25g each time to the naphthalene-X mixture which has been used above.
-
The molecular weight of substance X was calculated as an average of two determinations. [Kf for naphthalene is 6.8]
Results:
Table 1: Mass of test tube and naphthalene.
Table 2: Mass of substance X added for the first cooling process.
Table 3: Mass of substance X added for the second cooling process.
Table 4: Temperatures of naphthalene and substance X during cooling process.
Analysis and calculation:
- From Table 1:
Mass of naphthalene
= (mass of empty test tube + mass of naphthalene) – (mass of empty test tube)
= 35.7614 g – 30.7538 g
= 5.0076 g
- From Table 2:
Mass of substance X1
= (mass of empty test tube + substance X1) – (mass of empty test tube)
= 31.3103 g – 30.7538 g
= 0.5565 g
- From Table 3:
Mass of substance X2
= (mass of empty test tube + substance X2) – (mass of empty test tube)
= 30.9548 g – 30.7538 g
= 0.2010 g
-
The molecular weight of substance X for the first set of experiment is calculated, by adding 0.5g - 0.75g of substance X to naphthalene. (Given Kf of naphthalene is 6.8.)
= 75.0 C – 73.5 C
= 1.5 C
1.5 = 6.8
= 503.79 g mol-1
Molecular weight of substance X for the first set of experiment is 503.79 g mol-1.
The molecular weight of substance X for the second set of experiment is calculated, by adding 0.15g – 0.25g of substance X to naphthalene.
= 75.0 C – 74.0 C
= 1.0 C
1.0 = 6.8
-
= 6.8
Molecular weight =
= 272.95 g mol-1
Molecular weight of substance X for the second set of experiment is 272.95 g mol-1.
The molecular weight of substance X =
= 388.37 g mol-1
Hence, the molecular weight of substance X is approximately 388.37 g mol-1.
Discussion:
Naphthalene is a crystalline, aromatic, white, solid hydrocarbon with formula of C10H8 and its structure consists of two fused benzene rings. It is also known as the traditional, primary ingredient of mothballs. Naphthalene is volatile, forming an inflammable vapor, and readily sublimes at room temperature, producing a characteristic odor that is detectable at concentrations as low as 0.08 ppm by mass. Naphthalene has a freezing point of around 79°c – 81°c and a boiling point of 218 °c. A naphthalene molecule is derived by the fusion of a pair of benzene rings. It is classified as a benzenoid polycyclic aromatic hydrocarbon (PAH). Naphthalene is different from benzene; the length of carbon-carbon bonds in naphthalene is not the same. However it is similar to benzene in some way, which is it can undergo electrophilic aromatic substitution. For many electrophilic aromatic substitution reactions, naphthalene reacts under milder conditions than does benzene. For instance, although both benzene and naphthalene react with chlorine in the presence of a ferric chloride or aluminium chloride catalyst, naphthalene and chlorine can also react to form 1-chloronaphthalene even without a catalyst.
According to theory, the freezing point of naphthalene should be around 79°C – 81°C. However, the freezing point of naphthalene obtained in this experiment is 75°C. The result obtain is not very accurate as it may affected by some external factors. It may due to the improper techniques used in when carrying out the experiment in determining the temperature. In order to get a better result, we need to be aware of the technique used like the purity of naphthalene and the apparatus accuracy before carrying out the experiment.
In this experiment, we should handle and dealing with naphthalene with cautious due to its hazardous effects. This is because long term exposure to large amounts of naphthalene may lead to damages in our red blood cells. Some symptoms will occur if we exposed to large amounts of naphthalene, such as nausea, vomiting, diarrhea and blood found in the urine. It is indeed very harmful and hazardous to us if we inhaled too much of it. Besides, it may cause irritation to skin, eyes, respiratory tract and also allergic skin reaction. Besides, it may affect liver, kidney, blood and central nervous system too. Furthermore, naphthalene is flammable and it should be kept away from heat, sparks and flame. Some precaution steps must be taken when handle naphthalene. Lab coat and safety goggles included gloves should be worn when carrying out the experiment. We should avoid our eyes and skin from being contact directly with naphthalene. Moreover, mask should be worn to avoid inhaling the naphthalene in gaseous state.
Questions
-
How can the Kf value for a solvent be determined?
Several steps are required to determine the Kf value for a given solvent. First, identify the freezing point depression of the solvent. Next, calculate the molal concentration of the solution as it can be obtained by dividing the moles of solute with the mass of solvent in kilograms. Moles of solute can be obtained by dividing the mass of solute used in the experiment by molar mass of the solute. The mass of solvent in kilograms can be obtained directly from the amount of solvent used in the experiment. Lastly, use the formula:
ΔT = Kf · m
Substituting all the value obtained above to determine the Kf value for a given solvent.
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Compound A, C8H8N2, melts at 80.5 and compound B, C10H8, melts at 80.6. Compare Z which is given is the same as A or B. Suggest a simple method to determine the identity if Z.
The simple method used to identify substance Z is by using Freezing Point Depression method. First, we should identify and carry out an experiment to obtain the freezing points of both compound A and B. After that, we can dissolve the substance Z in both solvent of A and B in order to get two different solutions. The molal concentration of both the solution can be calculated after we obtain the freezing point of depression. Then, by using the molal concentration and the freezing point depression value obtained, use the formula of
ΔT = Kf x m to determine the freezing point depression constant for both compound A and B solvent. With both the Kf values of the compounds of A and B, this help us to calculate the molecular weight of compound Z by using the formula as shown as below:
ΔT = Kf m
= Kf
After the molecular weight of compound Z is calculated, we can start to determine the identity of Z after comparing the values obtained from compound A and B.
Conclusion:
The freezing point of naphthalene obtained in this experiment is 75°C. Molecular weight of substance X for the first set of experiment is 503.79 g mol-1 while for the second set of experiment is 272.95 g mol-1. From this two set of experiment, the molecular weight of substance X is calculated by getting the average value, which is 388.37 g mol-1.
Reference:
-
Freezing-point depression. (2009). Accessed on July 12, 2009.
-
Freezing Point Depression Constant. (2004). Accessed on July 14, 2009.
