Molar Mass Freezing Point Depression

Contents

1 Group Information
2 Objectives
3 Concepts
4 Background
5 Equations:
half dozen Materials
7 Take a chance Assessment
1. 7.ane BHT
2. 7.two Cetyl alcohol
3. seven.three Unknown substance
4. seven.four Water
8 Process
9 Results
1. 9.1 Data Observed
  1. nine.1.1 Masses
  2. 9.ane.two Temperatures
2. 9.two Information Calculated
ten Post-lab Analysis
11 Conclusion
12 Evaluation

Grouping Information

Lab Partners: Lester Uy and Niba Nirmal

Teacher: Ms. Couling

Objectives

Molar mass is useful to identifying an unknown substance; methods such equally using finding the freezing point depression of an unknown substance tin help in finding the identity. In this laboratory, nosotros will notice the tooth mass of an unknown substance that was combined with BHT using freezing bespeak depression. The given thermometer should be used to tape when the freezing points happen. We find the chiliad_fp and ΔT _fp of BTH and the unknown chemical compound to get the molar mass.

Concepts

Molality
Colligative Properties
Freezing Indicate Depression

Background

Knowing molar mass is a way to place a substance. There are several methods to discover information technology, but one way is using the formulas for colligative properties. These backdrop depend on the number of the solution present, not the blazon. While there are various colligative properties such equally the burning point, vapor pressure, and osmotic force per unit area, in this experiment freezing point depression is used to find the tooth mass of the unknown solute.

Equations:

*from laboratory PDF*

Materials

ii, half-dozen-Di-tert-butyl-4-methylphenol, BHT, 16 k
Cetyl alcohol, CH₃(CH₂)₁₄CH₂OH, 1 g
Unknown substance, 1 grand
Test tube, xviii x 150 mm
Beaker, 400-mL
Universal clamps, ii
Split rubber stopper with 1 pigsty
Timer, by seconds
Balance, 0.001 chiliad precision
Ring stand
Wire stirrer
Weighing paper or disc
Thermometer, preferably graduate to 0.1˚C
Hot plate or Bunsen burner
Ring clamp
Gauze with ceramic center

Take chances Assessment

*Think to always wash your easily before and after the experiment, wearable an apron and goggles, and gloves if bachelor.*

BHT

Actual proper name is half dozen-Di-tert-butyl-4-methylphenol. A white, crystal-similar cloth. Moderately toxic and irritant to skin. Avoid inhaling and ingesting in any way. Wipe with a newspaper towel or other absorbent objects if spilled.

Cetyl alcohol

A white, powdery textile in apartment shards.Flammable. Slightly toxic and a skin irritant. Avoid inhaling and ingesting in any way. Wipe with a paper towel or other absorbent objects if spilled.

Unknown substance

A white, fine, powdery material.Slightly toxic and a skin irritant. Avoid inhaling and ingesting in any mode. Wipe with a newspaper towel or other absorbent objects if spilled.

Water

This colorless liquid is not harmful. Contact is fine, but ingestion and inhalation are not recommended. Wipe with a paper towel or other absorbent objects if spilled. Nonflammable.

Procedure

We gathered and gear up all the materials according to the pictures in the lab PDF:

One time nosotros had everything, the materials were assembled similar the moving picture in a higher place with the stand up, heater, test tube and such. Nonetheless, we used an electronic thermometer continued to the laptop instead. This facilitates gathering the temperature information as we have the temperature per 2nd automatically recorded without doing likewise much transmission piece of work. We turned up the hot plate all the fashion up and waited for the temperature to become above 80˚C. While waiting, we stirred with the copper rod every bit the BHT began to cook and finally turn into liquid. Once nosotros got to fourscore˚C, nosotros pulled up the thermometer and test tube up the ring stand up so it wouldn't be in the beaker anymore. The heating pad was turned off. As the BHT cooled, we kept stirring it with the copper rod. Since class was almost over, we disconnected all electronics except the pad and put them abroad. We bundled the ring stand and accompanying components to the side for another 24-hour interval, with a warning to not impact it.

The next day, information technology turns out that nosotros misplaced the thermometer, so nosotros ended up breaking the test tube and starting all over once more. Information technology was too deep in the substance, so when we re-heated it afterward information technology had solidified, the test tube expanded until information technology broke. To go out the pieces of glass out safely, nosotros used tongs. The beaker was placed aside and replaced. The lab was prepare again, and we started from the beginning. This became our 2d trial, and the previous our failed first trial. After getting information for the BHT, the procedure higher up was repeated for two more times, one time with BHT plus cetyl alcohol and another with BHT plus the unknown.

Results

Data Observed

Masses

Object weighed	Trial 1*	Trial 2
Mass of empty test tube #i, g	nineteen.6585 g	nineteen.5425 m
Mass of test tube #one plus BHT, g	27.683 g	27.556 1000
Mass of BHT, one thousand	viii.045 g	8.0135 g
Mass of weighing newspaper, g		2.205 g
Mass of weighing newspaper plus cetyl alcohol, k		3.231 g
Mass of cetyl alcohol, g		1.0266 g
Mass of empty examination tube #2, g		19.119 one thousand
Mass of exam tube #2 plus BHT, g		27.1037 g
Mass of BHT, g		8.034 g
Mass of unknown		1.022 g

*Disregard Trial i data as this trial was messed upwards.

Temperatures

Freezing Point Table (data borrowed from Alma Chen)

Temperature in Celsius	BHT	BHT + cetyl alcohol	BHT + unknown
Freezing points, ˚C	67.ane˚C	57.8˚C	65.9˚C

BHT Graph

BHT + Cetyl Alcohol Graph

BHT + Unknown Graph

Data Calculated

	BHT	BHT + cetyl alcohol	BHT + unknown
ΔT_fp	-------------------	nine.thirty˚C	1.twenty˚C
Corrected Mass	0.00801 kg	--------------------	-------------------
m	-------------------	0.528 mol/kg	-------------------
m_fp	17.half dozen˚C/mol	--------------------	-------------------
Molar Mass	-------------------	--------------------	1833 g/mol

Post-lab Analysis

Our goal is to observe out the molar mass of the unknown. Knowing the equations in a higher place, and the data from our observations, we tin can solve for the molar mass of the unknown. Throughout the calculations, we must recall to convert to the units the equation requires if we oasis't done so too. To start, we look at our knowns and come across what we can calculate from there. From what we accept and then far, we can first discover the Δ for BHT + cetyl alcohol and BHT + unknown. Knowing our masses, we can notice the molality of the unknown. Nosotros plug these two values, the ΔT _fp value and molality, in order to solve for the tooth mass of the unknown.

Offset, we can see that the freezing points are already established. Therefore, nosotros can observe ΔT_fp for BHT + cetyl booze and BHT + unknown. This is because ΔT _fp is the change of the freezing point betwixt BHT and BHT + whichever substance. We use some unproblematic subtraction for this part.

State the knowns:

Freezing point of BHT: 67.1˚C

Freezing betoken of BHT+ cetyl booze: 57.8˚C

Freezing point of BHT + unknown: 65.9˚C

ΔT _fp for BHT + cetyl alcohol:

67.1˚C - 57.8˚C = 9.30˚C

ΔT _fp for BHT + unknown:

67.ane˚C - 65.9˚C = 1.twenty˚C

Having constitute the ΔT _fp , nosotros can move on to solving for the molality using the mass of cetyl booze, BHT, and the tooth mass of cetyl alcohol (calculated equally 242.43 thousand/mol). The formula for this is Equation 2 in the department "Equations", a rearranged form of the original molality equation (Equation ane).

Notice that all of the mass values are in grams. The units for the molality equation in the beginning of this page are not all grams. The equation is grams of solute over kilograms of solvent times the molar mass of the solute. The mass of BHT is in grams, and so we must change it into kilograms.

State the knowns:

BHT mass: 8.01 m

Using dimensional analysis:

8.01g

•

one kg

= 0.00801 kg

1 thousand g

State the knowns for Equation 2:

Mass of cetyl booze: 1.0266 g

Grams of BHT: 0.00801 kg

Molar mass of cetyl alcohol: 242.43 thousand/mol

Plug in:

thou = 1.0266 thou

(0.00801 kg) • (242.43 g/mol)

k = 0.528 mol/kg

Having found the molality, we take at present 2 knowns for Equation 3. We solve for k _fp :

Land the knowns:

ΔT_fp of cetyl alcohol: 9.30˚C

m: 0.528 mol/kg

Plug in:

ix.30˚C = g _fp 0.528

m_fp= 17.6˚C/mol

Now, nosotros know all necessary values to plug into Equation 4 and finally solve for the molar mass of the unknown substance.

Land the knowns:

k_fp:17.6˚C/mol

Mass of cetyl alcohol:i.0266 g

Mass of BHT: 0.00801 kg

ΔT_fp of unknown: one.20˚C

Plug in:

mm = 17.6˚C/mol • 1.0266g

0.00801 kg • 1.xx˚C

mm = 1833 g/mol

Conclusion

Later my calculations, I take arrived at the value of 1833.g/mol for the unknown substance'due south molar mass. This can be used to help identifying what exactly the unknown substance is. Looking at the graphs, all three substances show signs of supercooling, as the follow pattern (from lab PDF) appears in the graphs:

In our experiment, it was advantageous for u.s. to choose a solver that has a high value of k_sp. High values of m _sp mean that the values ofΔT_fpare farther apart, then our calculations are made more manageable. A low k _sp would hateful values ofΔT_fp that are very close in value, and information technology would be possible to accept them and then shut together that the electronic thermometer would show them as the aforementioned temperature to the nearest tenth. When trying to get a difference, nosotros and then become zero and and so be stuck in the postal service-lab analysis.

When analyzing the graphs, information technology tin can be seen that at points in which the BHT graph is more or less horizontal, the other ii graphs which comprise the cetyl alcohol and unknown substance are still sloping down. This is because the two solutions have lower freezing points than the puresolvent, as the tabular array above shows. So while the BHT gets to tis freezing point and the temperature levels out, the two solutions nonetheless need to cool down to reach their freezing points, and so the graph continues to go downwardly as they freeze. This happens at most 67-70˚C.

Evaluation

Sources of fault in our functioning in this experiment include weighing and forgetfulness.

Weighing is an mistake considering the scale kept going up 0.001, so measurements in the hundredths area may exist off, slightly affecting the results into a bit more than higher value. Because of the imprecision of the weighing, all masses are the least precise measurements.

Our performance is as well a source of error. The datakeeper never wrote downward at what temperature crystals form, which resulted in me having to infringe the values from someone else. Their values may not exist consistent with the way our amounts of BHT and cetyl alcohol/unknown substance reacted, which would result in different temperatures for when crystals grade. Their measurements may have also been affected by their sources of error.

There is the possibility of various sources of mistake, which did not occur in our lab, simply is withal dainty to know about anyways for futurity labs. Hither several scenarios are explained:

The thermometer read ane.4˚C also high.

A thermometer that reads likewise high would not count as a source of error in this experiment. As it tin be seen to a higher place, what was calculated was the deviation between the temperature of the BHT and the BHT + whatever substance. As long as all temperatures are increased past i.4˚C, the subtraction will result in the aforementioned difference.

Some of the solvent was spilled before the solute was added.

In other words, BHT is spilled before the solute, either the cetyl alcohol or unknown, was added. The lower amount of BHT would lower the results of all calculations done subsequently, so this is a definite source of fault. When computing the molar mass, the resulting lower values from the lower molarity would outcome in a lower molar mass than it should be.

Some of the solute was spilled afterward information technology was weighed and added to the solvent.

In other words, cetyl alcohol or the unknown substance is spilled subsequently information technology was weighed and added to BHT. This also tampers with results and is so a source of error. Also, this case happens to be the contrary of the event described above. Instead of the substance lowering values, it increasesvalues. The terminal molar mass of the unknown substance ends up beingness higher than it should exist.

Some of the solution was spilled subsequently the solute and solvent were mixed simply before the freezing point was determined.

In other words, after the BHT and cetyl booze or unknown substance were mixed, the combined substance spills. Because the solute and solvent spill in equal ratios, the molarity of the substance remains the same. The amount does not bear on the experiment in this instance, as long as the ratios of solvent to solute are kept the aforementioned. This is not a source of error as information technology does not bear upon the results.