LAB VISIT 5: CRYOSCOPY

CRYOSCOPY(freezing point)

Objective:

With this experiment our aim is obtaining a graph in which we will show the variation of the melting point of an aqueous solution as a function of its concentration and, also, calculating the molecular mass (mm) of one of the solute from the results that we will obtain.

Background information:

For understanding and developing this experiment we may know that the freezing point of a solution is without exception lower than the one of the pure solvent, with the proportional drop to the molality of the solution:

ATf = Kf * m

Solvent = 5 grams of water.

Materials:

• Distilled water (H20)
• Sugar (C12H22O11)
• Test tubes
• A digital thermometer
• Ice
• Salt (NaCl)
• Balance
• 250 mL beaker

Table:

Grams of sugar	Tf (ºC)	ATf (ºC)	Molality (mol/kg)
0.0	0.6	-----------------------	0.0
0.5	0.3	0.3	0.29
1.0	-1.9	2.5	0.58
1.5	-2.5	3.1	0.87
2.0	-4.5	5.1	1.16
2.5	-6.8	7.4	1.46

Calculations (molecular mass, moles and molality):

Sucrose : C12 H22 O11

Carbon: 12  Hydrogen: 1 Oxygen: 16

Molecular Mass=  12 (12) + 1 (22) + 16 (11) = 144 + 22 + 176 = 342 g/mol

Moles: mass/molecular mass

Molality = moles / kg of solvent

0.5 grams: (0.5 : 342)/ 0.005 = 0.001/ 0.005 = 0.29

1.0 grams: (1.0 : 342)/ 0.005 = 0.002/ 0.005 = 0.58

1.5 grams: (1.5 : 342)/ 0.005 = 0.004/ 0.005 = 0.87

2.0 grams: (2.0 : 342)/ 0.005 = 0.005/ 0.005 = 1.16

2.5 grams: (2.5 : 342)/ 0.005 = 0.007/ 0.005 = 1.46

Graphs:

Conclusion:

As we can see on the table, as the amount of sugar (in grams) increase, the freezing point is colder, so you need a colder temperature to freeze the solution. As we lower the temperature, the molecules have less energy to move. Adding sugar makes it harder to slow the molecules down, so the temperature has to be lower than pure water to freeze. Adding more sugar it takes more energy out. The sugar slow down the molecules and the temperature is lower than water. This way it makes more energy going out. We can see that in the first graph the results of the molality is increasing, when mass and molality increase relatively while in the second graph we can see how mass increase and the freezing point decrease. The second one is inversely proportional. Molality is higher because its related with the sugar, not with the temperature. In the molality column, we can see that as the amount of sugar increases, the molality increases, because in the formula the number of moles changed depending on the amount of sugar, that in this case was decreasing. In the other graph, its inversely proportional because its the relation of the grams of sugar and the freezing point, making it descent because as we first said, the sugar makes the freezing point be lower.

Lab session 3-2-2014

LAB SESSION 3-2-2014
PRECISION MEASUREMENTS MASS VS. VOLUME

Materials:
- Pipette
- Measuring Cylinder
- Test Tubes
- Test tube holder
- Scale
- Beaker
- Spatula
- NaCl
- Water
- Cyclohexane

PROCEDURE:

What happened? Why?
A) 10,0 mL of H2O have a mass of 10,0 grams.
B) 2,5 grams of NaCl have a volume of 1,0 - 1,5 mL.
C) 2,5 grams of NaCl in 10 mL of water shows a volume of 11,0 mL.

A) EXPERIMENT 1: 10,0 mL of H2O have a mass of 10,0 grams.

1) First, we measured the wieght of the empty measuring cylinder, which was 76,3
2) Then, we pour 10 mL of water with the pipette in the mesauring cylinder.
3) After that, we weight the measuring cylinder with the water in it.
4) Finally, we calculated the difference between the mass of the measuring cylinder with water and the mass of the empty measuring cylinder.

In our case, first we did it but the difference was 6,9 grams. The mass of the measuring cylinder with water in it was 83,2 grams and the measuring cylinder without water weight 76,3 grams. So the difference, that as firstly said was 6,9 grams was not correct because logically it needed to be 10 grams. So we had to do it again and on the second time, the mass without water was of 76,5 grams. The mass of the measuring cylinder with water was of 86,4 grams. This time, the difference was of 9,9 grams.

First time : 83, 2 - 76, 3 = 6, 9
Second time : 86, 4 - 76, 5 = 6, 9

EXPERIMENT 2:

Then, we did the same experiment but instead of water we used salt (NaCl)
1) This time our measuring cylinder weight 70, 8 grams.
2) Then we put 2,5 grams of NaCl in the measuring cylinder.
3) After that, we measured the mass of the measuring cylinder with the salt (NaCl) and weighted 73,4 grams.
4) Finally we calculated the difference (73,4 - 2,5) that was 2,6 grams, so that meant that the experiment was well done because the difference was very approximated to the grams of salt that we put in the measuring cylinder.

73, 4 - 70, 8 = 2, 6

B) 2,5 grams of NaCl have a volume of 1,0 - 1,5 mL.
In this experiment we used NaCl mixed with cylohexane (C6H12)
1) First, we took 2,5 grams of NaCl (measuring it first on the balance) and pour it on the measuring cylinder that weight 70,8 grams
2) Then, we pour 3,0 mL of cyclohexane (C6H12), measuring it first with the measuring pipette.
3) Finally, we calculated the final volume that was 3,0 mL.

In this experiment, the cyclohexane and the salt couldn't dissolve.

C) 2,5 grams of NaCl in 10 mL of water shows a volume of 11,0 mL.
In this third experiment also used NaCl mixed with water.
1) We put 2,5 grams of NaCl in the measuring cylinder.
2) Pour 10,0 mL of H2O.
3) The final volume was of 12,5 mL
4) The total mass was of 75, 6 grams.
In this experiment, the water and the salt could dissolve.


CONCLUSIONS:
In our results, we could prove that water could dissolve with sodium chloride (NaCl) but cyclohexane couldn't dissolve with NaCl. Water could dissolve with the salt (NaCl) because they have a positive and negative, so they are both polar substances, so they are ionic compounds. Polar substances attract each other, so in this case, they are easier to dissolve. On the other side, we could prove that hexane could not dissolve with NaCl because they are not polar substances, so they are not negative and positive charges. NaCl and cyclohexane are covalent bonds so its hard to dissolve them.
In this experiment, we have seen different changes in the volume; sometimes the volume stayed the same, however we had difficulties to prove that it had to be the same, for example in water. In the second experiment, the volume of the cyclohexane is 3,0 mL. When we mixed water with NaCl the final volume was of 12,5 because the amount of water, that first was 10,0 mL had dissolved with NaCl that wa 2,5 grams (the same as 2,5 mL) so the total volume was of 12,5. In the other case, cyclohexane and NaCl could not dissolve so it just rested the amount of cyclohexane, that was of 3,0 mL.

In this experiment, the measurements are not very accurate so this is because we could have had some problems during the experiment, such as not calculate well the volumes, or not do it very precisely so some measures were not very correct, as we had proved in the first experiment. However, they are realistic values and we proved them. To improve it we could do it more precisely with more time, concentration and ability. We could also impriove it by taking more pictures, that can complete our lab report.

Seeing the characteristics of Butyl acetate

Here is the bottle of Butyl acetate

Butyl acetate has the following formula: C6H12O6

It´s atomic mass therefore is the following: C= 12 H=1 O=16

Molecular mass= 72+12+96=180g/mol (A mol is 6.02x10^23 particles)

Here is it´s atomic model:

 Blue: Oxygen
White: Hydrogen
Black: Carbon

Pressure and Temperature relation

In this experiment we have seen Gay-Lussac´s law comparing pressure at different temperatures. As we may recall pressure and volume are proportionally related.

Initial pressure/Initial temperature=Final pressure/Final temperature

We did the experiment with logger pro and took the result of the pressure

Results with logger pro (KiloPascals)

15º C: 6,3 kPa
20ºC : 10,1 kPa
25ºC: 15,2 kPa
30ºC: 21.7 kPa

Table:

Temperature (ºC)	Pressure (KPa)
15.0	6.3
20.0	10.1
25.0	15.2
30.0	21.7

Graph:

CONCLUSION
As we can see in our results, as the temperature increases, the pressure also increases. Gay Lussac's law states that when there is more temperature, higher is the pressure because the particles move faster. This is because the temperature give energy to the partices, so when the temperature is higher, it gives ore energy to the partciles to move faster, causing more pressure.

Still, this results are not very accurate since when we took of the vacum pressure started increasing cause air was entering due to the fact that our equipment is not perfect. To make sure the changes in pressure were not only for air entering after doing all 4 temperatures we put it back in 15ºC and it dropped down to 10,5 kPa which shows that Temperature and pressure are directly related.
Here are some images from the experiment:

 Here we are checking pressure at 30ºC with Maria´s bath. Data are being taken to the Logger Pro program

This is the material we used to take out the air so our initial pressure was 0 torrents

VOLUME AND PRESSURE EXPERIMENT

VOLUME AND PRESSURE EXPERIMENT

X (volume:mL)	Y (Pressure: hPa)	Inverse pressure (hPa)
20	625	0,0016
25	375	0,002666667
30	100	0,01
35	-50	-0,02
40	-200	-0,005
45	-275	-0,003636364
50	-375	-0,002666667
55	-425	-0,002352941

CONCLUSIONS:

Watching our graphs and table we can conclude that the volume is inversely proportional to the pressure prooving Boyle-Mariotte´s law. Therefore, when the volume increases pressure decreases, and when pressure increase volume decrease.
With this information we can also understand how the pressure in gases may vary.
We conclude stating that both graphs represent, as we have said before, Boyle-Mariotte´s law which exemplify  how the pressure of a gas lays ot to decreasing as it volume increases.
The mathematical expression for this law is:

or

  P(pressure), V (volume), k (constant)

We may use this equation when we compare the selfsame but under different situations so therefore we can observe tha changes made upon it. Finally, with this we can conclude that this last equation is used during this experiment for watching the changes upon our gas.

Lab sesion 2: Comparing characteristics of different substances 4-10-2013

In this photograph we are testing the solubility of a substance in water. 

In this image we are testing if magnesium will melt after a minute being heated with a bunsen burner. In the end the result was that it did not melt so it´s melting point is high.

How to secure a Schlenk tube

In order to prepare a Schlenk tube, we need the elastic band to secure the tube. To do this, we put the elastic band in the way we see it in the video.