QUESTION: How do you determine the molar mass of the unknown, nonelectrolyte compound for each trial?
This is for a lab. My data is:
freezing point of solvent (water): -0.58
freezing point of unknown: -0.93
Mass of solvent (water): 19 ml
Mass of unknown: 1.045g
I know the formula to use is Tf=Kfm
So plugging that in, I got:
0.34=1.86*m
m=0.183
0.183=moles of solute/.019 kg
moles of solute = 3.477e-3
molar mass = 1.045g/3.477e-3
molar mass = 300.55
But I don't think this fits any of the possible compounds. Those being: Urea (60 g/mol), 2-deoxy-D-ribose (134 g/mol), D-ribose (150 g/mol), glucose (180 g/mol), sucrose (342 g/mol), alanine (89 g/mol), glycine 75 g/mol), or phenylalanine (165 g/mol).
I guess it could be sucrose? I don't think the data is bad, because the graph did the supercooling thing, and I did two trials of freezing the water. So the data should be right.
Can you see anything that I might have done wrong in my calculations?
First, the formula to use is delta T = Kf*m and delta T is 0.35 (not 0.34) Follow that through and you get about 292 for the experimental molar mass. None of the answers match. I think the freezing point of pure water at -0.58 C sounds fishy (it should be zero) BUI thermometers CAN be off by that much, I guess. After all, it's the DIFFEREMCE that counts and that difference is 0.35. I don't see anything wrong with your calculations; it has to be the data. One thing to remember is that this method of determining the molar mass is not and EXACT procedure. It can be off by 50% or so depending upon a lot of things. For non-volatile compounds,however, it is better than that but not error free. I would try decreasing/increasing those those molar mass of the compounds listed by +/- 10% and see if that gets close. For example 10% of 342 for sucrose is 342-34 = about308. So I might go with sucrose, also, especially when we consider that the 1.86 is an average of all compounds + you know there is the possibility of experimental error.
Based on the information provided, it appears that you have correctly calculated the molar mass of the unknown compound to be 300.55 g/mol. However, you are concerned that this value does not match any of the possible compounds provided (urea, 2-deoxy-D-ribose, D-ribose, glucose, sucrose, alanine, glycine, or phenylalanine).
To determine if there was an error in your calculations, let's review the steps:
1. Calculate the molality (m):
Tf = Kf * m
0.34 = 1.86 * m
m = 0.183 mol/kg
2. Calculate the moles of solute:
moles of solute = m * mass of solvent (kg)
moles of solute = 0.183 * 0.019 kg
moles of solute = 0.003477 mol
3. Calculate the molar mass:
molar mass = mass of solute / moles of solute
molar mass = 1.045 g / 0.003477 mol
molar mass = 300.55 g/mol
Based on these calculations, it seems that there might be some discrepancy between the expected molar masses of the potential compounds and the calculated molar mass of 300.55 g/mol.
This discrepancy could be due to experimental error, such as inaccurate measurements or contamination of the unknown sample. It is also possible that the unknown compound is not one of the compounds provided in the list.
To further investigate, you could review your experimental procedure, double-check your measurements, and consider conducting additional trials to validate the results. Additionally, you could consult with your instructor or classmates to see if they encountered similar issues or have any insights.
Ultimately, without more information or further experimentation, it may be difficult to determine the exact identity of the unknown compound.
To determine the molar mass of an unknown compound in this lab experiment, you are using the freezing point depression method. Here are the steps you followed:
1. You started with the freezing point formula: Tf = Kf * m
Tf is the freezing point depression, Kf is the molal freezing point depression constant (given), and m is the molality of the solution.
2. You substituted the given values: Tf = -0.93 - (-0.58) = -0.34 (since Tf is negative for the freezing point depression).
3. You rearranged the formula to solve for m: 0.34 = 1.86 * m.
4. You found the molality of the solute: m = 0.183 from the above equation.
5. To find the moles of solute, you divided the mass of the solute (1.045 g) by the molar mass in kg/mol: 0.183 = moles of solute / 0.019 kg.
6. You solved for the moles of solute: moles of solute = 3.477 x 10^-3.
7. Finally, you calculated the molar mass of the unknown compound by dividing the mass of the solute (1.045 g) by the moles of solute (3.477 x 10^-3): molar mass = 1.045 g / 3.477 x 10^-3 = 300.55 g/mol.
Now, let's determine if there might be any mistakes in the calculations:
1. It seems like you followed the correct steps in the calculations.
2. The molar mass you calculated (300.55 g/mol) does not match any of the given possible compounds. It is higher than the molar masses of all the listed compounds.
3. While the data appears to be accurate, it is possible that there was an error or inconsistency in the measurements or calculations.
Since the calculated molar mass does not match any of the listed compounds, review your experimental procedure, measurements, and calculations for any errors or inconsistencies. Double-check that the correct values and formulas were used at each step. It is also worth considering other possibilities or factors that might influence the deviation from the expected molar masses, such as impurities in the unknown compound or experimental limitations.