I need to calculate the molecular mass of ehtylene glycol based on the freezing point depression.
The teacher said that for glycol i=1 and to use the equation
m=delta T / Kf X i
Kf = -1.86C/m
so I did -2.5C / -1.86 X 1 = 1.344
I know that 1.344 can't be the weight for glycol but im not sure what to do now? Should i multiply by 1000?
Chemistry(Please help) - DrBob222, Tuesday, February 21, 2012 at 1:32am
1.344 is the molality.
Then molality = mols/kg solvent
Plug in kg solvent and m and solve for moles
Then mols = g/molar mass.
Rearrange to molar mass = g/mols. You know how much (grams) you used and you know mols, solve for molar mass.
Chemistry(Please help) - Hannah, Tuesday, February 21, 2012 at 12:47pm
the only info that I have is the mass of solute(g) and mass of solvent(kg) for glycol so im confused on what to use for mols?
Chemistry(Please help) - Hannah, Tuesday, February 21, 2012 at 12:54pm
Disregard question I understand now. I have to solve for mols.
Chemistry(Please help) - Hannah, Tuesday, February 21, 2012 at 12:59pm
so for molality I got 1.344 then I did
1.344 = mols/0.10kg = 0.1344 Im not sure if I did this step right
Then I did mm= g/mol
6.200 was the mass of the solute so
mm=6.200 / 0.1344 = 46.13.
I guess I made a mistake somewhere in my calculations because I thought that the mm of eythlene was around 62.
To calculate the molecular mass of ethylene glycol based on freezing point depression, you can follow the steps explained by DrBob222 in the conversation you shared.
1. Start with the given freezing point depression, which is -2.5°C.
2. Use the formula m = delta T / Kf x i, where delta T is the freezing point depression (-2.5°C), Kf is the freezing point depression constant (-1.86°C/m for ethylene glycol), and i is the van't Hoff factor (1 for glycol).
- (-2.5°C) / (-1.86°C/m) x 1 = 1.344 molality
Note: The molality (m) represents the number of moles of solute per kilogram of solvent.
3. Now, convert the molality to moles by multiplying it by the mass of the solvent in kilograms.
- 1.344 molality x 0.10 kg = 0.1344 moles
4. Finally, divide the mass of the solute (given as 6.200 grams) by the number of moles to calculate the molecular mass (mm) of ethylene glycol.
- mm = 6.200 g / 0.1344 moles ≈ 46.13 g/mol
According to your calculations, you obtained a molecular mass of approximately 46.13 g/mol. However, you expected the molecular mass of ethylene glycol to be around 62 g/mol. It's possible that a mistake was made in your calculations or that there was some discrepancy in the experimental data. I would recommend double-checking your calculations and confirming the accuracy of the given values to ensure the correct result.