Hey guys I did a lab and I have to answer the questions relating to the lab. I did question can someone please check if these are accurate
1. Calculation to Determine the molecular weight of unknown substance
Mass of unknown used: 2.0232 g
Mass of water used: 50.0 g (0.05kg)
Kf = 1.86°C kg/mole
Experimentally determined freezing point of unknown: -0.5°C
Initial Temperature = 9.20 C (had made a mistake)
Delta T= (-0.5) - 9.20
= -9.70
Solution = delta Tf/-Kf
= -9.70°C/-1.86°C kg/mole
= 5.2515 m (molality)
Molar mass = 2.0232g / 0.05 kg * 5.215 m
= 7.76 g/mol
I'm not sure how to do part B
2. Calculations to determine i for KCl solution:
GIVEN:
Delta t= t2-t1
= 0.00 -23.90
experimentally determined freezing point= 0.00
Molality KCl= 0.1
Kf= 1.86
The part
Solution=deltaTf/Kf
what is this? What is deltaTf? What was the initial freezing point of the solvent? If it was water, it was 0C. So deltatTf would be .5C
So that makes no sense to me.
So its
deltaTf = Kf * m
m= 9.70°C/-1.86°C kg/mole
= 5.2515 m (molality)
What i'm not sure about is, if
Delta Tf= in this equain is tfinal-t initial
or its the freezing point as ypu mentioned .5C
Delta tf is the CHANGE in freezing point. The initial freezing point was not 9C. Water freezes at 0C, and then when you add a solute, the freezing point is depressed some.
Let's break down your calculations and check their accuracy.
For question 1, you are correct in using the formula ΔT = (freezing point of solution) - (initial temperature).
ΔT = (-0.5°C) - (9.20°C)
ΔT = -9.70°C
Next, you correctly used the formula for molality (m):
m = ΔT/(-Kf)
m = -9.70°C/(-1.86°C kg/mole)
m = 5.215 m (molality)
Now, for the calculation of the molar mass, you used the formula:
Molar mass = (mass of unknown substance) / (mass of water used * molality)
Molar mass = 2.0232g / (0.05kg * 5.215m)
Molar mass = 7.76 g/mol
Your calculations for question 1 are accurate.
Moving on to question 2, you need to determine the van't Hoff factor (i) for a KCl solution. The formula for ΔT is the same as in question 1:
ΔT = (freezing point of solution) - (initial temperature)
In this case, it looks like the initial temperature was erroneously provided as 0.00°C. If the freezing point of the solution is also 0.00°C, then the change in temperature (ΔT) will be 0.00 - (-23.90) = 23.90°C.
Next, you have the molality of the KCl solution (m) and the Kf (freezing point depression constant) provided as 0.1 and 1.86, respectively. However, the formula you need to use to calculate the van't Hoff factor (i) is:
i = ΔT/(-Kf * m)
Using the values given:
i = 23.90°C / (-1.86°C kg/mole * 0.1m)
i = -128.49
It seems there might be a mistake in the given values or the formulas used for question 2, as a negative van't Hoff factor doesn't make physical sense.
Please double-check the provided information and formulas for question 2 to ensure accuracy.