Question

So I've seen a bunch of these questions recently so I put together a simple step by step example for freezing point depression let me know what you think!!

Answer 1

Question: 31.65 g of sodium chloride is added to 220.0 mL of water at 34 °C. How will this affect the freezing point of the water?

Assume the sodium chloride completely dissociates in the water.
Given: density of water at 35 °C = 0.994 g/mL
Kf water = 1.86 °C kg/mol

Answer 2

Solution:
To find the temperature change elevation of a solvent by a solute, use the equation:

ΔT = iKfm

where
ΔT = Change in temperature in °C
i = van 't Hoff dissociation factor
Kf = molal freezing point depression constant or cryoscopic constant in °C kg/mol
m = molality of the solute in mol solute/kg solvent.

Answer 3

Step 1: Calculate the molality of the NaCl

molality (m) of NaCl = moles of NaCl/kg water

From the periodic table
atomic mass Na = 22.99
atomic mass Cl = 35.45
moles of NaCl = 31.65 g x 1 mol/(22.99 + 35.45)
moles of NaCl = 31.65 g x 1 mol/58.44 g
moles of NaCl = 0.542 mol

kg water = density x volume
kg water = 0.994 g/mL x 220 mL x 1 kg/1000 g
kg water = 0.219 kg

mNaCl = moles of NaCl/kg water
mNaCl = 0.542 mol/0.219 kg
mNaCl = 2.477 mol/kg

Answer 4

Step 2: Determine the van 't Hoff factor

The van 't Hoff factor, i, is a constant associated with the amount of dissociation of the solute in the solvent. For substances which do not dissociate in water, such as sugar, i = 1. For solutes that completely dissociate into two ions, i = 2. For this example NaCl completely dissociates into the two ions, Na+ and Cl-. Another example could be CaCl2 which dissociates into three ions, Ca2+ and 2Cl-.
But, i = 2 for this example.

Answer 5

Step 3: Find ΔT

ΔT = iKfm

ΔT = 2 x 1.86 °C kg/mol x 2.477 mol/kg
ΔT = 9.21 °C

Answer 6

So the final answer is:
Adding 31.65 g of NaCl to 220.0 mL of water will lower the freezing point by 9.21 °C.

Answer 7

The normal freezing point of water is 0°C with the addition of NaCl the freezing point for this solution would be -9.21 °C.

Answer 8

nice job tara

Answer 9

Nice one... @taramgrant0543664

Answer 10

one other thing: remember that delta T is the temperature change not the final temperature that the substance was raised or lowered to. so if they ask for the final temperature you'll need to do some rearranging:

\[\Delta T = (Tf-Ti) \]

to get
\[\Delta T + Ti = Tf \]

Answer 11

Awesome work @taramgrant0543664

Answer 12

Thanks everyone! And yes @Photon336 thanks for pointing that out!

Answer 13

There should be a way for us to find these tutorials in the future though

Answer 14

There's one of the mods that runs a tutorial site thing on behalf of openstudy or in conjunction with or something along the lines I know that I can't remember which one it is though

Answer 15

@taramgrant0543664 do you know that mod's name? ah.. okay

Answer 16

ok.. thanks

Answer 17

Good one @taramgrant0543664 ... And yeah I've given a tutorial on genetics 2 days back as well.. Please check it out..:)

Answer 18

You did @shabbaranks genetics is my favourite bio course! I must have missed it normally I don't miss too much but ill definitely check it out!

Answer 19

after a while you cant bump up the tutorial to the top; i'm guessing you can just paste the link to your profile description?

Answer 20

good job :)

Answer 21

Brilliant work @taramgrant0543664 ! :D