# What is the molar mass of the unknown?

## Homework Statement

A 1.40 g sample of an unknown is dissolved in 8.10 g of lauric acid. The freezing point is depressed 1.23 oC. What is the molar mass of the unknown?

Delta T = Kf * m

## The Attempt at a Solution

Delta T = Kf * m
=> m = 1.23 / 3.9 = 0.32

mole solute = 0.32 * 0.0081 = 0.0026 moles.

=> Molar mass = 1.40 / 0.0026 = 540 g/mol = final answer.

Am I doing correct? Please let me know!

Borek
Mentor

1. Don't round down intermediate results (check that it changes molar mass by about 8g).
2. Don't ignore units in your calculations, they are much easier to follow when it is obvious what is what of what.

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1. Don't round down intermediate results (check that it changes molar mass by about 8g).
2. Don't ignore units in your calculations, they are much easier to follow when it is obvious what is what of what.

--

Thanks so much for your response!

I have edited my answer to:

Delta T = Kf * m => m = 1.23 / 3.9 = 0.32 m

mole solute = 0.32 * 0.0081 = 0.0026 mole

Molar mass of the unknown = 1.40 / 0.0026 = 538.46 g/mol .

Hope it better ;)

Borek
Mentor
This time you overdid - you don't have enough information to give 5 significant digits in the answer But you see that rounding errors were responsible for 538 being 530.

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