CHEMISTRY PLEASE HELP?!!!Determine the molar mass of a compound - given freezing point and masses of components in a solution?

Question

A company is studying the use of a compound as a potential antifreeze. When 0.243 g of the compound is dissolved in 25.0 g of water, the freezing point of the solution is -0.201 * Celsius.

 

a. Determine the molar mass of the compound.

b. If the empirical formula of the compound is C2H5O, determine the molecular formula of the compound.

 

 

I am really stuck and would greatly appreciate some help!! I have been working on problems similar to this, but this is the first time I have been asked to find the molar mass from the given data. Perhaps if I could analyze your steps and explanation, I will be able to learn how to solve problems like this. Your time and help is greatly appreciated!!!!

Answer 1

To solve this you need something called the Kf of water, which is 1.86 degree Celsius per molal. Molality is the number of moles solute in kg solvent. Multiply Kf by m  and you get the change in freezing point!

 

On this case it's useful to know that the freezing point of pure water is zero degree Celsius so the change in freezing point is -.201. We have 25 grams or .025 kg solvent. That allows us to find the moles solute. We know how many grams solute we have so divide that by the moles solute to get the molar mass. 

 

To get the molecular formula of the compound, find the molar mass of the empirical formula and divide the molar mass of the compound by that of the empirical. It should be a nice round number. Then scale the ratios of the empirical formula accordingly. For example if it was six, the compound had the formula C12H30O6.  Hope that helps!