Homework Help: chem

Posted by tomi on Friday, November 7, 2008 at 3:27pm.

a compound containing only boron, nitrogen and hydrogen was found to be 40.3% B, 52.2%N, and 7.5% H by mass. If 3.301g of this compound is dissolved in 50.00g of benzene, the solution produced freezes at 1.30 degrees celsius. If Kf(freezing point depression constant) for benzene is 5.12 degrees celsius per metre and the freezing point of benzene is 5.48 degrees celsius, what is the molecular weight of this compound?

chem - DrBob222, Friday, November 7, 2008 at 4:24pm
You can use the percents to determine the empirical formula.
Take 100 g sample which will give you
40.3 g C, 52.2 g N and 7.5 g H. Now divide each by the atomic mass of C, N, and H respectively, find the ratio in small whole numbers and you have the empirical formula.
Now use delta T = kf*molality to determine molality. Finally, knowing molality = # mols/kg solvent, you can determine the molar mass and from that the molecular formula and from that the molar mass.
Post your work if you get stuck.

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