Need help with Chemistry please....
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Need help with Chemistry please....

[From: ] [author: ] [Date: 12-05-27] [Hit: ]
5g of naphthalene and found it to be 84°C. After adding 3.1g of an unknown substance to the naphthalene, the freezing point was found to be 76.9°C. What is the molar mass of the unknown substance?......
I don't understand how to solve it so can you help me how to solve it please if you don't mind? I need help with only one question.

Here question:

In an experiment a student measured the freezing point of 20.5g of naphthalene and found it to be 84°C. After adding 3.1g of an unknown substance to the naphthalene, the freezing point was found to be 76.9°C. What is the molar mass of the unknown substance? (Kf for naphthalene is 6.8°C Kg/mol)

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delta(T) = iKfm (Since naphthalene is nonpolar, the van't Hoff factor, i, is just 1)
m = delta(T)/(Kf)
delta(T) = 84 - 76.9 = 7.1
m = 7.1/6.8 = 1.044 mol X/kg naphthalene

(3.1 g X)(1 kg naphthalene/1.044 mol X)(1000 g naphthalene/1 kg naphthalene)(1/20.5 g naphthalene) = 140 g/mol X (to correct sig figs)
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