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일반화학/[13장] 용액의 성질

128.2 g mol^-1 5.0% w/w 77.3℃ 80.2℃ Kf = 6.9℃ m^-1

by 영원파란 2022. 10. 1.
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128.2 g mol^-1 5.0% w/w 77.3℃ 80.2℃ Kf = 6.9℃ m^-1

 

 

A mixture is prepared that is 5.0% (w/w) of an unknown substance

mixed with naphthalene (molar mass = 128.2 g mol^-1).

The freezing point of this mixture is found to be 77.3℃.

What is the molar mass of the unknown substance?

The melting point of naphthalene is 80.2℃ and Kf = 6.9℃ m^-1.

 

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어는점내림, ΔTf

ΔTf = Kf × m

( 참고 https://ywpop.tistory.com/1920 )

 

 

 

m = ΔTf / Kf

= (80.2 – 77.3) / 6.9

= 0.4203 m

 

 

 

5.0% w/w = (5.0 g 용질 / 100 g 용액) × 100

---> 5.0 g 용질 + 95 g 용매(나프탈렌)

 

 

 

n = W / M 이고,

( 참고 https://ywpop.tistory.com/7738 )

 

몰랄농도 = 용질 mol수 / 용매 kg수 이므로,

 

0.4203 mol/kg = (5.0 g / M) / (95/1000 kg)

 

5.0 / M = 0.4203 × (95/1000)

 

M = 5.0 / [0.4203 × (95/1000)]

= 125.22 g/mol

 

 

 

답: 125.2 g mol^-1

 

 

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