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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