7.5 grams of a substance is dissolved in 75grams of water, the depression in freezing point is 1.2 degree celcius .what is the molar mass of the substance?
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12th
Chemistry
Solutions
Colligative Properties and Determination of Molar Mass
15.0 g of an unknown molecu...
CHEMISTRY
15.0 g of an unknown molecular material was dissolved in 450 g of water. The resulting solution was found to freeze at −0.34
o
C. What is the molar mass of this material? (K
f
for water=1.86 Kkg mol
−1
)
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ANSWER
(b) Given mass of solute =w
b
=15.0g
Molar mass of solute =M
b
=?
Mass of water =w
a
=450g
Freezing point of water =0
A
^
o
C=273K
Freezing point of solution =−0.34
A
^
o
K
=(−0.34+273)K
Depression in freezing point =∈"T
f
=273−(−0.34+273)=0.34K
K
f
for water = 1.86 K Kg mol
−1
Since, ΔT
f
=K
f
×m=K
f
M
b
×w
a
w
b
×1000
M
b
=
w
a
ΔT
f
K
f
×w
b
×1000
M
b
=
450×0.34
1.86×15×1000
=182.35g/mol