Q. 65.0( 1 Vote )
Calculate the freezing point of a solution containing 8.1 g of HBr in 100 g of water, assuming the acid to be 90 % ionized. [Given: Molar mass Br = 80 g/mol, Kf water = 1.86 K kg / mol]
Calculate the molality of ethanol solution in which the mole fraction of water is 0.88.
Mass of HBr given = 8. 1g
Mass of water = 100g
Percentage of ionization of acid = 90% so, α = 0. 90
Molar mass of HBr = 81 g
Kf = 1. 86 k kg/mol
As we know:
HBr dissociates to:
HBrH+ + Br-
To find the Van’t Hoff factor (i):
i = 1 +
The formula to find the
Substitute i, Kf,m in the equationAlso,
i = (1+α)
α = 0.9
= (1 +
= (1 +
= 3. 53
T0f- = 0
Tf = -3.534
Freezing point of the given solution is -3.534 °C
The mole fraction of water = 0. 88
Therefore, the mole fraction of ethanol will be = 1 - 0. 88 = 0. 12
The formula to find the mole fraction of a substance is =
0. 12 = (for ethanol) ……… (1)
0. 88 = (for water)………… (2)
Divide equation two by equation 1
7. 333 =
No. of moles of water in 1000g of water = = 55. 55
Therefore no. of moles of ethanol = = 7. 575
So, molality = 7. 575 mol/kg.
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