What is the freezing point of a solution that contains 36.0 g of glucose in 500.0 g of water (Kf for water is 1.86C/m. The molar
mass of glucose is 180.0 g per mole.) A 0.755
B -0.744
C 1.49
D-1.49
What is the molality of a solution of water and KCl if the boiling point of the solution is 103.07 degrees celcius. (Kb for water =0.512 C/m, KCl is an ionic compound)
A 0.300
B 0.600
C 3.00
D 6.00
PLEASEEE help! I have been having so much trouble with this. Have no idea where to even start!
1) The depression of the freezing point is a colligative property.
For non ionic compounds it may be calculated with the formula:
ΔTf = Kf * m
Where Kf is the cryoscopic constant of the water = 1.86°C/mol
And m is the molality, m = number of moles of solute / kg of solvent
You have 500.0 g of solvent = 0.5000 kg
And you can calculate the moles in 36.0 g of glucosa dividing by its molar mass:
number of moles = mass in grams / molar mass = 36.0 g / 180.0 g/mol = 0.2 mol
=> m = 0.2 mol / 0.5000 kg = 0.4 m
=> ΔT = 1.86°C/m * 0.4m = 0.744 °C.
Given that the freezing point of pure water is 0.0°C, the new freezing point is 0°C - 0.744 °C = - 0.744°C
Answer: - 0.744 °C
2) Elevation of boiling point is another colligative property.
It is calculate as:
ΔTb = i * Kb * m.
Whre i is van't Hoff constant, which accounts for the number of ions generated in ionic compounds.
Here, you do not have the i constant, but you can tell that each molecule of KCl dissociates into 2 ions (one K+ and one Cl-) which is to say that i = 2.
Kb is the boiling constant of the solvent, which here is water => Kb = 0.512 °C/m.
You also know the boiling point of the solution as 103.07°C. Given that the normal boiling point of pure water is 100.0°C, ΔT is 103.07°C - 100.0 °C = 3.07°C.
Then you can determine the molality, m = ΔTb / (i*Kb) = 3.07°C / (2*0.512°C/m) = 3.00 °C.
In order to perform the experiment, in identifying the
temperature of different kinds of liquid freeze—the needed equipment is the
thermometer. It is because the thermometer is an equipment that helps in
determining a temperature. The variable in the experiment would be the
temperature as this is not consistent for the effects or outcome will likely
change.