a solution is prepared by mixing 2.17g of an unknown non-electrolyte with 225.0g of chloroform. The freezing point of the resulting solution is -64.2 C . The freezing point of pure chloroform is -63.5 C and its kf= 4.68C m^-1 . What is the molecular mass of the unknown?
i know we must use molality and freezing point depression formula, but i do not know in what order.
To find the molecular mass of the unknown non-electrolyte, we can use the freezing point depression formula:
ΔTF = KF * m * i
Where:
ΔTF is the freezing point depression
KF is the freezing point depression constant for the solvent (chloroform)
m is the molality of the solution (moles of solute per kilogram of solvent)
i is the van't Hoff factor (1 for non-electrolytes)
First, let's calculate the molality (m):
m = moles of solute / mass of solvent in kg
Since we are given the mass of the unknown non-electrolyte (2.17g) and the mass of the chloroform (225.0g), we need to find the moles of the unknown non-electrolyte:
moles of solute = mass of solute / molar mass
To find the molar mass, we'll rearrange the equation as:
molar mass = mass of solute / moles of solute
Now, let's calculate the moles of solute:
moles of solute = 2.17g / molar mass
Next, we can calculate the molality (m):
m = moles of solute / (mass of solvent in kg)
mass of solvent in kg = 225.0g / 1000 = 0.225 kg
Finally, we can use the freezing point depression formula to find the molecular mass:
ΔTF = KF * m * i
Substituting the given values:
-64.2 C - (-63.5 C) = 4.68 C m^-1 * m * 1
Simplifying:
-0.7 C = 4.68 C m^-1 * m
Now, solve for m:
m = -0.7 C / 4.68 C m^-1 = -0.149 m
Now we can use the molality to find the moles of solute:
moles of solute = m * (mass of solvent / molar mass of solvent)
Combining the equations, we get:
moles of solute = 0.149 * (0.225 kg / molar mass)
Finally, rearrange the equation to solve for the molar mass:
molar mass = (0.149 * 0.225 kg) / moles of solute
Calculate the moles of solute and substitute it back into the equation to find the molar mass of the unknown non-electrolyte.
To find the molecular mass of the unknown non-electrolyte, you can use the formula for freezing point depression:
ΔTf = kf * m
where ΔTf is the change in freezing point, kf is the freezing point depression constant (which is given as 4.68 °C/m for chloroform), and m is the molality of the solution.
To calculate the molality (m) of the solution, you need to know the moles of the unknown non-electrolyte and the mass of the solvent (chloroform).
First, calculate the moles of the unknown non-electrolyte:
moles of solute = mass of solute / molar mass of solute
Given that the mass of the unknown non-electrolyte is 2.17 g, we can divide this by the molar mass (which we need to find) to calculate the moles.
Next, calculate the molality of the solution:
Molality (m) = moles of solute / mass of solvent (in kg)
The mass of the solvent is given as 225.0 g of chloroform. Convert this to kilograms by dividing by 1000.
Once you have the molality, you can use the freezing point depression formula to find the change in freezing point (ΔTf). The change in freezing point is given as -64.2 °C for the resulting solution minus the freezing point of pure chloroform (-63.5 °C).
Now you have all the values needed to use the formula ΔTf = kf * m. Rearrange the equation to solve for the molality (m):
m = ΔTf / kf
Then substitute the values for ΔTf and kf to calculate the molality (m).
Finally, you can calculate the molecular mass of the unknown non-electrolyte using the equation:
molar mass of solute = (mass of solute) / (moles of solute)
Substitute the values for the mass of the solute and moles of the solute to find the molecular mass of the unknown non-electrolyte.
When I come up with that dilemma I put doen the formula I need, plug in the values, see which unknown I don't have and work it from equation to equation. Here is the order you need.
delta T = Kf*m.
You have delta T (or can calculate it) and Kf so solve for m.
molality = mols/ kg solvent.
You have m and kg solvent, solve for mols.
Then mols = grams/molar mass. You have mols and grams, solve for molar mass.