Benzene (C6H6) has a normal freezing point of 5.50oC and a density of 0.88 g/mL. When 1.28 g of naphthalene (C10H8) is dissolved in 125 mL of benzene, the freezing point of the solution is 5.03oC. Determine the molal freezing point constant for this solvent.
mass = density*volume
mass benzene = 0.88 g/mL x 125 mL = ?
delta T = Kf*m
You know delta T.
m = mols/kg solvent;
mols naph = 1.28/molar mass naphthalene
kg solvent = ? g benzene/1000
Solve for Kf
To determine the molal freezing point constant for benzene, we can use the equation:
ΔTf = Kf * m
Where:
ΔTf = freezing point depression (in Kelvin)
Kf = molal freezing point constant (in K·kg/mol)
m = molality of the solution (in mol/kg)
In this case, we are given:
ΔTf = 5.50°C - 5.03°C = 0.47°C = 0.47 K (since 1°C = 1 K)
m = (moles of solute) / (mass of solvent in kg)
First, let's calculate the moles of naphthalene:
Molar mass of naphthalene (C10H8) = (12.01 g/mol * 10) + (1.01 g/mol * 8) = 128.17 g/mol
moles of naphthalene = (mass of naphthalene) / (molar mass of naphthalene)
= 1.28 g / 128.17 g/mol
≈ 0.01 mol
Next, let's calculate the mass of benzene:
mass of benzene = (density of benzene) * (volume of benzene)
= 0.88 g/mL * 125 mL
= 110 g
Then, let's convert the mass of benzene to kg:
mass of benzene in kg = 110 g / 1000
= 0.110 kg
Now, we can calculate the molality of the solution:
m = (0.01 mol) / (0.110 kg)
≈ 0.091 mol/kg
Finally, we can calculate the value of Kf:
Kf = ΔTf / m
= 0.47 K / 0.091 mol/kg
≈ 5.16 K·kg/mol
Therefore, the molal freezing point constant for benzene (C6H6) is approximately 5.16 K·kg/mol.
To determine the molal freezing point constant for benzene, we can use the formula:
ΔT = Kf * m
Where:
ΔT is the change in freezing point (in this case, the difference between the normal freezing point and the observed freezing point).
Kf is the molal freezing point constant for benzene.
m is the molality of the solution, which is defined as moles of solute per kg of solvent.
First, we need to calculate the moles of naphthalene (C10H8) dissolved in benzene.
Given mass of naphthalene = 1.28 g
To calculate the number of moles, we'll use the molecular weight (MW) of naphthalene:
MW of naphthalene = 2(C) + 8(H) = 128 g/mol
Number of moles of naphthalene = mass / MW = 1.28 g / 128 g/mol
Next, we need to calculate the molality of the solution.
Molality (m) = moles of solute / mass of solvent (in kg)
The given volume of benzene is 125 mL, which is equivalent to 0.125 L.
Density of benzene = 0.88 g/mL
Mass of benzene = density * volume = 0.88 g/mL * 0.125 L = 0.11 g
We need to convert the mass of benzene to kg:
Mass of benzene (in kg) = 0.11 g / 1000 g/kg
Finally, we can calculate the molality:
Molality (m) = moles of naphthalene / mass of benzene (in kg)
Now that we have all the required values, we can calculate the change in freezing point (ΔT).
ΔT = (normal freezing point - observed freezing point)
Substituting the values, we can solve for the molal freezing point constant (Kf) using the formula:
Kf = ΔT / m
Now you have all the steps to follow in order to solve for the molal freezing point constant for this solvent.