Tf (acetic acid) = 16.1°C
Tf (with 3.00g of solute in 10.00mL of acetic acid) = - 2.5°C
Use the data to calculate the molecular mass of the solute (Acetic acid: Kf = 3.9°C•kg/mol; D = 1.05 g/mL)
After finding molality I was completely stuck on what to do next..
OK, you have molality = m
m = mol/kg solvent. You have m and kg solvent, substitute and solve for mol.
Then mol = g/molar mass. You have mol and grams, solve for molar mass.
Could you please explain the steps? I am still unsure about what I am doing...
The steps are there. Just follow my instructions What's confusing?
.You say you have m. Then m = mols/kg solvent. The problem tells you kg solvent is 10 mL acetic acid. density of the acid iis 1.05 g/mL so
mass = density x volume = 1.05 g/mL x 10 mL = 10.5 grams or 0.0105 kg.
m = mols/0.0105 kg and mols = m x 0.0105 = ? mols. Plug in your m to find mols. Then mols = grams/molar mass. You have just calculated mols. The problem tells you grams = 3.00. Solve for molar mass.
To calculate the molecular mass of the solute, we can use the freezing point depression equation:
ΔTf = Kf * m
Where:
ΔTf = freezing point depression (Tf of pure solvent - Tf of solution)
Kf = cryoscopic constant (given as 3.9°C•kg/mol)
m = molality of the solution (moles of solute per kilogram of solvent)
First, let's calculate the molality (m) of the solution using the given data:
1. Determine the mass of the solvent:
Using the density of acetic acid (D = 1.05 g/mL) and the volume of acetic acid (10.00 mL):
Mass = Volume * Density
Mass = 10.00 mL * 1.05 g/mL
2. Determine the moles of solute:
Moles = mass / molar mass
Since we don't know the molar mass of the solute yet, let's use a variable "M" for now.
3. Determine the molality:
Molality (m) = moles of solute / mass of solvent (in kg)
To convert the mass of the solvent to kg, divide it by 1000.
Now, to find the molecular mass of the solute, we need to rearrange the freezing point depression equation to solve for the moles of solute:
ΔTf = Kf * m
Rearranging, we get:
Moles = ΔTf / (Kf * m)
Using the given data, ΔTf = -2.5°C and Kf = 3.9°C•kg/mol, and we've calculated the molality (m).
Now, substitute the values into the equation to find the moles of solute:
Moles = -2.5°C / (3.9°C•kg/mol * molality (m))
Finally, once you have the moles, you can calculate the molecular mass (M) using the following equation:
Molar mass (M) = mass / moles
Substitute the moles of solute into the equation, and you will find the molecular mass of the solute in grams/mol.