How do you find the molar mass in this experiment?

Question

Here's the information that's given:

A 1.60 g sample of a large boimolecule was dissolved in 13.0 g of carbon tetrachloride. The boiling point of this solution was determined to be 77.85°C. Calculate the molar mass of the biomolecule. For carbon tetrachloride, the boiling point constant is 5.03°C·kg/mol, and the boiling point of pure carbon tetrachloride is 76.50°C.

I don't know how to go about solving this. I feel like I learned an equation or something but I can't think of it... Thanks in advance.

Answer 1

Here are the equations you need:

no. of moles= mass/MW

change in B.P= - boiling point elevation constant * molality

molality = moles of solute/ mass (in Kg) of solvent

basically in the presence of the solute, the BP will be higher than for the pure solvent, so just plug in the values and find molality, and then from that get the moles of solute (moles of the biomolecule) then find the Mr using the moles to mass conversion formula (moles= mass/Mr

Answer 2

b.p. elevation = 77.85 - 76.50 = 1.35 deg C

b.p. elevation = 1.35 = 5.03 x molality

molality = 1.35 / 5.06 = 0.2684 m

(1.60 g / molar mass) / 0.013 kg = 0.2684

1.60 g/ molar mass = 0.003489

molar mass = 458.6 g/mol

    

Answer 3

you're going to need the equation for boiling point elevation, which is

ΔTb = Kb · mB

where ΔTb is the change in boiling point of the solvent (in this case, carbon tetrachloride) after adding the solute, Kb is the boiling point constant of the solvent (5.03°C·kg/mol), and mB is the molality of the solution.

So lets first solve for the molality of the equation-

mB = ΔTb/Kb= (77.85-76.50)/5.03 = 0.27 mol/kg

So this means there is 0.27mol of the biomolecule in every kg of carbon tetrachloride. Since you know the mass of carbon tetrachloride, you can easily find the total number of moles in solution-

0.27mol/kg X 0.013kg = 0.00351mol.

Now you know there is 0.00351 moils of the biomolecule in 1.6g of it. Divide the grams by moles to get the molar mass.

1.6/0.00351 = 456 g/mol