The vapor pressure of chloroform is 173.11 mm hg at 25°c. how many grams of aspirin, c9h8o4, a nonvolatile, nonelectrolyte (mw = 180.1 g/mol), must be added to 244.0 grams of chloroform to reduce the vapor pressure to 171.05 mm hg ? chloroform = chcl3 = 119.40 g/mol. the boiling point of water, h2o, is 100.000 °c at 1 atmosphere. kb(water) = 0.512 °c/m in a laboratory experiment, students synthesized a new compound and found that when 11.55grams of the compound were dissolved in 243.4 grams of water, the solution began to boil at100.071 °c. the compound was also found to be nonvolatile and a non-electrolyte. what is the molecular weight they determined for this compound?

a) There must be added 4.465 grams of aspirin

b) The unknown has a molecular weight of 342.2 g/mol

Explanation:

Step 1: Data given

The vapor pressure of chloroform is 173.11 mm Hg at 25°C

a nonvolatile, nonelectrolyte (MW = 180.1 g/mol), must be added

Mass of chloroform = 244.0 grams

The vapor pressure is reduced to 171.05 mm Hg

chloroform = CHCl3 = 119.40 g/mol.

The boiling point of water, H2O, is 100.000 °C

Step 2: Calculate moles chloroform

244.0g CHCl3/ 119.40g/ mol  = 2.0436 mol CHCl3

step 3: Calculate moles aspirin

Moles aspirin = x grams / 180.1 g/mol

(p0 - p)/p0

⇒ p0 is the initial pressure = 173.11 mm Hg

⇒ p = the reduced pressure = 171.05 mm Hg

(p0 - p)/p0  = (173.11-171.05)/173.11  = (xg/180.1 g/mol) / (2.0436 mol + (xg/180.1g/mol))

0.0119 mol = (xg/180.1 g/mol) / (2.0436 mol + (xg/180.1g/mol))

0.02432 + (0.0119x/180.1) = (x/180.1)

0.02432 =(( x - 0.0119x)/180.1)

0.9981x = 4.38

x = 4.465 grams

There must be added 4.465 grams of aspirin

b)  What is the molecular weight they determined for this compound?

Step 1: Define molar mass of the compound:

Molar mass = mass / moles

Molar mass compound = 11.55 grams / moles

Step 2: Calculate ΔT

ΔT = Tsoln - Tpure

ΔT = 100.071 - 100.00 = 0.071 °C

Step 3: Calculate m

ΔT = m*Kb

0.071°C = m*0.512 °C/m

m = 0.071  °C / 0.512 °C/m

m = 0.13867 m  = 0.13867 mol/kg

Step 4: Calculate number of moles of the compound

Moles = mass of water * m

Moles = 0.2434 kg * 0.13867 mol/kg

Moles = 0.03375 moles

Step 5: Calculate molar mass

Molar mass  = mass / moles

Molar mass compound = 11.55 grams / 0.03375 moles

Molar mass compound = 342.2 g/mol

The unknown has a molecular weight of 342.2 g/mol (and can be sucrose. It has a molar mas of 342.2 g/mol, is nonvolatile and non-electrolyte.).