An Erlenmeyer flask that weighs 93.667g has a quantity of liquid vaporized in it

Question

An Erlenmeyer flask that weighs 93.667g has a quantity of liquid vaporized in it at 96.4oC and a pressure of 1583.8 torr. The mass of the cooled flask is 94.345g. When the flask is filled with water, the mass of the flask and water is 268.92g.

(a). Determine the molecular weight of the unknown liquid.   g/mol    **

(b). Given the following compounds:

Liquid             Molecular Weight (g/mole)

Pentane                     72.2

Acetone                     58.1

Methanol                   32.0

Hexane                      86.2

Ethanol                      46.1

Propanol                    60.1

What is the identity of the unknown liquid?

Explanation / Answer

Given information:

Mass of erlenmeyer flask = 93.667 g

Temperature = 96.4 deg C

Pressure = 1,583.8 torr

Mass of cooled flask = 94.345 g

Mass of flask + water = 268.92 g

a)

Quantity of vaporized liquid = 94.345 g - 93.667 g = 0.678 g

Quantity of water = 268.92 g - 93.667 g = 175.25 g

Volume of water = 175.25 g x density (1 mL / 1 g) = 175.25 mL = 0.175 L

Use the ideal gas equation, PV = nRT, to solve for the molecular weight:

P = 1,583.8 torr = 2.08 atm

V = 0.175 L

n = mass / molecular weight = m/M

R = 0.0821 atm*L / mol*K (gas constant)

T = 96.4 deg C = 369.55 K

PV = nRT ... sub in m/M for n

PV = (m/M)RT ... rearrange the equation to solve for M

PV = mRT / M

PVM = mRT

M = mRT / PV

M = (0.678 g) (0.0821 atm*L / mol*K) (369.55 K) / (2.08 atm) (0.175 L)

M = 56.5 g/mol

b)

The closest molecular weight is acetone.

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