Question

A student was assigned the task of determining the identity of an unknown liquid. The student weighed a clean, dry 250-mL Erlenmeyer flask. The student completely filled the flask with water. He then proceeded to weigh the flask containing the water.

Mass of 250-mL Erlenmeyer flask: 78.639 g

Mass of 250-mL Erlenmeyer flask and water: 327.039 g

Assuming the density of water is 1.00 g/mL at ambient temperature, what is the exact volume (in mL) of the 250-mL Erlenmeyer flask?

Next, the student added 5.2 mL of an unknown liquid to the dry 250-mL Erlenmeyer flask (same flask used in Question 1). He placed the flask in a boiling water bath and left it there until the liquid completely vaporized. The student measured the temperature of the boiling water bath to determine the temperature of the gas inside the flask. The temperature of the water bath was 99.3 degrees Celsius. Atmospheric pressure was 751 torr when the liquid completely vaporized. What is the temperature of the gas in Kelvins?

The student removed the 250-mL Erlenmeyer flask containing the gas vapor to let it cool to room temperature. When the gas had completely condensed, he weighed the flask and the liquid. Mass of 250-mL Erlenmeyer flask and liquid: 79.118 g What is the mass of the liquid in the flask? What is the molar mass of the unknown liquid in grams per mole? what is the identity of the unknown liquid?

Answer 1

First we calculate the mass of water that fills the flask:

mH2O = mFlask + H2O - mFlask = 327.039 - 78.639 = 248.4 g

We calculate the volume of water, referred to that mass:

VH2O = 248.4 g * (1 mL / 1 g) = 248.4 mL = VFlask

We calculate the temperature in Kelvin:

T (K) = T (ºC) + 273.15 = 99.3 + 273.15 = 372.45 K

We calculate the pressure in atmospheres:

P = 751 Torr * (1 atm * 760 Torr) = 0.988 atm

We calculate the volume in liters:

V = 248.4 mL * (1 L / 1000 mL) = 0.2484 L

We calculate the moles of the gas:

n = P * V / R * T = 0.988 atm * 0.2484 L / 0.08216 atm * L / mol * K * 372.45 K = 8.02x10 ^ -3 moles

We calculate the mass of the liquid:

m Liq = mLiq + Flask - mFlask = 79,118 - 78,639 = 0.479 g

We calculate the molar weight of the liquid:

M = m / n = 0.479 / 8.02x10 ^ -3 = 59.73 g / mol

The results should be collated, with a proposed table, to determine what the compound is.