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13 Dec 2019

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 ** Write answer in decimal form to one decimal place, NO units in answer**

(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?

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Reid Wolff
Reid WolffLv2
17 Dec 2019

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Related questions

What is the molar mass of the unknown liquid? (Step 5) Use the table in the experiment print out to identify the liquid.

Atmospheric pressure = 765 mmHg

Mass of empty flask = 120.30g

Mass of flask with liquid at the end = 121.00g

Temperature = 100oC

Volume of flask = 285mL

Volatile Liquid

Molecular Weight (g/mol)

Boiling Point (°C)

Acetone

58.1

56.5

2-Butanone

72.1

79.6

Cyclohexane

84.2

80.7

Ethanol

46.1

78.5

Ethyl Acetate

88.1

77.1

1-Heptene

98.2

93.6

Hexane

86.2

69.0

Methyl Acetate

74.1

57.0

2-Propanol

60.1

82.3

A certain volatile hydrocarbon (a binary compound of carbon and hydrogen) is found to be 92.3% carbon by mass. In a seperate experiment, utilizing the Dumas method, a 4.00 mL pure liquid sample of this hydrocarbon is vaporized in a 125 mL Florence flask when the barometric pressure is 768.0 torr. After the exess gas escapes the temperature is measures as 98.0 C. The flask and contents are subsequenlty cooled to 25 C (densitry water at 25 C = 0.997044 g/mL ) and the vapor condenses to a liquid. The flask is then empetied cleaned and filled with water. When weighed on the balance, the difference in weight between the flask filled to the brim with water and the dry empty flask at 25 C is 128.12 g

The empty flask - fitted with a foil cap pierced with a pinhole - weighs 25.3478g

The weight of the flask and contents is found to be 25.6803g

Determine the Emperical formula, the volume that the vapor occupied in the Florence flask and the molecular formula of this hydrocarbon.

Please give me Post lab questions 2 and 3 at the bottom.

Experiment 7 - Molar Mass of a Volatile Liquid

Introduction. Given the mass of vapor at conditions of known pressure, volume, and temperature, one can determine the molar mass of the vapor. The Ideal Gas Law mathematically relates the quantities of pressure (P in atm), volume (V in liters), and temperature (T in Kelvin) and the quantity of gas (n in moles). Using the expression PV=nRT where R equals the Ideal Gas Constant 0.0821 L*atm/K*mol, one can determine the quantity of gas under given conditions of pressure, volume and temperature. The number of moles of gas is found by

PV

RT

n =

Because the molar mass (MM) is defined as the mass of one mole of substance in units of g/mol, the molar mass of the vapor can be determined by

MM= mass of vapor in grams/moles of vapor

In this experiment, an unknown volatile liquid is heated in a boiling water bath and is vaporized. The vapor forces air from the flask until the pressure within the flask equals the barometric pressure of the lab. As with all gases, the vapor occupies the entire volume of its container. The temperature of the vapor equals the temperature of the boiling water bath. After 8-10 minutes, the vapor is cooled. The mass of the condensed liquid is determined and the molar mass is calculated as described above.

Notes:

Check out unknown from stockroom

Record unknown number

Before weighing your condensed vapor, thoroughly dry the outside of the flask and make sure that no water is trapped under the foil cover

Do not rinse your flask with water between trials

Dispose of waste in appropriate container

Procedure. You will record your data and calculations in your lab notebook.

Obtain an unknown liquid and a 3 inch aluminum foil square. You will need two 600 or 800 mL beakers, and 3 or 4 boiling chips.

Add 3 or 4 boiling chips to the water in an 800 mL beaker and heat the water to the boiling point. Continue heating for 8 to 10 minutes after the water comes to a rolling boil. The temperature of the boiling water is 100.0oC. This will be the temperature of the vapor. Convert the temperature to Kelvin and record in your lab notebook.

800 mL Beaker

250 mL Flask


11.

Chemical Molar Mass

1-chlorobutane 92.6 g/mol

t-butyl chloride 92.5 g/mol

trichloroethylene 131 g/mol

2-propanol 60.1 g/mol

Ethyl acetate 88.1 g/mol

n-hexane 86.2 g/mol

cyclohexane 84.2 g/mol

2-butanone 72.1 g/mol

Post Lab Questions: Answer the following questions using complete sentences. Include them after your conclusion.

1. If a lab group did not keep the flask submerged in boiling water for a full ten minutes, and some unknown never vaporized, how would that affect the calculated molar mass? In other words, would it make the calculated value too high or too low? Explain why.

2. While performing this experiment, ten lab groups have ten different unknown volatile liquids with a range of molar masses. If all lab groups have exactly the same size of flask (255 mL), and all groups took their unknown vapor to the same temperature (100.0oC) and pressure (760.0 mmHg), how would the number of molecules within the different flasks compare? Would it vary with molar mass? How? Explain.

3. A student has determined that an unknown liquid is either 1-chlorobutane or t-butyl chloride. What other physical property(s) could be used to determine the identity of the unknown?

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