Why isn\'t it necessary to weigh the amount of liquid initially put into the fla

Question

Why isn't it necessary to weigh the amount of liquid initially put into the flask? Explain. A certain volatile hydrocarbon(binary compound of carbon and hydrogen) is found to be 92.3% carbon, by mass. In separate experiment, utilizing the Dumas method, a 4.00mL purr liquid sample of this hydrocarbon is vaporized in a 125 mL Florence flask when the barometric pressure is 768.0 tort. After the excess f gas escapes, the temperature is measured as 98.0 degree C. The flask and contents are subsequently cooled to 25 degree C= 0.997044 g/mL) and the vapor condenses to a liquid. The flask is then emptied, cleaned, and filled with water. When weighed on a balance, the difference in weight between the flask filled to the brim with water and the dry empty flask at 25 degree C is 128.12 g. The empty flask-fitted with a foil cap pierced with a pinhole-weighs 25.3478 g The weight of the flask and contents is found to be 25.6803g Please determine the following, The empirical formula of this hydrocarbon. The volume that the vapor occupied in the Erlenmeyer flask(inL).The molecular formula of this hydrocarbon.

Explanation / Answer

Step 1

Empirical formula :

The hydrocarbon is 92.3% carbon by mass.

Thus mole percent of carbon will be = 92.3/atomic mass of C = 92.3 / 12 = 7.7

Mass percent of hydrogen will be = 100 - 92.3 = 7.7

Mole percent of hydrogen will be = 7.7 / 1 = 7.7.

Since C:H mole ratio is 1:1 , the empirical formula of given hydrocarbon will be 1:1.

Step 2 : Finding out moles of given substance:

Barometric pressure is given as 768 torr, which is the atmospheric pressure , we have to convert this into units of atm. Since 1 atm = 760 torr

768 torr x 1 atm / 760 torr = 1.01 atm.

Difference in weight between water filled and empty flask = 128.12g

We will convert this mass of water into L to find the exact volume of flask (density of water is given).

128.12 g / 0.997044 g ml-1 = 128.5 mL = 128.5 mL x 1L/1000mL = 0.1285 L

Now we will apply ideal gas equation to find out number of moles

PV = nRT

n = PV/ RT = 1.01 atm x 0.128 L / 0.0821 atmLmol-1K-1 x 371 K = 0.00426 moles.

Step 3:

Weight of the content in flask = Wt of fask with content - Wt of empty flask = 0.3325 g

Molecular mass of substance = 0.3325 g / .00426 moles = 78.05 g/mol

Emprical formula mass = 12 +1 = 13

Molecular mass / Empirical formula mass = 78 / 13 = 6.

Thus Molecular formula of substance is C6H6 (Benzene ).

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