A flask with a volume of 2.60 L , provided with a stopcock, contains ethane gas

Question

A flask with a volume of 2.60 L , provided with a stopcock, contains ethane gas (C2H6) at a temperature of 291 K and atmospheric pressure 1.013×105 Pa . The molar mass of ethane is 30.1 g/mol . The system is warmed to a temperature of 398 K , with the stopcock open to the atmosphere. The stopcock is then closed, and the flask cooled to its original temperature.

Part A

What is the final pressure of the ethane in the flask?

Express your answer using three significant figures.

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Part B

How many grams of ethane remaining in the flask.

Express your answer using three significant figures.

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Explanation / Answer

Apply ideal gas equation PV = nRT to find the number of moles

the number of moles of a gas is

n = P1*V1 / R*T1

= (1.013*10^5 pa)*(2.60*10^-3)/(8.314 J/mole k)*(291 k)

= 0.108 mol

the initial mass of the ethane in the flask is

m = (0.103 mol)*(30.1 g/mol)

= 3.1 g

Part (a)
the volume and number of moles remains constant after the cock is closed

the final pressure of the ethane is

P2 = (T2/T1)*P1

= (291 k/398 k)*(1.013*10^5 pa)

= 7.406*10^4 pa

Part (b)

the reaming number of moles of the gas in flask is

n' = P1*V1 / R*T1

= (7.406*10^4)*(2.60*10^-3)/(8.314)*(291)

= 7.958*10^-2 moles

the remaining mass of the ethane in the flash is

m' = n'*M

= (7.958*10^-2)*(30.1)

= 2.395 g

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