A) A certain amount of chlorine gas was placed inside a cylinder with a moveable

Question

A) A certain amount of chlorine gas was placed inside a cylinder with a moveable piston at one end. The initial volume was 3.00 L and the initial pressure of chlorine was 1.60 atm. The piston was pushed down to change the volume to 1.00 L. Calculate the final pressure of the gas if the temperature and number of moles of chlorine remain constant. Express your answer with the appropriate units: Pfinal= ?

B) A cylinder with a moveable piston contains 2.00 g of helium, He, at room temperature. More helium was added to the cylinder and the volume was adjusted so that the gas pressure remained the same. How many grams of helium were added to the cylinder if the volume was changed from 2.00 L to 4.50 L? (The temperature was held constant.) Express your answer with the appropriate units. Mass of helium added= ?

Explanation / Answer

A) we know that

PV = nRT

given

moles and temperature are constant

so

PV = constant

so

P1V1 = P2V2

1.6 x 3 = P2 x 1

P2 = 4.8

so

the final pressure of the gas is 4.8 atm

B)

we know that

moles = mass / molar mass

so

initial moles of He = 2/ 4 = 0.5

now

PV = nRT

given pressure and temperature are constant

so

V/n = constant

V1/n1 = V2/n2

so

2 / 0.5 = 4.5 / n2

n2 = 1.125

now

mass = moles x molar mass

so

final mass = 1.125 x 4

final mass of He = 4.5

so

mass of He added = final - inital

mass of He added = 4.5 - 2

mass of He added = 2.5

so

2.5 grams of Helium were added

C) the reaction is

6Li + N2 ---> 2 LI3N

given

59.5 ml of N2 at STP

so

PV = nRT

1 x 59.5 x 10-3 = n x 0.0821 x 273

n = 2.6546 x 10-3

now

from the reaction

moles of Li required = 6 x moles of N2

moles of Li required = 6 x 2.6546 x 10-3

moles of Li required = 0.0159

now

mass = moles x molar mass

so

mass = 0.0159 x 7

mass of Lithium = 0.111 g = 111 mg

so

111 milligrams of Lithium is required

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