How Much Oxygen Does a Medical Cylinder Hold at Different Pressures?

[qspartan570]

Homework Statement

An oxygen cylinder for medical use contains oxygen at a pressure of 140 atm, at room temperature (20^$\circ$C). The cylinder measures 20 cm in diameter and 110 cm in length. How many kilograms of oxygen does this cylinder contain? What will be the volume of the oxygen if it is allowed to expand slowly to normal atmospheric pressure, at room temperature?

Homework Equations

ideal-gas law pV=nRT
pV=NkT

The Attempt at a Solution

V_cylinder= ∏r²h=(∏)(10 cm)²(110 cm)=3.45 cm³

m_oxygen= 3.45 cm³ X 1.141 g$\bullet$cm³=3.94 g or 0.00394 kg

According to the book the answer to this part is 6.4 kg

What did I do wrong?

 

[ehild]

Take care of the units, and recalculate the volume.
How did you get the mass? Note that the density of gases depend on pressure and temperature.

ehild