Chemestry
posted by Monica on .
A cylinder with a movable piston contains 2.00 of helium, , 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 to 4.30 ? (The temperature was held constant.)

A cylinder with a movable piston contains 2.00g of helium, , 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 2.00L to 4.30L ? (The temperature was held constant.)

I would use PV = nRT first. You know V, n, R, and T, so you can determine p.
Second part of the problem says p stays constant. Use PV = nRT again. Here you know the old p (since it stays constant), the new V, along with R and T. Calculate new n and grams He from that.
T doesn't change; just make up a convenient T. I would make it something easy like 300 K. 
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