Calculated how many grams of methane (CH4)?

Question

Calculated how many grams of methane (CH4) are in a sealed 80 mL flask at room temperature (22ºC) and 780 mm Hg of pressure

Answer 1

PV=nRT
plug in variables and solve for n and you get 0.0034 moles

MW of CH4 = 16.043 g/mol

0.0034 mol x 16.043 g/mol = 0.054 grams

Answer 2

First, we have to get the methane to STP, which is 273K and 760mmHg. The grams will wait until we get there. Atomic weights: C=12 H=1 CH4=16

80mL x (273K/298K) x (780mm/760mm) x (1molCH4/22400mLCH4) x (16gCH4/1molCH4) = (80)(273)(780)(16)/(298)(760)(22400) = 0.054g CH4

The 80mL CH4 is given. To convert to STP, the temperature goes down, so volume goes down, so we need to multiply by a number less than one to do that. Hence 273K/298K. The pressure goes down, so the volume must go up, so we need to multiply by a number greater than one, hence 780/760. The 22400mL comes from the molar volume of a perfect gas.