By calculating it's number of GROUP........
answer: 3.40625 moles
Explanation: see the attached pics
PV = nRTP is pressure, V is volume in L, n is number of moles, R is the gas constant,and T is temperature in K
(1.5 atm)(1 L) = (n)(.08206)(301K)
n = .06 moles in one liter
If there are 3.9 grams in .06 moles then
1/.06 x 3.9 = 64.2 grams per mol
K value of water is 1x 10^-14