Question

How many grams of water vapor (H2O) are in a 10.2-liter sample at 0.98 atmospheres and 26°C?

Answer 1

Those this is based off the equation:
pv=nrt

for the most part it is a simple equation. all you have to do is plug stuff in. however, when looking for grams it is not as clear. the thing to look at is which of these values are we missing.
p = 0.98 atm
v = 10.2 L
n = ?
r = constant value
t = 26 C or 299 K

So we are missing n. this value has the units of moles. We we know that
molar mass = grams / moles

So if we can plug in the numbers to find n, then we can use that value and the molar mass to determine the grams. so:
0.98 * 10.2 = n *0.08206 * 299
n = 0.407 moles

the molar mass of h20 is 18.0. So
grams = molar mass * moles
grams = 18 * 0.407
grams = 7.33 grams.

however since atmospheres only has 2 sig figs, so will out answer. So
g = 7.3