H2O decomposes to hydrogen and oxygen as describe in the following reaction:
2H20 (1) ► 2H2 (9) + O2 (9)
What is the mole fraction of hydrogen gas in the products?
a 0.5
b 0.33
C 0.4
d 0.67

When a gas is collected over water, one must subtract the vapor pressure of water from the total pressure in order to find the pressure of the gas itself. Thus, 1.05 atm - 0.0278 atm = 1.0222 atm = pressure of the O2 gas.

PV = nRT and solve for n to determine moles of O2 collected.
n = PV/RT = (1.0222 atm)(0.5 L)/(0.0821 Latm/Kmol)(296 K) note the use of Kelvin for temperature.
n = 0.02103 moles O2
mass of O2 = 0.02103 moles x 32 g/mole = 0.67 grams (to 2 significant figures)

Mg + 2H(NO)3 ⇒ Mg(NO3)2 + H2