Water can be formed according to the equation 2H2 + O2 2H2O if 12.0 L of hydrogen is reacted at STP exactly how many liters of oxygen at STP would be needed to allow complete reaction?

In order to find the number of liters, we use the ideal gas law PV=nRT.

Where:

P = Pressure
V = Volume
n = Number of moles
R = Gas constant (#~~ 0.08206 L·atm·mol^-1·K^-1#)
T = Temperature (Kelvin)

First, we find the number of moles of hydrogen by manipulating the equation to find the number of moles.

#n=(PV)/(RT)#

Then we plug in the known values for standard temperature and pressure into the equation.

In Standard Temperature and Pressure:

P = 1
T = 273.15

So the equation solving for the number of moles of hydrogen gas looks like:

#n = (1*12)/(0.08206*273.15)#

so the number of moles of hydrogen gas is

#n~~ 0.535# moles

Now that we know the number of moles of hydrogen gas in 12 L we can determine the number of moles of oxygen needed.

According to the stoichiometric equation of the formation of water 2 moles of hydrogen are used for every mole of oxygen gas. Therefore we can divide the number of hydrogen moles by two to find the exact amount of oxygen needed in moles to complete the reaction.

#0.535/2=0.2675# moles

We can use the ideal gas law again to find the number of moles of gas in terms of liters.

Changing the equation to solve for liters we get:

#L=(nRT)/P#

Then we plug in the values for STP and the number of moles of oxygen to get the number of liters of gas.

#L=(0.2675*0.08206*273.15)/1#
#L = 6#

Therefore the number of liters required to complete the reaction is 6 L of oxygen gas.