How do you do gas stoichiometry problems at STP?

2 Answers
Mar 19, 2014

Gas stoichiometry problems relate the number of moles of reactants and products, so use the ideal gas law at standard temperature and pressure (273.15 K and 1.00 atm) to solve for the number of moles of gas at a specified volume, V:

n=PVRT

If V is given in liters, then use the gas constant

R=0.082054LatmmolK

At STP (Standard Temperature and Pressure 273 K and 1 atm) we are able to use Avogadro's number of 22.4 L per mole of gas or
22.4 L/mole as our conversion factor in the stoichiometry.

EXAMPLE

How many liters of hydrogen gas are necessary to react with 10 grams of nitrogen to produce ammonia at standard temperature and pressure?

We begin with a balanced chemical equation

N2+3H22NH3

Now using the given value of 10 grams of nitrogen we will convert using stoichiometry

gramsN2molN2molH2LitersH2

10gN2x1molN228gN2=0.357molN2
(remember N2 , 2 x 14 g = 28 g)

Use the mole ratio from the balanced chemical equation to convert moles of N2 to moles of H2

0.357molN2x3molN21molN2=1.071molH2

Now use Avogadro's number 22.4 L/mol to convert moles of H2 to Liters of H2.

1.071molH2x22.4LH21molH2=23.99LH2

Remember, we can only use the 22.4 L/mole value if the reaction is taking place at Standard Temperature and Pressure (STP = 0 C and
1 atm).

Here's a video that might be helpful: