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# How do you do gas stoichiometry problems at STP?

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 .

**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

##N_2 + 3H_2 -> 2NH_3##

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

##grams N_2 -> mol N_2 -> mol H_2 -> Liters H_2##

##10 g N_2 x (1 mol N_2)/(28 g N_2) = 0.357 mol N_2## (remember ##N_2## , 2 x 14 g = 28 g)

Use ratio from the balanced chemical equation to convert moles of ##N_2## to moles of ##H_2##

##0.357 mol N_2 x (3 mol N_2)/(1 mol N_2) = 1.071 mol H_2##

Now use Avogadro's number 22.4 L/mol to convert moles of ##H_2## to Liters of ##H_2##.

##1.071 mol H_2 x (22.4 L H_2)/(1 mol H_2) = 23.99 L H_2##

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: