In some aquatic ecosystems, nitrate (NO3–) is converted to nitrite (NO2–), which then decomposes to nitrogen and water.

Question

As an example of this second reaction, consider the decomposition of ammonium nitrite:

NH₄NO₂(aq) ==> N₂(g)+2H₂O(l)

What would be the change in pressure in a sealed 10.0 L vessel due to the formation of N₂ gas when the ammonium nitrite in 3.00 L of 1.40 M NH₄NO₂ decomposes at 25.0°C?

Answer 1

How many moles of NH₄NO₂ are present?

3.00 L x 1.40 mol/L = 4.20 moles

From the balanced equation for the decomposition of NH₄NO₂, 1 mol decomposed to produce 1 mol N₂(g)

Thus, moles N₂ gas produced = 4.20 moles N₂(g)

Now, use the ideal gas law to find the pressure when 4.20 moles N2 gas is produced in a 10 L vessel.

PV = nRT

P = nRT/V = (4.20 mol)(0.0821 Latm/Kmol)(298K) / 10.0 L = 10.3 atm

This is the pressure in the 10.0 L vessel.

If you want the change in pressure, do the same calculations using a 3.00 L vessel, find the pressure and compare it to the pressure in the 10 L vessel. That will give you the change in pressure.