Everything Science Grade 11

CHAPTER 5. THERMAL PROPERTIES AND IDEAL GASES 5.6

2 NaN 3 (s)→ 2 Na (s) + 3N 2 (g)

1. Calculate the mass of N 2 (g) needed to inflate a sample airbag to a volume of 65 dm^3 at
   25 ◦C and 99 , 3 kPa. Assume the gas temperature remains constant during the reaction.


2. In reality the abovereaction is exothermic.Describe, in terms of the kinetic molecular
   theory, how the pressurein the sample airbag will change, if at all, as thegas temperature
   returns to 25◦C.

SOLUTION

Step 1 : Look at the information you have been given, and the informationyou still

need.

Here you are given thevolume, temperature and pressure. You are required to

work out the mass of N 2.

Step 2 : Check that all the unitsare S.I. units

Pressure: 93. 3 × 103 Pa

Volume: 65 × 10 −^3 m^3

Temperature: (273 + 25) K

Gas Constant: 8 , 31 J· K−^1 mol−^1

Step 3 : Write out the Ideal Gas formula

pV = nRT

Step 4 : Solve for the requiredquantity using symbols

n =

pV

RT

Step 5 : Solve by substituting numbers into the equation to solve for ’n’.

n =

(99, 3 × 103 Pa)× (65× 10 −^3 ) m^3

8 , 31 J· K−^1 mol−^1 × (273 + 25) K

Step 6 : Convert the number ofmoles to number of grams

m = n× M

m = 2, 61 × 28

m = 73, 0 g

Step 7 : Theory Question

When the temperature decreases the intensity ofcollisions with the wallsof the

airbag and between particles decreases. Therefore pressure decreases.

Exercise 5 - 5

1. An unknown gas haspressure, volume and temperature of 0.9 atm, 8 � and 120◦C respectively.
   How many moles of gasare present?