<u>Answer:</u> The net chemical equation is given below.
<u>Explanation:</u>
To determine the net chemical equation, we will look into the intermediate steps:
<u>Step 1:</u> Formation of ammonia from hydrogen and nitrogen gas.
 ......(1)
        ......(1)
<u>Step 2:</u> Formation of nitric acid from ammonia and oxygen gas.
 .....(2)
      .....(2)
The net chemical equation for the formation of nitric acid from hydrogen gas, nitrogen gas and oxygen gas follows by adding both the equations, we get:

By Stoichiometry of the reaction:
1 mole of hydrogen gas reacts with 2 moles of oxygen gas and 3 moles of hydrogen gas to produce 1 mole of nitric acid, 1 mole of water and 1 mole of ammonia.
Hence, the net chemical equation is given below.
 
        
             
        
        
        
Half-life time of a reaction is time at which reactant concentration becomes half of its initial value.
Half-life of the first order reaction is 20 min. Rate constant can be calculated as follows:

The rate expression for first order reaction is as follows:

initial number of molecules of reactant are  , time is 100 min thus, putting the values to calculate number of reactant at time 100 min,
, time is 100 min thus, putting the values to calculate number of reactant at time 100 min,
![0.03466 min^{-1}=\frac{2.303}{100 min}log\frac{[10^{20}]}{A_{t}}](https://tex.z-dn.net/?f=0.03466%20min%5E%7B-1%7D%3D%5Cfrac%7B2.303%7D%7B100%20min%7Dlog%5Cfrac%7B%5B10%5E%7B20%7D%5D%7D%7BA_%7Bt%7D%7D)
On rearranging,

Or,

Therefore, number of molecules unreacted will be 
 
        
             
        
        
        
No because it is a gas so (in space) so, it would most likely stay in one place. Your welcome!