Please help explain the approach to these questions, thanks.

Here is the complete planning for question one
Use the method of grams to moles and moles to moles of product.For every equation apply the percent yield on every part.

let us assume X g of ammonia is needed.

Planning

Xg NH3 --> moles of  NH3 --->Moles of NO--->50% of the amount of NO --->moles of NO2---> 50% of the amount of NO2 ---->moles of HNO3 --->25% of the amount of HNO3 ---->grams of HNO3 = 126g HNO3.


( X g NH3)(1 mole of NH3)/(17 g NH3) . (4 mole of NO)/(4 moles of  NH3) .  50/100.  (2 mole of NO2)/(2 molesNO) .  50/100 .  (1 mole o HNO3)/(2 moles NO2 ) . 25/100 .  (63.01 g  HNO3 )/(1 mol of HNO3 )
 =  126 g
Solve this algebra equation for x .
X = 1088 g NH3

Here both the containers have same volume , number of molecules and same temperature .So the pressure in both containers before mixing is same i.e  1 atm( given for hydrogen gas).
P = nRT/V
However when the total volume changes to 6L after mixing them then pressure also changes accordingly.
Use Boyle's law to know new pressure when only volume is changing from 2L to 6L.
P₁V₁=P₂V₂
P₁ = 1 atm (given in the question)
V₁ = 2L
V₂ = 6L
P₂ = P1V1/V2 = 0.33 atm
P (total) = P (H₂ gas) + P (O₂ gas) = 0.333 atm + 0.333 atm = 0.67 atm