Hi, I'm confused about what they're discussing in this question. Could you explain? I'm also confused about how you would describe the shape. And I am a bit confused about the question in general.

Shape of the graph indicates that after 40 g of Na used ,adding more Na has no effect on the reaction. Production of more NaCl stops and it means the other reactant Cl₂ is completely used up. So Cl₂ becomes the Limiting reactant.

b) When 20 g of Na is added,Cl₂ is the excess reactant and we can calculate the amount of NaCl formed using 20 g Na.
20 g of Na ----> moles of Na------> moles of NaCl ----> g NaCl
Use balanced equation to know the molar ratio.
C.40 g of the Na completely reacted with all Cl₂ .We can use this information to get mass of Cl2 in each container.
40g Na ---> moles of Na-----> moles of Cl₂ ----> g of Cl₂

d) Mass of NaCl formed at 50 or 60 g of Na added is the same as of 40g of Na.Reaction stops after reaction with 40 g of Na .
40 g of Na ----> moles of Na------> moles of NaCl ----> g NaCl
e) In part b Na is the Limiting reactant and Cl is in excess.
Convert -20g Na ---> moles of Na-----> moles of Cl₂ ----> g of Cl₂ (used)
Subtract this amount from the amount of Cl2 calculated in C.[/b]
In d Cl₂ is the limiting reactant
so mass of Cl₂ will be zero after reacting with 40 g of Na .So mass of Na left will be (50-40) g.