If 0.42 mole of helium occupies 9.9 L, how many moles must be added to inflate the
balloon to 17.0 L?

0.301 moles

Avogadro's hypothesis:  

In the same T,P and V, the gas contains the same number of molecules  

So the ratio of gas volume will be equal to the ratio of gas moles  

Given

n₁=initial mol of Helium : 0.42

V₁=initial volume : 9.9 L

V₂=final volume : 17 L

Required

number of moles added

Analysis

Use formula :

Solution

moles added :

0.721 - 0.42 =0.301 moles

Paraphrase

0.301 moles must be added to inflate the  balloon to 17.0 L

it is the most commonly accepted one