The moles of B that will be needed to convert 2 moles of A into as many moles of C as possible is 6 moles
Explanation
3A +9B → 5C
The moles of B are calculated using the mole ratio.
That is; from the equation above the mole ratio of A:B is 3:9
If the moles of A required is 2 moles therefore the moles of B
= 2 x9/3= 6 moles