Kitty F.

# Why are we multiplying by eleven or why are we not multiplying 4.5 moles of heptane by 22??

Original question: In a particular reaction, 4.5 moles of heptane burn in the presence of oxygen. C7H16 + 11O2 -> 7CO2 + 8H2O How many moles of oxygen will burn?

You have 4.5 moles of heptane. You also know that for every 1 mole of heptane that burns, 11 moles of oxygen will burn. So, 4.5 moles * 11= 49.5 moles of oxygen, or 50. if you are considering sig figs.

WHY are we not multiplying 4.5 by 22?
When I total the amounts of the individual elements on each side, I count 22 oxygens... not eleven.

REACTANTS:
Carbon: 7
Hydrogen: 16
Oxygen: 22

PRODUCTS:
Carbon: 7
Hydrogen: 16
Oxygen: 22

So why are we multiplying by eleven or why are we not multiplying 4.5 moles of heptane by 22??

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