In level 3, you balanced the equation NH3 + O2 → NO2 + H2O. What number did you place in front of water?

Balance nitrogen and hydrogen first: using 4 NH3 gives 4 NO2 and 6 H2O, since 4 NH3 contains 12 hydrogen atoms. The products then contain 14 oxygen atoms total, so 7 O2 balances oxygen. The balanced equation is 4 NH3 + 7 O2 → 4 NO2 + 6 H2O, so the number in front of water is 6.

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