What is the molecular formula for compound with the ratio of of 1 mole of nitrogen per 2 moles of oxygen if the formula mass for the compound is 138 g/mol?

Question

Hey! Please help, I have no idea how to do this. :)

Answer 1

Hello, Adrianna,

The phrase "1 mole of nitrogen per 2 moles of oxygen" means, for this problem, the mystery compound has twice as many oxygens as it has nitrogens. What is inferred, although not stated, is that the compound only has nitrogens and oxygens. Otherwise, we'd need to start with NO₂ and try adding other elements to see if any combination results in a molar mass of 138 g/mol. Neither of us has the time, nor inclination, to do that.

NO₂ has a molar mass of 46.0 g/mol. We need 138 g/mol. Since we concluded that the mystery compound only has O and N atoms, in the specified ratio of NO₂, we just need to divide 138 by the molar mass of the NO₂ unit:

(138 g/(mole Mystery Compound))/(46 g/mole NO₂ unit) = 3 NO₂ units/Mystery Compound

So the mystery compound has the formula (NO₂)₃, or N₃O₆.

Bob