Chemistry moles and molars problem

Question

Write two different stoichiometric coefficients that you could use when doing conversions involving the following chemical equation: 4 Na + O2 → 2 Na2O (+ 2 points if you can create three)

Answer 1

The equation is 4 Na + O2 -> 2 Na2O, and it is already balanced (4 Na and 2 O on each side).

A stoichiometric conversion factor is just a ratio built from any two coefficients. With three substances there are three distinct pairings, so you can get the bonus:

1. 4 mol Na / 1 mol O2
2. 4 mol Na / 2 mol Na2O (which reduces to 2 mol Na / 1 mol Na2O)
3. 1 mol O2 / 2 mol Na2O

Each one can also be flipped upside down, so you really have six usable factors. Which orientation you write depends on what you want to cancel: put the unit you are converting FROM on the bottom.

An example of each in use

If you start with 3.0 mol Na:

3.0 mol Na x (1 mol O2 / 4 mol Na) = 0.75 mol O2 needed
3.0 mol Na x (2 mol Na2O / 4 mol Na) = 1.5 mol Na2O produced

Notice the "mol Na" cancels in both because I wrote it in the denominator. That is the entire trick to dimensional analysis - you are not memorizing which way the ratio goes, you are choosing the orientation that cancels the unit you have.

The one thing to be careful about

These ratios are valid only for THIS balanced equation. If you rewrote it as 2 Na + 1/2 O2 -> Na2O, every factor changes, even though it is the same chemistry. Always balance first, then read your ratios off the coefficients you actually wrote down.