Question

If you have 200 grams of `Na_2O`, how many moles of `Na_2O` do you have?

Answer 1

You have `3` and a bit moles of sodium oxide.

Explanation:

`"Moles "= " Mass"/"Molar mass"` `=` `(200*g)/(61.98*g*mol^-1)` `=` `??" moles"`

The signal advantage of doing calculations this way, i.e. including units in the calculation is that it adds an extra level of validation. I wanted an answer in `"moles"`. The calculation as given me an answer in `"moles"`, i.e. `(200*cancelg)/(61.98*cancelg*mol^-1)` `=` `?mol`. Why? Because `1/x^-1` `=` `1/(1/x)` `=` `x`

This is an example of dimensional analysis. You can get very good at it and introduce another level of checking your answser.