Question

How many moles are in a 42.5 g sample of magnesium oxide `MgO`?

Answer 1

Approx. `1.0*mol`

Explanation:

Oxygen has an atomic mass of `16.0*g*mol^-1`; magnesium has an atomic mass of `24.3*g*mol^-1`. We add these masses together, and we get a molar mass for `MgO` of `40.3*g*mol^-1`.

How many `MgO` formula units does such a mass represent?

To find the molar quantity, we simply divide the mass by the molar quantity, to get an answer in `"moles"`:

`"Moles of MgO"` `=` `(42.5*cancelg)/(40.3*cancelg*mol^-1)=1.05*mol`.

The answer is dimensionally consistent; we wanted an answer in moles. We got an answer in `1/(mol^-1)=1/(1/(mol))=mol` as required..........

How many formula units of `MgO` does such a mass represent?