Relative Formula Mass

I'd like to work out the Relative Formula Mass of 2MgO.

On freesciencelessons youtube channel it says the Relative Formula Mass is just 16 + 24 = 40 (because we don't use the big numbers at the front of the compound)

However, on other websites like BBC Bitesize, it says to take the number 2 into account and so do 40 * 2 = 80.

I'm not sure which is correct, do I ignore the number 2 in front of the MgO or do I use it to calculate the Relative Formula Mass?

Reply 1

Scanjo63

You DO NOT take into account the "2" in front of MgO, the formula mass is 40. Not 80. I think what you might be trying to work out is the mass of 2 molws of magnesium oxide which would be 80g.
https://www.science-revision.co.uk/Ar_Mr_mole.html

Reply 2

*LifeHappens*