# GCSE Chemistry - 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? Original post by *LifeHappens*
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?

i am not a gcse student but i think the first one (40 one) is the answer of just 1 MgO

so 40*2 =8
so 80 is the answer i suppose
as it asks to calculate 2MgO , you should take that into consideration probably. Original post by Alm_xo
i am not a gcse student but i think the first one (40 one) is the answer of just 1 MgO

so 40*2 =8
so 80 is the answer i suppose
as it asks to calculate 2MgO , you should take that into consideration probably.

I agree with you, most websites and revision guides say that the relative formula mass is 80, not just 40 - I was just confused as to why freesciencelessons said we don't take big numbers into account but he might have meant something else, not sure.

Anyways, thank you  Original post by *LifeHappens*
Anyways, thank you I suggest following the majority's choice and confirming with your teachers )