The RFM (Relative Formula Mass) of the substance H2O2 is:
H2 = 1 x 2
= 2
O2 = 16 x 2
= 32
32 + 2 = 34
USE THIS FORMULA:
*Mass = Moles x RFM*
Part A:
Mass = Moles x RFM
= 2.5 x 34
= 85 g
(You can always rearrange this formula to suit the question)
Part B:
Moles = Mass/RFM
= 30/34
=0.8823529........ (Need a calculator haha)
=0.88 (Rounded up to 2 decimal places)
I hope I helped, ask if you have any questions.