A group 2 metal, M, is reacted with water so that M + 2H2O --> M(OH)2 + H2. 0.162g of metal produces 97.0cm3 of gas at RTP. Identify M.

1 mole of gas is produces for 1 mole of metal used.

n (moles) = volume/24000             ----   This is because 1 mole of a gas occupies 24000cm3 of space, so it's a simple ratio to work out moles(H2).

= 97/24000

= 0.004042 moles.

For group 2 metal, we want to work out the RFM, so we use:
 

RFM = grams/moles               ---  Rearranged from moles = grams/rfm

= 0.162/0.004042

= 40.1 gcm-3

This corresponds to calcium.