Question

the mass of 112 cm2 of methane at stp

Answer 1

Methane : CH4  : molecular weight = 16
So one mole of methane weighs 16 gm.

At STP, one mole of  an ideal  gas occupies 22.4 litres. 
112 cm³ of methane contains :  112 / 22,400 = 0.005 moles.
   mass of the gas = 0.005 * 16 gm = 0.080 gm = 80 milligrams

Answer 2

1 cc= 1ml
112 cc=112ml= 0.112L

no.of moles= $\frac{Vol. in L at STP}{22.4}$
n=0.112/22.4=0.005 mol

1 mole of Methane weighs 16gm
So, 0.005 mole weighs 16X0.005=0.08gm