The  molar  mass  of    a, b and  c at  STP is calculated  as  below
At  STP  T  is always=   273 Kelvin and ,P= 1.0 atm 
by  use of  ideal  gas  equation  that  is  PV =nRT
n(number   of moles) = mass/molar mass  therefore  replace   n  in  the  ideal   gas  equation
that  is Pv = (mass/molar mass)RT
multiply  both side  by molar  mass  and  then  divide  by  Pv  to  make  molar mass the  subject of the  formula
that is  molar  mass =  (mass x RT)/ PV
 density is always = mass/volume
therefore  by  replacing  mass/volume  in   the equation  by  density the equation
molar  mass=( density  xRT)/P  where R  =  0.082 L.atm/mol.K
the  molar mass  for  a
= (1.25 g/l  x0.082 L.atm/mol.k  x273k)/1.0atm = 28g/mol
the molar  mass of b
=(2.86g/l  x0.082L.atm/mol.k   x273  k) /1.0  atm  = 64  g/mol
the molar  mass of c
=0.714g/l  x0.082  L.atm/mol.K  x273 K) 1.0atm= 16 g/mol
therefore  the 
   gas  a  is  nitrogen N2   since 14 x2= 28 g/mol
   gas b =SO2  since  32 +(16x2)= 64g/mol
  gas c =   methaneCH4  since  12+(1x4) = 16 g/mol
        
             
        
        
        
<h3>Water</h3>
<em>Specific heat is defined as the amount of heat one gram of a substance must absorb or lose to change its temperature by one degree Celsius.</em>
-  For water, this amount is one calorie, or 4.184 Joules.