The densities of gases a, b, and c at stp are 1.25 g/l, 2.86 g/l and 0.714 g/l, respectively. calculate the molar mass of each substance. identify each substance as ammonia (nh₃), sulfur dioxide (so₂), chlorine (cl₂), nitrogen (n₂) or methane (ch₄).

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

B. 2

Explanation:

It's important to know what distinguishes an element and also the role that each sub particle of an atom plays. Understanding the concept of isotopes would be your way out in this particular question.

So what are isotopes?

Isotopes are simply atoms of an element with the same atomic number (number of protons) with different number of mass number (sum of number of protons and neutrons). This means the difference between isotopes are the number of neutrons but they are all atoms of the same element.

Examples of isotopes of Hydrogen include; Protium (Number of neutron = 0), Deuterium(Number of neutron = 1) and Tritium (Number of neutron = 2).

So we now unnderstand that what differentates element is the number of portons and not the neutrons. Back to the table, the atoms have two different number of protons ( 6 and 7). This means the correct option is B. 2