# The vd for a certain gas is 70. If the molecular formula for this gas is #("CO")_x#, then what is #x#?

##### 1 Answer

Thanks @michael-2 for clarifying what "vd" is.

The **vapor density** of a compound is:

#\mathbf(rho_"vap" = ("mass of n molecules of gas")/("mass of n molecules of H"_2(g)))#

Let's take

#rho_"vap" = (M_(r,"gas"))/(M_(r,"H"_2))# where

#M_r# is therelative molar massof the substance in#"g/mol"# .

So, since we know that

#color(green)(M_(r,"gas") = M_(r,"H"_2) * rho_"vap")#

#= (2xx"1.0079 g/mol")xx70#

#=# #color(green)("141.106 g/mol")#

Based on that, and the molar mass of

#color(blue)(x) = (M_(r,"gas"))/(M_(r,"empirical formula"))#

#= ("141.106 g/mol")/("28.01 g/mol")#

#= 5.04 ~~ color(blue)(5)#