Knowing the fact that #color(red)("no. of mole" (eta)=("mass"(m))/("Molar mass"(M))#, just rearrange the formula to find the mass; so that,
#m=etaxxM#
The standard formula of #"density"(rho)=("mass"(m))/("Volume"(V))#. If #eta# is given in the problem, the variable #m# can be substituted with #etaxxM# as reflected in the relationship above. This way, the density formula through the use of #eta# will be presented as;
#rho=(etaxxM)/V#