One mole of stuff represents #6.022140857(74)xx10^23# individual items of that stuff.
Explanation:
#1##mol# of manganese has a mass of #54.94*g#.
Thus, #"Mass"# #=(1.02xx10^24" atoms manganese")/(6.022xx10^24" atoms of manganese "mol^-1)xx54.94*g*mol^-1#