Answer:
n = 0,295 mol
Explanation:
I'm pretty sure there is no such thing as cobalt (I) dichromate, so I'm using cobalt (II) dichromate here.
To get the amount of substance, calculate the mass divided by the molar mass:
n = m / M
n = 81 g / 274,92 g/mol
n = 0,295 mol
