Chemistry: A Molecular Approach (3rd Edition)

Its formula is $Co_{3}(PO_{4})_{2}.8H_{2}O$
The cation of cobalt(II) phosphate octahydrate has a charge of (II), i.e it is $Co^{2+}$. The anion is phosphate, i.e $PO_{4}^{3-}$. To balance the charges, 3 $Co^{2+}$ combine with 2 $PO_{4}^{3-}$. Then it also has 8 (octa) water molecules of hydration. So, its formula is $Co_{3}(PO_{4})_{2}.8H_{2}O$