To determine the mass of 0.640 moles of cobalt (Co), you need to use the molar mass of cobalt, which is approximately 58.93 grams per mole.
Now, you can calculate the mass:
Mass (grams) = Moles × Molar Mass
Mass (grams) = 0.640 moles × 58.93 grams/mole ≈ 37.75 grams
So, the mass of 0.640 moles of cobalt is approximately 37.75 grams.