What is the formula of the cobalt(II) sulfate hydrate?

Question

A student uses gravimetric determination to find the water of hydration of a hydrated sample of cobalt (II) sulfate. The student’s sample contained 0.0098459 moles of anhydrous cobalt(II) sulfate (CoSO4) and 0.068921 moles of water (H2O). What is the formula of the cobalt(II) sulfate hydrate?

Answer 1

Hey Ooyeon!

So we know the moles of each of the compounds in the problem: the moles of water, as well as the moles of CoSO4. CoSO4 *(_H2O) would be the formula of the hydrate form of CoSO4. When we are given moles and asked to find a formula, think of a ratio between the moles of the parts of the total compound. All we have to do for this problem is divide the moles of anhydrous CoSO4 and H2O by the smallest number of moles, which in this case would be the 0.0098459 moles of anhydrous CoSO4:

0.0098459 mol CoSO4/ 0.0098459 mol CoSO4= 1 mol CoSO4

0.068921 mol H2O/0.0098459 mol CoSO4= 7 mol H2O

This division tells us that for every one CoSO4, we have 7 molar equivalents of H2O. Therefore, the formula of cobalt(II) sulfate hydrate would be CoSO4 *(7H2O).

Hope this helps!