Chemistry chapter 3 homework question

Question

Epsom salts, a strong laxative used in veterinary medicine, is a hydrate, which means that a certain number of water molecules are included in the solid structure. The formula for Epsom salts can be written as MgSO₄*xH₂O, where x indicates the number of moles of H₂O per mole of MgSO₄. When 5.061 g of this hydrate is heated to 250^oC, all the water of hydration is lost, leaving 2.472 g of MgSO₄. What is the value of x?

Answer 1

First, we can find the mass of H₂O lost and from that, convert it to moles of H₂O that were present.

5.061 g - 2.472 g = 2.589 g of H₂O

moles H₂O = 2.589 g H₂O x 1 mol H₂O/18 g = 0.1438 moles H₂O

moles MgSO₄ = 2.472 g MgSO₄ x 1 mol MgSO₄/120.4 g = 0.0205 moles MgSO₄

Now we find the ratio of H₂O to MgSO₄: 0.1438 mol/0.0205 moles = 7.01

We now know that the value of x = 7 and the formula for the compound is MgSO₄•7H₂O