In an experiment, a student was assigned to find the formula of an unknown hydrate that weighed 15.67g. The anhydrate, XCO3 (molar mass = 84.32 g/mol), weighed 7.58 g after all the water was driven off. Determine the moles of water per one mole of anhydrate.

5.0 moles of water per one mole of anhydrate

Explanation:

To solve this question we must find the moles of the anhydrate. The difference in mass between the dry and the anhydrate gives the mass of water. Thus, we can find the moles of water and the moles of water per mole of anhydrate:

Moles Anhydrate:

7.58g  * (1mol / 84.32g) = 0.0899 moles XCO3

Moles water:

15.67g - 7.58g = 8.09g * (1mol / 18.01g) = 0.449 moles H2O

Moles of water per mole of anhydrate:

0.449 moles H2O / 0.0899 moles XCO3 =

5.0 moles of water per one mole of anhydrate

answer is: c. the experiment shows that the red substance experienced a chemical change.

this is chemical change (chemical reaction), because two new substances are formed (metalig liquid and gas), the atoms are rearranged and the reaction is followed by an energy change.

this is example of chemical decomposition.

chemical decomposition (analysis) is the separation of a single chemical compound into its two or more elemental parts or to simpler compounds.  

chemical decomposition is opposite of chemical synthesis.