Washing soda, a compound used to prepare hard water for washing laundry, is a hydrate, which means that a certain number of water molecules are included in the solid structure. Its formula can be written as Na2CO3⋅xH2O, where x is the number of moles of H2O per mole of Na2CO3. When a 1.279 −g sample of washing soda is heated at 25 ∘C, all the water of hydration is lost, leaving 0.474 g of Na2CO3.

Though incomplete in statement, the question requires the student to find x, that is, the number of molecules of water of crystallization.

10

Explanation:

The masses of both anhydrous and hydrated salt was provided in the question as 0.474g and 1.279g respectively. These values are now substituted into the equation shown in the solution in the image attached. Solving that equation for x, the number of molecules of water of crystallization in Na2CO3.xH2O gives the answer as 10 as shown in the image attached.