Washing soda, a compound used to prepare hard water, is a hydrate, which means that a certain number of water molecules are included in its solid structure. Its formula can be written as Na₂CO₃ ⋅ xH₂O, where x is the number of moles of H₂O per mole of Na₂CO₃.

A 6.51 g sample of washing soda is heated at 125 °C, and all the water of hydration is lost, leaving 1.48 g of Na₂CO₃. What is the value of x?