Final answer:
To calculate the mole ratio of water to anhydrous salt, determine the number of moles of each by dividing their masses by their respective molar masses and then divide the number of moles of water by the number of moles of anhydrous salt.
Step-by-step explanation:
To calculate the mole ratio of water to anhydrous salt, you must first determine the number of moles of each substance involved in the reaction. To do this, divide the mass of each reactant by its respective molar mass. For instance, if you have a hydrated compound like AlCl3·6H2O, and you want to determine the mole ratio of H2O to AlCl3, you would calculate the moles of H2O and AlCl3 separately and then divide them accordingly.
As an example, suppose you have 100 grams of hydrated AlCl3·6H2O and you heat it to obtain the anhydrous AlCl3. If the molar mass of AlCl3 is approximately 133.5g/mol and that of H2O is 18g/mol, you would divide the mass of water lost on heating by 18 to find the moles of water and divide the remaining mass of anhydrous salt by 133.5 to find the moles of AlCl3. The mole ratio would be the moles of H2O to the moles of AlCl3.