How do you determine the formula of a hydrate?

The formula of a hydrate is determined by calculating the ratio of water molecules to the anhydrous compound in the hydrate.

To determine the formula of a hydrate, you need to find out the number of moles of water and the number of moles of the anhydrous compound in the hydrate. This is done by heating the hydrate to remove the water of hydration and then weighing the remaining anhydrous compound. The difference in mass gives the mass of water.

Firstly, weigh a clean, dry crucible and lid. Add a known mass of the hydrate to the crucible and weigh again. Heat the crucible gently at first, then more strongly until the hydrate has decomposed completely. This is indicated by a constant mass when the crucible and contents are cooled and weighed. The difference in mass before and after heating gives the mass of water lost.

Next, calculate the number of moles of water lost and the number of moles of the anhydrous compound left behind. The molar mass of water is approximately 18 g/mol. Divide the mass of water lost by this number to find the number of moles of water. Similarly, find the molar mass of the anhydrous compound (from the periodic table) and divide the mass of the anhydrous compound by this number to find the number of moles of the anhydrous compound.

Finally, find the simplest whole number ratio of moles of water to moles of the anhydrous compound. This is done by dividing both numbers by the smaller of the two. The result is the number of water molecules per formula unit of the anhydrous compound, which gives the formula of the hydrate. For example, if the ratio is 5:1, the formula of the hydrate is written as M•5H2O, where M is the formula of the anhydrous compound.

Remember, safety is paramount when conducting this experiment. Always use tongs to handle hot equipment, and ensure that the crucible is cooled before weighing.