Question

how does the law of conservation of matter and mole ratio relate to the composition of a hydrate

Answer 1

The law of conservation of matter state that- chemical reaction in an isolated system, the total mass of the reactants or starting materials must be equal to the mass of the products.

A hydrate is an ionic compound that contains water molecules in its structure. To determine the formula of a hydrate experimentally, we must calculate the mole: mole ratio of the water portion compared to the anhydrate portion. An anhydrate is the substance that remains after the water is removed from a hydrate. When a hydrate is heated the water molecules are driven off as steam, leaving behind the water-free anhydrate.

Ex. A hydrate of magnesium sulfate has a mass of 6.76 g. After heating for several minutes the mass of the anhydrate is found to be 3.30 g. using this information to determine the formula for the hydrate.

The mass of the water driven off:

Mass of hydrate – mass of anhydrate = mass of water

6.76 g – 3.30 g = 3.06 g

Convert the mass of anhydrate to moles:

3.30gm of MgSO₄ /1 * 1mol MgSO₄ / 120.36 g MgSO₄ = 0.0274 mole anhydrate

Convert the mass of water to moles:

3.06 gm of H₂O/1 * 1mol H₂0 / 18 g H₂O = 0.17 mole water

The mole H₂O to mole MgSO₄ ratio =

0.17 mole water/ 0.0274 mole MgSO_{4 =} 6 mole of water/ 1mole of MgSO₄

Since the compound contains 6 moles of water for every 1 mole of anhydrate

the formula is MgSO₄ · 6 H₂O