Question

UNKNOWN NUMBER Data Trial 1 Trial 2 Mass of the crucible and cover 28.2569 Mass of crucible, cover and sample 28.9369 2806309 30.7449 30.330g 30.328 Mass of crucible, cover and salt after first heating 29.7969 Mass of crucible, cover and salt after second heating 28.7979 Mass of crucible, cover and salt after third heating Calculations Chemical formula of anhydrous salt (given) Mass of hydrate (sample) Mass of anhydrous salt Mass of water liberated Percentage H20 in the hydrate Percentage anhydrous salt in the hydrate Casou Ga Sou 6.689 | 2.1149 Oosung lo 6989 0.134 g 0.4169 20.44% 19.687. 79-56%. 80-32% 0.584 moi 0.590 mol 1. 14 mol 1.09 mol 1.95 mol 1.85mol 2 males Casoye 2H2O Moles of anhydrous salt in 100 g of hydrate Moles of water in 100 g of hydrate Moles of water per mole of anhydrous salt Average Moles of water per mole of anhydrous salt Formula of the Hydrate

Answer 1

1) The experiment performed gives precisely, the waters of crystallization present in a given sample of hydrated salt, by merely dehydrating it, without melting the salt. Since this physical change involves a change in mass, the law of mass conservation is obeyed and thus, the moles of the salt obtained after dehydration, when used to determine the actual mass of calcium sulfate present in the hydrated sample gives the mass difference, from which the moles of water molecules in the sample is calculated. This in turn, is used to determine the no.of water molecules.

The given hydrated calcium sulfate has been found to be a dihydrate from the performed experiment. Though the experiment is simple, its results are arrived at by applying a wide array of fundamental chemistry relations. This can also be extrapolated to find the precise identity of certain chemicals in an impure sample by sheer mass difference when the species causing the mass difference is known. The technique is known as Differential Scanning Calorimetry, and is widely used analytical technique.

An advanced application of the relations in this experiment, in a similar manner is the mass spectrometry.

2) $CaSO_4.2H_2 O _{(s)} \overset{heat}{\rightarrow} CaSO_4 \; _{(s)} + 2 \; H_2 O _{(g)}$

3) If a hydrated salt sample is not heated strongly enough to remove all the water in it, then the mass difference seen after heating will be lower than the actual. This will result in a percentage of water, lower than the actual and in turn, will give a lower moles of water per mole of anhydrous salt.