Salts are substances that have the property of readily allowing water molecules to attach themselves to its crystalline structure. The water molecules that combine with the salt can result from exposure to humid conditions. When a substance becomes saturated with water, it is said to be hydrated. A salt that becomes hydrated is called an ionic hydrate. The forces holding the water molecules and crystal lattice together can either be hydrogen bonds, coordinate covalent bonds, or ion-dipole forces. Through heat, these bonds can be separated and the original salt can be recovered. In this experiment, an unknown ionic hydrate was heated in order break any bonds between water molecules and the salt’s crystal lattice to recover the original salt. Then when given the molar mass of the unknown salt, the ratio of H2O to salt in the original hydrate was able to be figured out.
An empty crucible was first heated with a Bunsen burner to get rid of any moisture that would add weight to the crucible. It was then placed in a desiccator to cool down, while not absorbing any moisture from the air. Once cool, the empty crucible was then weighed and the mass was recorded. Next 1 g of unknown ionic hydrate was placed into the crucible and this new mass was recorded. The crucible with ionic hydrate inside was then heated with its lid partially on for 20 minutes. The heat started off low and was steadily increased throughout the 20 minutes. The crucible and ionic hydrate were then allowed to cool in the desiccator. Once cool, they were weighed and this mass was recorded. The crucible and ionic hydrate were heated once again to remove any excess moisture left from the hydrate. The hydrate and crucible were cooled and weighed as before, and if its new mass was less than 0.003 g different than the previous weigh, it was considered to be completely free of water. The remaining product was the anhydrous salt.
Identification code of unknown: B
Mass of empty crucible: 8.766 g
Mass of hydrate sample: 1.057 g
Mass of crucible plus sample before heating: 9.823 g
Mass of crucible and sample…
- After first heating and cooling: 9.674 g
- After second heating and cooling: 9.674 g
Mass of water lost: 0.149 g
Mass percent of water in sample: 14.1 %
Molar mass of anhydrous salt: 110. g/mol
Mass of anhydrous salt: 0.908 g
Number of moles of anhydrous salt remaining in the crucible: 0.00825 mol
Number of moles of water lost: 0.00827 mol
Average number of moles of water per mole of hydrate: 1 mol
Formula of hydrate: X ∙ H2O
In order to find the mass of the crucible and sample together, I simply added the mass of the empty crucible and mass of the hydrate alone. To find the mass of water lost, I subtracted the mass of the crucible and sample after the second heating from the mass of the crucible and sample before heating. In order to find the mass percent of water in the sample, I divided the mass of water lost (0.149 g) by the original mass of the hydrate sample (1.057 g) and then multiplied that answer by 100 to find the percent. To find the mass of anhydrous salt, I subtracted the mass of water lost from the mass of the hydrate sample. To find the number of moles of anhydrous salt remaining in the crucible, I took the mass of anhydrous salt (0.908 g) and divided that by its molar mass (110. g/mol). I performed that same operation to find the number of moles of water lost using a molar mass of 18.01528 g/mol for H2O. Lastly, to find the number of moles of water per mole of hydrate, I divided the number of moles of anhydrous salt remaining by itself and by the number of moles of water lost. This came out to a 1:1.002 ratio, which is very close to 1:1.
My final equation for my hydrate came out to almost exactly a whole number ratio. This could mean I performed the experiment very well, or that I simply got lucky and my numbers ended up coming out even. There is no way for me to check this because I don’t know the formula of the anhydrous salt, which would tell me what ionic hydrate it usually forms. If I knew the formula of the anhydrous salt, then I would be able to check my work and make sure my results came out correctly.
Overall, I think the experiment went smoothly. I was able to heat the crucible without turning it red, which could have caused some of the hydrate to burn away. I only had to reheat the crucible once before I got a constant mass. I did not have any trouble with that step, as I know some people did. Everything went as planned and I am fairly confident my formula is correct.