|Purpose:||To determine the percent by weight of water hydrated to a salt.
To establish for formula of a hydrated salt.
|Principles:||Many salts occurring in nature or purchased from chemical suppliers are hydrated; that is, a number of water molecules are bound to the ions in the crystalline structure of the salt. The number of moles of water per mole of a particular hydrated salt is usually a constant. For example, ferric chloride is purchased as FeCl3.6H2O, not as FeCl3; cupric sulphate as CuSO4.5H2O, not as CuSO4. For some salts, heat removes these water molecules:|
|Na2CO3.10H2O --------> Na2CO3 + 10 H2O|
|whereas in others, they cannot be removed, no matter how intense the heat, e.g., FeCl3.6H2O.|
|In sodium carbonate decahydrate, 10 moles of water molecules are bound to each mole of Na2CO3 or 180.20 g of H2O per 105.99 g of Na2CO3. The percent H2O in the hydrated salt is|
X 100% = 62.97% H2O
180.20 g + 105.99 g
|This experiment determines the percent by weight of water in a hydrated salt and its formula.|
|Procedure:||Completion of at least 3 trials is suggested for this experiment. To use your time most efficiently, obtain 3 crucibles and lids, identify each crucible and lid as a matched pair, and simultaneously perform the experiment in triplicate. While one crucible is cooling, another sample can be heated.|
|1.||Support a clean crucible and lid on a clay triangle and heat with an intense flame for 5 minutes. Allow it to cool. If the crucible is dirty, add a few millilitres of 6 M HNO3. (CAUTION: Avoid skin contact, flush immediately with water) and evaporate to dryness. Weight the fired crucible and lid. Handle the crucible and lid with the crucible tongs for the rest of the experiment; do not use your fingers.|
|2.||Add at most 3 g of an unknown hydrate to the crucible and weight it, the lid, and the sample.|
|3.||Return the crucible with the sample to the clay triangle and set the lid off the crucible's edge to allow evolved gases to escape.|
|4.||At first, heat the sample slowly and then gradually intensify the heat. Do not allow the crucible to become red hot. This could cause the anhydrous (dried) salt to decompose. Heat the sample for 15 minutes. Cover the crucible with the lid, cool to room temperature, and weight it, the lid, and the sample.|
|5.||Reheat the sample for 5 minutes. Reweigh it. If the second weighing disagrees by anything over 2% with the first, repeat the heating until a constant weight is achieved.|
|1. A hydrated MgSO4 salt, weighing 3.211 g, is heated in a crucible until reaching a constant weight. The weight of the anhydrous MgSO4 is 1.570 grams.|
|a) Calculate the percent of water in the hydrated MgSO4
|____________ % H2O|
|b) Calculate the moles of H2O removed and the
moles of anhydrous MgSO4 remaining in the crucible.
____________ mol H2O
____________ mol MgSO4
|c) What is the formula for the hydrated MgSO4? ____________|
|2. Anhydrous CaCl2 is used as a dessicant in
dessicators, i.e. it removes water from the air within the dessicator and
forms a hydrated salt. A 16.43 g mass of anhydrous CaCl2
weighed 21.75 grams after being left in a dessicator for several months.
What is the formula of the hydrated CaCl2 salt?
|3. In today's experiment what error in the data is likely
to occur if the hydrated salt is heated too strongly?
|4. How long should the hydrated salt be heated in
removing the water?
|Data Sheet - FORMULA of a HYDRATE|
|Name of the salt ___________________________
1. Mass of crucible and lid ___________ _____________ ____________
2. Mass of crucible, lid and
hydrated salt ___________ _____________ ____________
3. Mass of crucible, lid, and
1st weighing ___________ _____________ ____________
2nd weighing ___________ _____________ ____________
3rd weighing ___________ _____________ ____________
1. Mass of hydrated salt ___________ ____________ _____________
2. Mass of anhydrous salt ___________ ____________ _____________
3. Moles of anhydrous salt ___________ ____________ _____________
4. Mass of water lost ___________ ____________ _____________
5. Moles of water lost ___________ ____________ _____________
6. Percent by weight of volatile
7. Average % H2O in hydrated salt ______________________
8. Mole ratio of anhydrous salt to
9. Formula of the hydrate
|1. If some volatile impurities are not burned off
in Step 1 but are removed in Step 4 is the mass of the anhydrous salt to
high or too low? Explain.
|2. What happens to the sample's reported percent water
if the salt decomposes yielding a volatile product?