1.2 Water of crystallisation

Summary

This video by M sjcam explores the concept of water of crystallization, demonstrating the difference between hydrated and anhydrous salts using copper sulfate pentahydrate as an example. It outlines the experimental procedure to determine water content in a hydrated salt, including steps like measuring mass, heating to constant weight, and calculating moles to find the water of crystallization. The video concludes with an example calculation for barium chloride dihydrate and discusses the assumptions made during the experiment.

Takeaways

• 💧 The water of crystallization refers to the fixed number of water molecules in one formula unit of a salt.
• 🔬 Hydrated salts, such as copper sulfate pentahydrate, contain water molecules as part of their structure, whereas anhydrous salts do not.
• 🔥 Heating a hydrated salt can cause it to decompose into an anhydrous salt and water vapor.
• 📝 The naming of hydrated salts is based on the number of water molecules they contain; for example, 'pentahydrate' indicates five water molecules.
• 🔬 An experimental procedure to determine water of crystallization involves measuring the mass of a crucible, heating the sample, and weighing until a constant mass is achieved.
• ⚖️ The mass of the empty crucible and lid must be measured before adding the sample to determine the mass of the hydrated salt.
• 🔥 Heating the sample to constant mass is crucial to ensure all water of crystallization is driven off.
• 📉 The mass of water lost during heating represents the water of crystallization in the sample.
• 📊 Converting the mass of the anhydrous salt and the water lost into moles helps in determining the ratio of components in the hydrated salt.
• 🧪 The formula for the hydrated salt is derived from the mole ratio of the anhydrous salt to the water molecules.
• 🤔 Assumptions in the experiment include the loss of all mass being due to water, complete removal of water of crystallization, no water absorption by the crucible, and no further decomposition of the anhydrous salt.

Q & A

What is water of crystallization?

Water of crystallization refers to the fixed number of water molecules present in one formula unit of a salt.

What is the difference between copper sulfate pentahydrate and anhydrous copper sulfate?

Copper sulfate pentahydrate is the hydrated form of the salt with five water molecules attached, whereas anhydrous copper sulfate is the dehydrated form obtained after heating, which lacks the water molecules.

Why is the salt called copper sulfate pentahydrate?

The name 'copper sulfate pentahydrate' indicates that there are five water molecules associated with each formula unit of the salt, hence the term 'penta' for five.

What happens when copper sulfate pentahydrate is heated?

Upon heating, copper sulfate pentahydrate decomposes to form anhydrous copper sulfate and water vapor.

What is the purpose of heating to constant mass in the experimental procedure?

Heating to constant mass ensures that all the water of crystallization has been driven off and the mass of the anhydrous salt remains stable, indicating the end of the dehydration process.

How is the mass of an empty crucible and lid measured in the experimental procedure?

The mass of an empty crucible and lid is measured first as a baseline before adding the sample, to later determine the mass of the hydrated sample accurately.

What is the significance of recording the mass of the sample after each heating step?

Recording the mass after each heating step helps to monitor the loss of water and to confirm when the mass has stabilized, indicating that all water of crystallization has been removed.

How can the formula for a hydrated salt be determined from experimental data?

By comparing the masses of the hydrated sample, the water driven off, and the anhydrous salt, and then converting these masses to moles, the ratio of components in the hydrated salt can be determined, leading to the formula.

What is the formula for the hydrated salt BaCl2·xH2O derived from the given data?

The formula for the hydrated salt is determined to be BaCl2·2H2O, indicating barium chloride dihydrate.

What assumptions are made in the experiment to determine water of crystallization?

The assumptions include that all mass lost is due to water loss, all water of crystallization is driven off, the crucible does not absorb water, and the anhydrous barium chloride does not decompose further.