Water of crystallization of BaCl2.xH2O

Material:

• Crystallized barium chloride

Procedure:

1. Wear safety googles.
2. Weight the crucible together with the lid. Record the mass.
3. Add 3g crystallized BaCl2 into the crucible. Weigh the crucible with the hydrated salt. Record the mass.
4. Put a clay triangle on the tripod. Keep the lid slightly open to make sure the water vapor can escape. Put the crucible on the clay triangle. Always use tongs to handle the crucible from this point onwards.
5. Leave the air hole half open in order to have a medium flame to heat the crucible for the first 5 minutes. Then open the air hole and heat the crucible strongly for another 5 minutes. Turn off the Bunsen burner, leave the crucible to cool for about 2 minutes and check the mass of the crucible with lid and its content. Record this mass (m1).
6. Heat the crucible again for 5 minutes. Check the mass (m2). If m2 = m1, this means that all the water has been removed. Otherwise, heat the crucible further and stop to check the mass again every 5 minutes.
7. When it is certain that all the water has been heated off, let the crucible cool for about 2 minutes and weigh the crucible with lid and the anhydrous salt. Record this mass.
8. Repeat the whole procedure and change the mass of crystallized BaCl2 added to 3.5g and 4g for each of the repeat.

Diagram of lab set up:

• DATA COLLECTION AND PROCESSING:

Data collection:

FORMULAE:  

  Mass of water lost =   Mass of the crucible, lid and the hydrate (2)

                                – Mass of crucible, lid and anhydrous salt (5)

  Mass of anhydrous salt = Mass of hydrate – Mass of water lost

Data processing:

Equation:                                 

BaCl2.xH2O → BaCl2 + xH2O

As seen from the equation, the number of water molecules in the crystallized BaCl2 is the number of moles of water lost. Therefore, once we find the ratio of the number of moles of anhydrous BaCl2 and the number of moles of water lost, we can find the empirical formula of the crystallized BaCl2 .

*1 repeat:

Mass of water lost = 44.9 – 44.4 = 0.5 (g)

• The number of moles of water =  (moles)

Mass of anhydrous salt = 3.1- 0.5 = 2.6(g)

• The number of moles of anhydrous salt =  (moles)

*2nd repeat:

Mass of water lost = 40.8 – 40.2 = 0.6 (g)

• The number of moles of water =  (moles)

Mass of anhydrous salt = 3.5 – 0.6 = 2.9 (g)

• The number of moles of anhydrous salt =  (moles)

*3rd repeat:

Mass of anhydrous salt = 4 – 0.6 = 3.4 (g)

• The number of moles of anhydrous salt =  (moles)

Mass of water lost = 45.8 – 40.2 = 0.6 (g)

• The number of moles of water =  (moles)

All the three repeats give the same result, in which the ratio:

Moles of BaCl2 : Moles of H2O

   1 : 2

• CONCLUSION AND EVALUATION:

Conclusion:

• This supports the hypothesis, which is pure prediction from the reality.
• From the processing of raw data above, we can conclude that the ratio of barium chloride and water in the crystal is 1 to 2.
• Therefore, the sample crystal is gypsum, i.e. a hydrate in which 2 water molecules are present for each unit of BaCl2. In other words, the empirical formula of the crystallized barium chloride provided in the lab was BaCl2.2H2O. As the experiment is repeated 3 times with different initial mass, the result is reliable.
• This means:

• This is the percentage composition of water in the initial hydrate

% 

• This is the percentage composition of barium chloride in the initial hydrate.

Evaluation:

Percentage error = 

In this case, all the uncertainties for recorded mass in the table below is  because it’s the result of the subtraction of values with uncertainties , which are mass of the crucible + lid + hydrate and mass of crucible + lid + anhydrous salt.

Evaluation of the results:

• The percentage error of the mass of salt, either as a hydrate or anhydrous salt is relatively small.
• However, the percentage error for mass of water lost is very large. This is a result of the small mass loss of water (as seen in the formula, the value of the reading is inversely proportional to the percentage error). The mass of water lost, in return, is affected by the mass of hydrate used at the beginning as these two variables are related: for each 1 mole of crystal used, 2 moles of water is lost.

Evaluation of the procedure:

• When the crucible is taken out from the cupboard to weigh in step 1, the mass could have been different from that after 2 minutes for cooling after heating. This can affect the mass of the crucible and its content after heating.

• In the data table, the mass of crucible and lid for 2nd repeat is different from that in the 1st and 3rd repeat. When carrying out the experiment, for fear of insufficient time, I used 2 different crucibles at the start, labeled A (m=41.8) and B (m=37.3), for 3.1 and 3.5 g of the sample respectively (after the whole process is done for 3.1 g of the crystal sample, the same crucible A is used for the repeat using 4g of the sample). As the time set for each of the crucible is approximately 2 minutes, crucible A possibly couldn’t have cooled down as much as crucible B did and haven’t come back quite close to the initial mass. This can affect the value of the mass of crucible + lid + anhydrous salt, which in turn affects the value of the mass of anhydrous salt.

• In the procedure, the lid of the crucible was slightly open in order for water vapor to escape. However there is still fume coming out at the end of each repeat. This might have trapped some of the water inside the crucible which can affect the mass of crucible + lid + content after heating.

• Unexpected: During the 3rd repeat, the Bunsen burner went out several times for no clear reason (as the Bunsen burner is in the same place as that in 1st repeat, and the flame wasn’t extinguished during the 1st repeat). This disrupts the process of heating. The crucible needed to be moved to another Bunsen burner. During that time, part of the water vapor trapped inside the crucible, with decrease in temperature, condenses, making the mass of crucible and its content increase. This partly explains why the mass of water lost in 3rd repeat is the same as that in the 2nd repeat  (0.6g) where as the initial mass of crystal used was different.

Improvements:

• Use greater initial mass of the crystallized BaCl2 in order to lessen the percentage error for the variables as much as possible. Suggestion: 5g – 6.5g – 8g.

• Include one more step at the beginning of this experiment. Heat the empty crucible with lid on first for 5 minutes with the medium flame, then with the strong flame for 5 more minutes. Let it cool for 2 minutes. Then take the reading for mass of the crucible and lid. As this is the same process as what we do to the crucible when it has the content in, the mass is more reliable.

• Use only 1 crucible throughout all the repeats.

• Open the lid for 1-2 seconds for each time the crucible is heated to ensure most of the water vapor is given off.

• Make sure the air-conditioner where the balance is put and where the flame is burning is off. This can avoid the unequal heating and also inaccuracies of the reading on the balance.

Sources:

Analytical Chemistry for Technicians -  

Chemistry 122: Percent water in a hydrate