[VISUAL] The common mineral trona, sodium sesquicarbonate, is used in detergents and in glass making — Physical Chemistry — Thermodynamics Chemistry Question

[VISUAL]

The common mineral trona, sodium sesquicarbonate, is used in detergents and in glass making. The mineral is composed of sodium carbonate, sodium bicarbonate, and water . The objective of this experiment is to determine the formula of the mineral.

To determine the formula, three experiments can be done. The first is the titration of a sample of the compound to determine the relative amounts of carbonate and bicarbonate ions.
x CO₃ 2–(aq) + x H⁺(aq) → x HCO₃–(aq)
(x + y) HCO₃–(aq) + (x + y) H⁺(aq) → (x + y) H₂CO₃(aq)

A second experiment can be done in which the sample is thermally decomposed to sodium carbonate, carbon dioxide, and water.
x Na₂CO3·y NaHCO3·z H₂O (s) → Na₂CO₃ (s) + (y/2) CO₂ (g) + [(y/2) + z] H₂O (g)

Finally, a third experiment can be done in which the sample is reacted with aqueous HCl.
x Na₂CO3· y NaHCO3·z H₂O(s) + (2x + y) HCl(aq) → (2x + y) NaCl(aq) + (x + y) CO₂(g) + 6y (x + y + z) H₂O (l)

Combining the results of these three experiments will give the values of x, y, and z.

Note: This experiment was adapted from N. Koga, T. Kimura, and K. Shigedomi, J. Chem. Educ., 2011, 88, 1309.

Chemicals and Reagents
* Sodium sesquicarbonate
* HCl (aq), hydrochloric acid
* Indicators for titration (phenolphthalein and methyl orange)

Equipment and Glassware
* Analytical balance (± 0.0001 g)
* Volumetric flask, 100 cm₃
* Volumetric pipette, 10 cm₃
* Pipette bulb or pump
* Erlenmeyer flask, 100 cm₃ (3)
* Burette, 50 cm₃
* Burette stand
* Hot plate
* Ice water bath
* Bunsen burner
* Crucible
* Crucible tongs
* Beaker, 100 cm₃ (3)

Procedure

A. Titration of Sodium Sesquicarbonate (SSC) with Hydrochloric Acid
(All mass measurements should be done to the maximum allowed number of significant figures.)
In this part of the experiment you will determine the relative amounts of carbonate ion and bicarbonate ion in a sample of SSC. You will titrate a sample with standardized HCl solution to a phenolphthalein endpoint, which indicates when the carbonate ion has been converted to bicarbonate ion.
x CO₃ 2–(aq) + x H⁺(aq) → x HCO₃–(aq)

Then the resulting solution is further titrated with standardized HCl to a methyl orange endpoint where bicarbonate ion, from the SSC sample and from the first titration step, has been titrated.
(x + y) HCO₃–(aq) + (x + y) H⁺(aq) → (x + y) H₂CO₃(aq)

1. Dissolve a weighed amount of SSC (about 2.5 g) in distilled water in a 100.0 cm₃ volumetric flask. Mix thoroughly and fill with water up to the mark.
2. Transfer 10.0 cm₃ of the SSC solution to a small Erlenmeyer flask using a 10 cm₃ transfer pipet.
3. Add several drops of phenolphthalein solution to the titration sample.
4. Titrate the sample using standardized HCl (~0.1 mol dm–3, known exactly) until the solution turns colorless. Record the volume of the standard solution of HCl required for the titration as V₁ cm₃.
5. Add several drops of methyl orange indicator to the solution from step 4. (The solution will become light yellow.)
6. Titrate the sample solution with standardized HCl until the solution turns red or red-orange. (Note: Students often have problems seeing the methyl orange end point. You should consider doing a test sample before trying to carry out an exact titration.)
7. Add boiling chips to the sample solution and heat the solution to boiling for 1 or 2 min. Cool to room temperature (using a water bath). If the sample solution turned back to yellow, repeat the procedures (6) and (7). If the red coloration did not change, record the volume of the standard solution of HCl required for the second titration as V₂ cm₃.
8. Repeat the procedures (2)–(7).

B. Thermal Decomposition of Sodium Sesquicarbonate
In this portion of the experiment you will determine the percent mass loss on heating a sample of sodium sesquicarbonate. You can combine the results of the thermal decomposition with the titration results in Part A to determine x, y, and z.
1. Record the mass of a crucible or small evaporating dish.
2. Add approximately 1 g to the crucible or evaporating dish and then weigh the sample and dish or crucible precisely.
3. Gently heat the crucible or evaporating dish with a burner flame for 3 min. Then heat with a hotter flame until decomposition is complete. (Be careful that solid pieces do not escape the dish.)
4. After cooling the crucible or dish to room temperature, determine the total mass precisely.
5. Repeat the steps (1) – (4).

C. Reaction of Sodium Sesquicarbonate with Acid
In the third portion of the experiment you will confirm a value for z in x Na₂CO₃ · y NaHCO₃ · z H₂O by decomposing the sample with acid and calculating the percent mass loss in that reaction.
1. Weigh about 0.5 g of SSC (s) and record the mass precisely.
2. Transfer about 20 cm₃ of 1 mol dm–3 HCl solution into a beaker and record the total mass of beaker and HCl precisely.
3. Add SSC to the dilute HCl little by little, avoiding splashing of the solution.
4. After adding all the SSC, allow the solution to stand for 5 min or so.
5. Record the total mass of the beaker and the resultant solution precisely.
6. Repeat the procedures (1) – (5).