Double-Titration

Chemistry Experiment
Double Indicator Titration 1

Aim
The purpose of this experiment is to find the composition of a sample of sodium carbonate mixture by titration.

Introduction
The reaction between sodium carbonate and hydrochloric acid occurs in 2 stages, with the formation of hydrogen carbonate ion as the intermediate product.

Na2CO3(aq) + HCl [pic] NaHCO3(aq) + NaCl(aq)
NaHCO3(aq) + HCl(aq) [pic] NaCl(aq) + H2O(l) + CO2(g)

[pic]

Requirements
Hydrochloric acid (0.5M), watch glass, spatula, 250 cm3 beaker, wash bottle of distilled water, glass rod, 250 cm3 volumetric flask, filter funnel, dropper, burette, stand, beakers X 2, 100 cm3 pipette, 25 cm3 pipette filler, sodium carbonate mixture, 250 cm3 conical flasks X 3, white tile, phenolphthalein, methyl orange indicator

Procedure
Part I

1. Transfer 10.00g of the sample onto the watch glass and weigh it to the nearest 0.01g.

2. Prepare a solution of the sample in 250 ml volumetric flask.

3. Using the funnel, rinse the burette with the hydrochloric acid and fill it with the same acid. Do not forget to rinse and fill the tip. Record the initial burette reading in the ‘Trial’ column of the Results Table.

4. Using a pipette filler, rinse the pipette with some of the sample solution and carefully transfer 25.00cm3 of the solution to a clean 250cm3 conical flask.

5. Add 2–3 drops of the phenolphthalein indicator solution.

6. Run hydrochloric acid from the burette into the flask, with swirling, until the solution just turn color (Pink to colorless). Recor d the final burette reading.

7. Refill the burette with the hydrochloric acid, and again record the initial burette reading to the nears 0.05 cm3 (one drop).

8. Add 2–3 drops of the Methyl Orange indicator solution.

9. Run hydrochloric acid from the burette into the flask, with swirling, until the solution just turn color (Yellow to Orange). Record the final burette reading. (The first flask may be used as a trial run, because you will probably overshoot the end-point.)

10. Repeat steps 4–9 at least three times more.

11. Empty the burette and wash it carefully immediately after the titration, especially if it has a ground glass tap.

Results and Calculations
Using your data, you can fill in the Results Tables.
Result Table I

|Concentration of the hydrochloric acid |mol dm–3 |
|Molar mass of sodium hydrogen carbonate |g |
|Molar mass of sodium carbonate |g |
|Mass of bottle and contents before transfer, m1 |g |
|Mass of bottle and contents after transfer, m2 |g |
|Mass of the sample, m = (m1 - m2) |g |
|Volume of solution, V |dm3 |

Result Tables II & III

| |Trial |1st |2nd |3rd |
|Final Burette Readings(I)/ cm3 | | | | |
|Initial Burette Readings(I)/ cm3 | | | | |
|Volume used(I)/ cm3 | | | | |
|Mean volume of HCl(aq) used(I)/ cm3 | |
| | | | | |
|Final Burette Readings(II)/ cm3 | | | | |
|Initial Burette Readings(II)/ cm3 | | | | |
|Volume used(II)/ cm3 | | | | |
|Mean volume of HCl(aq) used(II)/ cm3 | |

Calculation
Calculate the composition of the sample by mass.

END