G overall 2hcl na 2 co 3 2nacl h 2 o co 2 it is

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2(g) Overall: 2HCl + Na 2 CO 3 2NaCl + H 2 O + CO 2 It is important to note that the stoichiometry between HCl and Na 2 CO 3 is a 2 to 1 ratio. Today, the indicator phenolphthalein (pH range: 8.3~10) and bromocresol green (BCG, pH range: 3.8~5.5) will be used. BCG changes from blue to green, and then yellow. The color at the end-point however is green . If you titrate to the yellow color, you have gone too far. When Na 2 CO 3 reacts with HCl, carbon dioxide (CO 2 ) is produced. This will give us an incorrect endpoint. To correct this, we must remove the CO 2 by boiling the solution when it first starts to turn green. The solution will turn back to blue and then can be titrated to the correct endpoint. II. Procedure a. Standardization of HCl : The TA will provide you a fresh known number of moles of standardized HCl solution. b. Preparation of soda ash unknown : Soda ash unknown has been dried in the oven for at least two hours before the lab starts. Bring one clean and dry weighing bottle to obtain soda ash unknown from TA, and keep in the desiccator.
36 c. Determination of total carbonate in soda ash : Rinse the burette with deionized water, and then rinse with 5-10 mL portion of standardized HCl solution. Fill the burette with HCl solution. d. Using the weigh-by-difference technique, accurately weigh out three soda ash unknown samples of 0.2 to 0.25 g each on a weighing paper (or dish), and transfer quantitatively into 250 mL Erlenmeyer flasks. Note 1: It is NOT a good idea to dissolve the three samples all at once and let them sit. Because sodium carbonate will slowly react with water to give sodium bicarbonate (NaHCO 3 ). e. When you are ready to carry out a titration, dissolve the sample in 25 mL of water, add three drops of phenolphthalein, and titrate with standardized HCl until the solution is colorless. Then add 3 drops of bromocresol green indicator and continue the titration until the color changes from blue to green ( premature endpoint ). At this point, the solution contains a little bicarbonate and a large amount of carbon dioxide. Boil the solution for 5 minutes to remove carbon dioxide. The solution will turn back to blue at this point. Cool to room temperature you may use an ice bath if available and continue the titration dropwise until the color changes to green ( endpoint ). Report % sodium carbonate in the unknown sample.

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