Post Lab 4 CHM3120C - Sindy SaintClair Lab Partner Pauline...

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Sindy SaintClair Lab Partner: Pauline TA: Siqi Sun Lab 4. Volumetric Determination of Impure Sodium Carbonate (Na 2 CO 3 ) CHM 3120C. 001
P a g e | 2 Introduction Determining the final amount of carbonate (Na 2 CO 3 ) in impure sodium carbonate (soda ash), a titration is conducted using standardized HCl solution. Aqueous HCl is considered to be a rather strong acid thus resulting in nearly complete dissociation (H+ and Cl-). Because of this, when HCl is utilized in a titration, H+ is the titrant. Carbonate in aqueous solution has the ability to accept protons, thus causing it to behave as a base (Seely) . As soon as carbonate receives the H+, a bicarbonate ion is produced: Na 2 CO 3 (aq) + HCl (aq)==== NaHCO 3 (aq) + NaCl (aq) This is just the beginning of the reaction for the titration. The bicarbonate can still accept another proton. This reaction makes carbonic acid which deteriorates to sodium chloride, water and carbon dioxide: NaHCO 3 (aq) + HCl (aq) ==== NaCl (aq) + H 2 O (l) + CO 2 (g) These reactions convey a 2 to 1 ratio of HCl to Na 2 CO 3 . This is shown in the following equation: Overall : 2HCl + Na 2 CO 3 ====== 2NaCl + H 2 O + CO 2 It is important to note that the first two chemical equations exhibit the two equivalence points that take place in this reaction. Thus, phenolphthalein and bromocresol green are two indicators that are used to witness the endpoints (Clark, 2013) . Procedures
P a g e | 3 Qualitative Observations Once the first endpoint was reached (1 mole of HCl per 1 mole of carbonate) the solution with phenolphthalein changed from pink to colorless. Three drops of BCG were added to the Na2CO3 solution, causing it to turn blue. The titration continued, as it continued, the solution changed from teal to green. However, the green hue did not last long before changing quickly to yellow. Once the solution was boiled, the yellow color darkened, but did not go back to green. This occurred in all three trials. Charts, Graphs, & Diagrams Trial 1 Trial 2 Trial 3 Mass of weighing paper 0.2259 grams 0.2292 grams 0.2227 grams Mass of weighing paper and unknown #052 0.4382 grams 0.4418 grams 0.4375 grams Mass of unknown sample 0.2129 grams 0.2126 grams 0.2148 grams Volume of HCl (initial buret reading) 0 10.65-mL 13.20-mL Obtain a fresh known number of moles of standardized HCl solution Obtain soda ash unknown

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