Chem lab Essay

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Part A:
3. 41 mL of HCl was added to 2.01 g of sodium benzoate solution and a thick solution formed with a white precipitate. After the pH of the solution reached 2, the solution with precipitate was separated by vacuum filtration and dried. After the first drying period the mass of the watch glass and solid was 27.50 g. After the second drying, the solid and watch glass weighed 27.38 g. A constant mass was reached after the second drying as the change in mass was 0.44% indicating a change in mass less that 0.5%. The final mass of the recovered solid was 0.97 g, which in comparison to the theoretical yield of 1.25 g showed a 77.6% yield. Melting point of the recovered solid was also determined to be 118.8-122.8 °C, which shows there may have been some impurity in the solid.
Part B:
A -10-260 °C thermometer was calibrated using 3 calibration points. The freezing point mark was calibrated using crushed ice and distilled water and a final temperature of 0.15°C was recorded with a -0.15°C calculated correction factor (see Table 1). For the boiling point, distilled water was boiled and the thermometer was placed in the vapour and a temperature of 102.1°C was recorded. The correction factor was calculated to be -2.2 °C, taking into account the atmospheric pressure of 757.4 mm Hg. For the 3rd calibration point the melting point of a sample of succinic acid was determined using a melting point apparatus. The melting point range was determined to be 185.4-189.2°C, giving an average melting point of 187.3°C with a correction factor of 0.70°C. See Figure 1. Thermometer Calibration Graph for a plot of the temperatures recorded vs. their correction factors.
Discussion:
Part A:
Sodium benzoate is used to preserve certain foods against bacteria, yeasts and molds and is used to prolong their shelf life. This compound, when ingested has the potential to mix with stomach acid and undergo a chemical reaction. In a lab, sodium benzoate can be combined with hydrochloric acid (HCl) to simulate the stomach environment. The purpose of this experiment is to determine when these two substances are combined, whether a new substance will be formed.
It was predicted that a new substance would form from the combination of sodium benzoate and HCl because when combined, a reaction takes place where protons will transfer from the acid (HCl) to the base (sodium benzoate) and will form benzoic acid, which being water insoluble, will precipitate from the solution. If the pH is low enough, this reaction should occur and since HCl is a strong acid, it is predicted this will occur. In the lab, when 2.01 g of sodium benzoate solution was combined with 3.41 mL HCl (which was when a pH of 2 was reached, similar to the pH of stomach acid), a precipitate did indeed form. 0.97 g of solid was recovered and a melting point range was determined to be 118.8-122.8°C, which supported the formation of benzoic acid.

In conclusion, the lab experiment showed the formation of a new substance, which supported the initial hypothesis statement. The combination of sodium benzoate with HCl forms benzoic acid, a new substance, which may or may not have heath effects on those ingesting sodium benzoate as a preservative in foods.
Part B:
Lab thermometers differ in accuracy and therefore need to be calibrated. Three points were used to calibrate a thermometer including freezing point, boiling point and melting point. The temperatures measured were compared to literature values and a correction factor for the thermometer was determined accounting for any errors in thermometer temperature measurement. These points were plotted onto a graph so that future temperatures taken by the same thermometer can be corrected according to the determined correction factors from the thermometer calibration.
For the freezing point a temperature of 0.15°C was measured, which gave a correction factor of -0.15 in comparison to the literature value of the freezing
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