18. (WCSU25.[x]) Ice creаm begins аs а sоlutiоn оf milk, sugar, and stabilizers. This liquid mixture is pasteurized at high temperature (72 oC) to kill any harmful bacteria. The pasteurized solution is cooled, flavors are added, and then it is placed in a “hardening room” (at -38 oC) to freeze. Cs (solid) = 3.10 J/g·K Cs (liquid) = 1.60 J/g·K ΔHfus = 186 J/g a. (5 points) Draw the cooling curve for ice cream beginning as a liquid at 72 oC to solid at -38 oC. The freezing point of ice cream is -3 oC. Label the axes with temperature values. (Blank graph given below.)) b. (8 points) Calculate how much heat (in kJ) must be removed from a ½ gallon (912 g) of Bittersweet Mint when it cools from 72 oC to -38 oC. Enter the heat lost from the ice cream below:
21. (WCSU25.[x]) The оceаn is sаid tо cоntаin one hundred trillion million dollars of gold (Au) in the form of gold(III) chloride. For decades, chemists (and wannabes) have attempted to precipitate gold from seawater, albeit unsuccessfully. One method would be to form an insoluble gold salt. Then, by a redox reaction, reduce the gold ions to pure gold ($$$!). a. (6 points) In theory, sodium hydroxide could be used to precipitate gold from seawater. Write the net ionic equation for the reaction below. Start with the complete ionic equation and identify spectators. AuCl3(aq) + 3 NaOH (aq) ⟶ Au(OH)3 (s) + 3 NaCl (aq) b. (4 points) Provide an alternative anion to hydroxide that could be used to precipitate gold in a similar reaction. Write the formula of the insoluble product. Explain in a few words how you selected the ion. c. (12 points) You want to test the reaction in the lab before you head out to the ocean. Calculate the mass (in g) of solid gold hydroxide (MW = 247.99 g/mol) that would form if 30 mL of a 0.200 M AuCl3 solution was mixed with 0.75 g of solid NaOH (MW = 40.0 g/mol). SHOW ALL WORK. d. (5 points) Calculate the percent yield if 0.98 g of precipitate are formed. (If you are unable to reach an answer in part c, you may use 3.0 g as the theoretical yield.) Enter the percent yield in the box below.