A 20. mL sample of 0.50 M HC2H3O2(aq) is titrated with 0.50 M NaOH(aq). Which of the following best represents the species that react and the species produced in the reaction?

A student is studying the composition of brass, an alloy made from copper and zinc. He places a piece of brass into excess nitric acid, and the copper reacts to make copper (II) nitrate.  To determine the concentration of the copper (II) ions in solution, he uses colorimetric (spectrophotometric) analysis.  The student first determines the appropriate wavelength to use to analyze the copper (II) ion. Then he analyzes a series of solutions of copper (II) of known concentration with a spectrophotometer that has a 1.00 cm cuvette to generate the following data.  Trial Concentration (M) Absorbance 1 0.10 0.28 2 0.20 0.56 3 0.40 1.12 4 0.50 1.41 unknown Unknown from brass sample 1.66 He determines the appropriate wavelength to analyze iron thiocyanate is 635 mn. What is the frequency of this wave? Use the data in the table to calculate the molar absorptivity constant for the copper (II) ions.  What is the concentration of copper (II) ions in the unknown solution?  If the solution of copper (II) ions analyzed had a volume of 100.0 mL, how many grams of copper reacted with the nitric acid? Show your work. Note: Your answer(s) must be provided in this text box. If you are unable to show your work adequately using the Rich Text Editor, complete your work on scratch paper. Upload a photo of your work in the “Comments” of this exam immediately after you have completed and submitted the exam. You can access the “Comments” through the grade book. Answers on scratch paper alone will not be scored.

A student pipetted five 25.00–milliliter samples of hydrochloric acid and transferred each sample to an Erlenmeyer flask, diluted it with distilled water, and added a few drops of phenolphthalein to each. Each sample was then titrated with a sodium hydroxide solution to the appearance of the first permanent faint pink color. The following results were obtained. Volumes of NaOH SolutionFirst Sample35.22 mLSecond Sample36.14 mLThird Sample36.13 mLFourth Sample36.15 mLFifth Sample36.12 mL Which of the following is the most probable explanation for the variation in the student’s results?

A 0.35 g sample of Li(s) is placed in an Erlenmeyer flask containing 100 mL of water at 25°C. A balloon is placed over the mouth of the flask to collect the hydrogen gas that is generated.   After all of the Li(s) has reacted with H2O(l), the solution in the flask is added to a clean, dry buret and used to titrate an aqueous solution of a monoprotic acid. The pH curve for this titration is shown in the diagram below   A graph plots Base Added, in m L, on the horizontal axis, from 0.0 through 30.0, in increments of 10.0, and, p H on the vertical axis, from 0.0 through 14.0, in increments of 2.0. The graph plots a curve with an increasing trend that rises through the following estimated points: (0, 3.5), (10, 4.5), (20, 5.5), (25, 8), (30, 12), and (34, 12.4). What will be the effect on the amount of gas produced if the experiment is repeated using 0.35 g of K(s) instead of 0.35 g of Li(s) ?

A fuel cell is an electrochemical device, much like a battery. However, a battery is self contained. In other words, it contains all the reactants needed to produce electricity. In contrast, a fuel cell requires a constant supply of one or more reactants.  A methanol fuel cell uses methanol as the main fuel source, and oxygen flows in from the outside air. While some race cars used to burn methanol in an internal combustion engine, a fuel cell converts the stored energy in the methanol into an electrical current instead of a flame. CH3OH(l) + O2(g) → CO2(g) + H2O(l) Identify the oxidation number of the following elements: The carbon in CH3OH The oxygen in CH3OH The oxygen in O2 The carbon in CO2 The oxygen in CO2 The oxygen in H2O Based on your answers in part A, identify which substance in the reaction is oxidized and which is reduced. Balance this equation using whole numbers. Then identify the coefficients in front of each substance in the reaction. CH3OH = O2 =  CO2 = H2O = Show your work. Note: Your answer(s) must be provided in this text box. If you are unable to show your work adequately using the Rich Text Editor, complete your work on scratch paper. Upload a photo of your work in the “Comments” of this exam immediately after you have completed and submitted the exam. You can access the “Comments” through the grade book. Answers on scratch paper alone will not be scored.

What is the molarity of I-(aq) in a solution that contains 34 g of SrI2 (molar mass 341 g) in 1.0 L of the solution?

A student is separating silver from an alloy containing silver and copper. He places 2.023 g of the alloy in excess nitric acid, where the silver reacts according to this reaction: Ag(s) + HNO3(aq)  → H2(g)  + AgNO3(aq) After all the metal is completely reacted, he adds excess sodium chloride to the silver nitrate solution, creating a white precipitate.  The mass of the rinsed and dried precipitate is 1.430 g Write a balanced net ionic equation for the precipitation reaction. Calculate the moles of precipitate produced. What is the percent, by mass, of the silver in the alloy?  The student was expecting the alloy to be 60.0% silver (by mass). What was his percent yield of precipitate?  Show your work for all calculations. Note: Your answer(s) must be provided in this text box. If you are unable to show your work adequately using the Rich Text Editor, complete your work on scratch paper. Upload a photo of your work in the “Comments” of this exam immediately after you have completed and submitted the exam. You can access the “Comments” through the grade book. Answers on scratch paper alone will not be scored.

When the equation below is balanced and all coefficients are reduced to their lowest whole-number terms, the coefficient for O2(g) is . . . C10H12O4S(s) + . . . O2(g) → . . . CO2(g) + . . . SO2(g) + . . . H2O(g)

Reaction 1: CaC2(s) + 2 H2O(l) → C2H2(g) + Ca(OH)2(s)Reaction 2: NaOCl(aq) + 2 HCl(aq) → Cl2(g) + NaCl(aq) + H2O(l)Reaction 3: C2H2(g) + Cl2(g) → C2H2Cl2(g) Reaction 2 occurs when an excess of 6 M HC1(aq) solution is added to 100. mL of NaOCL(aq) of unknown concentration. If the reaction goes to completion and 0.010 mol of Cl2(g) is produced, then what was the molarity of the NaOCL(aq) solution?

C3H8(g) + 4 Cl2(g) → C3H4Cl4(g) + 4 HCl(g) A 6.0 mol sample of C3H8(g) and a 20. mol sample of Cl2(g) are placed in a previously evacuated vessel, where they react according to the equation above. After one of the reactants has been totally consumed, how many moles of HCl(g) have been produced?