A student uses visible spectrophotometry to determine the concentration of CoCl2(aq) in a sample solution. First the student prepares a set of CoCl2(aq) solutions of known concentration. Then the student uses a spectrophotometer to determine the absorbance of each of the standard solutions at a wavelength of 510 nm and constructs a standard curve. Finally, the student determines the absorbance of the sample of unknown concentration. The figure presents a graph in the first quadrant of a coordinate plane. The horizontal axis is labeled “Molar Concentration of C o with a positive 2 charge, aqueous,” and the numbers 0 through 0.100, in increments of 0.020, are indicated. The vertical axis is labeled “Absorbance,” and the numbers 0.10 through 0.60, in increments of 0.10, are indicated. The line of best fit is drawn. The line begins where the axes meet. It moves steadily upward and to the right passing through the points 0.020 Molar and 0.11 absorbance, 0.040 Molar and 0.22 absorbance, 0.060 Molar and 0.33 absorbance, 0.080 Molar and 0.44 absorbance, and 0.100 Molar and 55 absorbance. There are 5 data points along the line. The data points at 0 Molar, 0.020 Molar, 0.065 Molar, and 0.100 Molar are on the line. The data point at 0.050 Molar is just below the line. The student made the standard curve above. Which of the following most likely caused the error in the point the student plotted at 0.050 M Co2+ (aq)?

Select the choice that best fits this statement: An oxidation-reduction reaction that is also a synthesis reaction

Which of the following compounds at 25°C and 1.0 atm contains an element in a +1 oxidation state?

2 F2(g) + 2 NaOH(aq) → OF2(g) + 2 NaF(aq) + H2O(l)   A 2 mol of F2(g) reacts with excess NaOH(aq) according to the equation above. If the reaction is repeated with excess NaOH(aq) but with 1 mol of F2(g), which of the following is correct?

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?