An ionic crystal is added to water and starts to dissolve as shown in the particulate representation below. Based on the orientation of the water molecules, what can be assumed about the charge of the hydrated particle indicated by the arrow? The figure shows a particle diagram of an ionic solid made of alternating large shaded circles and medium unshaded circles. Water molecules are represented by a small shaded circle and two smaller black circles. Ten water molecules surround the undissociated ionic crystal. To the right, one large shaded circle is surrounded by 5 H 2 O molecules oriented with the smaller black circles toward the large shaded circle. The medium unshaded circle is surrounded by 4 H 2 O molecules oriented with the small shaded circle toward the medium unshaded circle. An arrow points to the medium unshaded circle.

A student is assigned the task of determining the mass percent of zinc in a sample of brass, which is an alloy of copper and zinc. She places a piece of brass into excess hydrochloric acid, HCl. The copper does not react with hydrochloric acid, but the zinc does.Once the brass is completely reacted away, she precipitates out the zinc using excess NaOH according to this reaction:  Zn2+(aq) +  2OH-(aq) →  Zn(OH)2(s) The precipitate is filtered, washed, dried, and weighed to constant mass in a filter crucible. The data are shown in the table below. Mass of sample of copper-zinc alloy (brass) 2.36 g Mass of dry filter crucible 29.12 g Mass of filter crucible and precipitate (first weighing) 31.07 g Mass of filter crucible and precipitate (second weighing) 30.45 g Mass of filter crucible and precipitate (third weighing) 30.45 g Calculate the number of moles of Zn(OH)2 precipitate collected. Calculate the mass percent of zinc in the brass. Answers must be written in the space provided. Show your work.  Note: 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 provided on scratch paper alone will not be scored.

What is the maximum number of moles of Al2O3 that can be produced by the reaction of 0.40 mol of Al with 0.40 mol of O2?

Suppose a solution was too concentrated for an accurate reading with the spectrophotometer. The concentrated solution was diluted by placing 1.00 mL of the concentrated solution in 4.00 mL of water. The solution was then placed in the spectrophotometer, an absorbance was obtained, and after a few calculations the molar concentration was calculated to be 3.5 × 10-6 M. What was the concentration of the original stock solution before dilution? (Assume volumes are additive.)

How many grams of CaC12 (molar mass = 111 g/mol) are needed to prepare 100. mL of 0.100 M C1- (aq) ions?

5 H2O2(aq) + 2 MnO4-(aq) + 6 H+(aq) → 2 Mn2+(aq) + 8 H2O(l) + 5 O2(g) In a titration experiment, H2O2(aq) reacts with aqueous MnO4-(aq) as represented by the equation above. The dark purple KMnO4 solution is added from a buret to a colorless, acidified solution of H2O2(aq) in an Erlenmeyer flask. (Note: At the end point of the titration, the solution is a pale pink color.) Which element is being oxidized during the titration, and what is the element’s change in oxidation number?

When the equation below is balanced and the coefficients are reduced to the lowest whole-number terms, what is the coefficient of H2O(l)? . . . LiHCO3(aq) + . . . H2SO4(aq) → . . . Li2SO4(aq) + . . . H2O(l) + . . . CO2(g)

C6H12O6 + 6 O2 → 6 CO2 + 6 H2O   The reaction between C6H12O6 and O2 is represented by the balanced equation above. In an experiment, 0.30 mol of CO2 was produced from the reaction of 0.05 mol of C6H12O6 with excess O2. The reaction was repeated at the same temperature and in the same container, but this time 0.60 mol of CO2 was produced. Which of the following must be true?

5 H2O2(aq) + 2 MnO4-(aq) + 6 H+(aq) → 2 Mn2+(aq) + 8 H2O(l) + 5 O2(g) In a titration experiment, H2O2(aq) reacts with aqueous MnO4-(aq) as represented by the equation above. The dark purple KMnO4 solution is added from a buret to a colorless, acidified solution of H2O2(aq) in an Erlenmeyer flask. (Note: At the end point of the titration, the solution is a pale pink color.) Which of the following best describes what happens to the pH of the H2O2 solution as the titration proceeds? 

Compare the mass percent of Cu values that were determined by the two different experimental methods for finding the molarity of the brass solution. Which method do you think will provide a more accurate value? Justify your answer.