Unknown Solid Postlab 1

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University of Virginia *

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1411

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Chemistry

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Jan 9, 2024

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Unknown Solid Postlab 1 This document has 2 pages. Show all of your work for each question. Sources do not need to be cited using any particular format, but be sure to give enough information that someone could find the source if they wanted to (e.g. URL, textbook title, author, and page number). To Submit Your Work: Go to Gradescope. You will be uploading your work, so be sure to save as a .pdf before uploading. Gradescope does not accept .docx files! For Mastery on this assignment, you must Master 12 or more rubric items. Questions have one rubric item each. For the following 3 problems, identify the solid from the possible unknowns list. If there is not enough information, identify as much as possible (either the cation or the anion). 1. Addition of silver nitrate gave a precipitate, addition of NaOH resulted in a strong choking smell, addition of BaCl 2 yielded a precipitate, and addition of HCl resulted in no reaction. a. NH4Cl; The strong choking smell (ammonia) that occurred upon the addition of NaOH suggests the presence of NH4 ions. The precipitate that occurred with the addition of AgNO3 suggests the presence of a halide ion (the only one present in the unknown salt list is chloride) 2. A flame test resulted in no colored flame. Addition of NaOH resulted in formation of a precipitate, addition of barium chloride resulted in no reaction, and addition of HCl resulted in no reaction. a. MgCl2. All the cations except for Mg in the unknown salt list will produce a color in the flame test. Ca is red, Na is orange, K is lilac, and NH4 is faint green (https://www.soinc.org/sites/default/files/uploaded_files/flametest.pdf). The non-reaction to barium chloride indicates there are no sulfate ions present. The last option is MgCl2 3. A flame test resulted in a brick red flame and addition of barium chloride resulted in no reaction. a. Calcium cations are present. Only calcium out of the ions in the unknown will produce a brick red color in the flame test. The non-reaction to barium chloride indicates there are no sulfate ions present, but there’s not enough information for a positive identification of the negative ion.

For the following 2 problems, determine which qualitative test could differentiate between the two compounds. Defend your choice by stating the expected results for each compound. Possible qualitative tests are: addition of silver nitrate, addition of HCl, addition of barium chloride, addition of NaOH, and flame test. 4. KCl and NaCl a. Flame test. If K cations are present, the flame will turn a lilac hue. If Na cations are present, the flame will turn a color in the range of yellow-orange to orange. 5. NaNO 3 and NaCl a. Addition of silver nitrate. If Chloride ions are present, a white precipitate will form. If NO3 ions are present instead, no precipitate will form. For the following 6 problems, write the balanced reaction, including states of matter (aq), (ppt), (l), or (g). If there is no reaction, simply write “no reaction”. The reactants might not be balanced to start with. 6. HCl(aq) + CaCl 2 (aq) → No reaction 7. 2HCl(aq) + AgNO 3 (aq) → AgCl (ppt) + HNO3 (aq) 8. HCl(aq) + Na 2 CO 3 (aq) → 2NaCl (aq) + CO2 (g) + H2O (l) 9. CaCl 2 (aq) + AgNO 3 (aq) → Ca(NO3)2 (aq) + 2AgCl (ppt) 10. CaCl 2 (aq) + Na 2 CO 3 (aq) → 2NaCl (aq) + CaCO3 (ppt) 11. 2AgNO 3 (aq) + Na 2 CO 3 (aq) → Ag2CO3 (ppt) + 2NaNO3 (aq) And now for the big finale: a 4-bottle problem. You have four bottles of reactants dissolved in water. They are the four compounds from your balancing reactions problems above: HCl, CaCl 2 , AgNO 3 , and Na 2 CO 3 . But the labels of all four bottles have come off! It’s up to you to figure out which compound is in which bottle. You temporarily label the bottles A, B, C, and D. Then you mix a few drops of each bottle with a few drops of each of the other bottles. Here are the results: 12. Which compound is in bottle A? What evidence do you have? Bottles Mixed Result A and B No reaction A and C precipitate A and D bubbles Bottles Mixed Result B and C precipitate B and D precipitate C and D precipitate

a. HCl. When mixed with Na2CO3, CO2 gas is produced, which will create the bubbles seen in the experimental reaction. When HCl is mixed with CaCl2, no reaction will occur, such as in A’s observed non-reaction with B. 13. Which compound is in bottle B? What evidence do you have? a. CaCl2. When mixed with HCl, no reaction occurs, such as in the mixture between A and B. When mixed with the other two substances, CaCl2 will form a precipitate, much like it is seen in the mixtures between B and C and B and D 14. Which compound is in bottle C? What evidence do you have? a. AgNO3. It forms precipitates with all the other compounds, much like it is seen in the mixtures between C and A, C and B, and C and D. 15. Which compound is in bottle D? What evidence do you have? a. Na2CO3; when mixed with A, bubbles are formed. This indicates that a gas was formed. When mixed with HCl, Na2CO3 forms CO2 gas as one of its products. In all of D’s other mixtures, a precipitate is formed. Na2CO3 forms a Ag2CO3 precipitate when mixed with AgNO3, and a CaCO3 precipitate when mixed with CaCl2.

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