Solve the following problems, showing your work and using correct units and significant figures. 5 points each

Transcription

1 Chemistry I Zimmer Name QUIZ: PERCENT COMPOSITION & EMPIRICAL FORMULA Solve the following problems, showing your work and using correct units and significant figures. 5 points each 1. While mining for zinc you isolated 30.00g of the ore zinc nitrate, what is the mass of zinc in this sample? 2. Working as a forensic chemist on an investigation, you found a g sample of a lead compound at the scene and sent it to the lab to get analyzed. Looking over the results, you found it to be made up of 9.65 g of lead and 0.99 g of oxygen. Determine the empirical formula AND name for this compound.

2 3. A patient in the hospital who is dehydrated needs 0.75moles of Na ions to restore his electrolyte level. You have a 1.2M NaCl solution, how many milliliters of this solution should you administer to the patient? 4. When an ionic compound is added to water it will either dissolve or not. If it doesn t dissolve this is because the solid is ( insoluble / soluble ). The particles will be momentarily distributed through the liquid as you swirl the beaker, but it will form a ( heterogenous / homogenous ) mixture. 5. If, instead, the solid is does dissolve, the result is a ( solution / suspension ). The resulting mixture appears ( opaque / transparent ). In this mixture the ionic solid is considered the ( solvent / solute ), and the water is considered the ( solvent / solute ). In order for this to occur (ionic bonds / hydrogen bonding attractions / both ) must break, the energy required to accomplish this is called the ( lattice energy / hydration energy). This process involves an ( increase / decrease / neither ) in potential energy of the system, which is ( favorable / unfavorable ). The second step is the formation of ( ion-dipole / dipole-dipole / dispersion ) attractions between the solute particles and the solvent particles. Since this process involves the formation of attractions it causes an ( increase / decrease ) in the potential energy. The structure formed by water becoming attracted to either the cation or anion of the ionic compound is referred to as a. This is easy to remember because the energy gained when these form is called ( lattice energy / hydration energy ). If the final solution feels warm when you mix it up, than the the solution will ( absorb / release ) heat, which would make the process ( exothermic / endothermic ).

3 Unit 5 Mixed Review Problem Worksheet Percent Composition, Empirical Formula, Molecular Formula, Molarity Unit Conversion: 1000mg = 1g, 1000 ml = 1L 1. Calculate the moles and grams of solute in each solution. 1) 1.0 L of 0.50M NaCl 0.50 mole and 29 g of NaCal 2) 5.0 x 10 2 ml of 2.0M potassium nitrate 1.0 mole and g of KNO3 3) 250 ml of 0.10M calcium chloride 2.5 x 10-2 mole and 2.8 g of CaCl2 4) 2.0 L of 0.30 M sodium sulfate 0.60 mole and 85 g of Na2SO4 2. What is the molarity of a salt solution made by dissolving mg of NaCl in water to make 2.00mL of solution? 2.40 M 3. How many moles of H2SO4 are in 2.50 L of a 4.25 M aqueous solution? 10.6 mole 4. Find the mass of the given element in each of the following compounds: 1) Bromine in 50.0 g of potassium bromide 33.6 g 2) Gold in g of gold (III) selenide Au 5. A sample of silver carbonate has a mass of 62.4 g. What mass of each element could be obtained by decomposing this sample? 48.8 g of Ag, 2.72 g of C, 10.9 g of O 6. A sample of copper chloride is found to contain 5.46 g of copper and 6.10 g of chloride. What is its percent composition? 47.2 Cu, 52.8 Cl 7. A student wants to make a M aqueous solution of silver nitrate using g of silver nitrate. What should be the final volume of the solution? L 8. Find the percent composition for the following: 1) tin (IV) sulfate Sn, S, O 2) lithium acetate 10.5 Li, C, 4.6 H, O 9. Determine the empirical formula for the compounds with the given information: 1) 36.2 aluminum and 63.8 sulfur Al2S3 2) 93.5 niobium and 6.50 oxygen Nb5O2 3) A compound composed of g of copper, g of fluorine CuF2 4) A compound composed of g of manganese, g of nitrogen, and g of oxygen Mn(NO3)2 10. Determine the molecular formula for the following: 1) A compound with molar mass of 54g and empirical formula B2H5 B4H10 2) A compound with molar mass of g and 56.6 K, 8.7 C, 34.7 O K2CO3

4 Unit 5 Mixed Review Problem Worksheet Percent Composition, Empirical Formula, Molecular Formula, Molarity 3) A compound with molar mass g containing 0.70 g of chromium, 0.65 g of sulfur, and 1.30 g of oxygen Cr2(SO4)3

5 TOTAL POINTS: 42 POINTS PRELAB (10PTS) INCLUDING DATA TABLE PROCEDURE NOTES (4pts) CALCULATIONS (8pts) POSTLAB (20pts) Answer key for empirical formula lab questions: 1. Reaction would be MUCH slower because particles in solids only vibrate and cannot collide as frequently in order to transfer electrons. 2. Iodine is much heavier than zinc, and so fewer atoms are present if they are present in equal masses. Furthermore, it turns out that 2x as many iodine atoms are needed to make ZnI2. 3. (no single right answer) masses of previously wet/dissolved substances are probably less accurate than dry ones, because of the risk that some water remains. Masses measured directly in the tube they originated in are probably more accurate than masses that required a transfer step. 4. (depends on class data) if a student had outlier data, they should remark on problems in their procedure 5. statement that gives both experimental and theoretical percent Zn, and compares the 2 6. Note: analysis of error sources may vary depending on which calculation method the student used for finding Zn and I. a. no effect on comp, just may affect rate of reaction b. I2 still remaining in solution will leave during heating step (dangerous). Less zinc consumed, less ZnI2 formed. If Zn is determined using only zinc and product masses, no effect on result. If grams of iodine are used, then the I will be overestimated. c. Mass of product too high, mass of zinc consumed too high also. Will result in Zn being higher because smaller mass is more affected by the increase. d. Leftover zinc would be contaminated by zinc iodide, make mass of zinc consumed look lower than actual and mass of product also lower than actual. Will result in Zn being lower. e. Mass of leftover zinc too high, mass of zinc consumed too low, Zn too low f. Masses of leftover zinc and/or zinc iodide too high. In the former case, see answer to part e. In the latter case, Zn would be too low. g. Mass of product would be lower as iodine vapor leaves tube. comp of whichever element they start their calculation with would be too high, and the other would be too low if they calculated it by subtraction.

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7 Chemistry 1 Name THE ROLE OF ENERGY AND ENTROPY IN DISSOLVING 1. Draw particle- level representations of A) solid sodium chloride and B) aqueous sodium chloride, meaning dissolved in water. A 2. Describe the key characteristics of your drawings in words. Make sure to reference the spacing, organization, and bonding that your drawings illustrate, as well as the motion of the particles that doesn t show on paper. In A, Na+ and Cl- ions should be touching, in an alternating crystal lattice pattern and in a 1:1 ratio, with ionic bonds between them. The ions would be vibrating in place. In B, the ions should be separated from each other and spread out in the water, making ion- dipole bonds with the water (Na+ bonded to the O atom in water, Cl- bonded to the H atoms in water). They are no longer organized in a lattice, and their movement includes rotation and translation. B 3. What bonds or forces are broken during the process of NaCl dissolving in water? Ionic bonds between Na+ and Cl-, hydrogen bonds between water molecules 4. What bonds or forces are formed during this process? Ion dipole bonds between Na+ and water, and Cl- and water 5. What bonds or forces are broken during the process of sugar (a polar molecular compound) dissolving in water? Hydrogen bonds between sugar molecules, and hydrogen bonds between water molecules 6. What bonds or forces are formed during this process? Hydrogen bonds from sugar to water

8 7. Lead(II) sulfate does not dissolve in water. How could you explain this observation? The ion- dipole bonds that Pb2+ and SO 4 2- ions could form with water aren t strong enough to make up for the strength of the ionic bonds that must be broken. In other words, the dissolving process would be too endothermic, and the entropy increase of the system would not be enough to make up for the large endothermic energy change. 8. Would you expect stearic acid (a solid nonpolar molecular compound) to dissolve in water? Why or why not? No, because it can only make LDFs. Since it is solid, these LDFs must be pretty strong (i.e. stearic acid is a large molecule). Water is small, and therefore bad at making LDFs. Therefore, stearic acid molecules are more stable bonded to each other than bonding to water. Again, the dissolving process would be too endothermic, so it doesn t occur. 9. For each pair of substances, circle the one that has higher entropy: a) Ice cube OR liquid water b) Table salt OR curry powder (a mix of spices) c) CH 4 (g) OR C 6 H 14 (l) d) Diamond OR silly putty e) 2 mol H 2 O OR 5 mol H 2 O (at the same temperature) f) nitrogen gas OR air 10. During each change described, does the entropy of the system increase or decrease? a) Evaporation of water increase b) Stirring sugar into your coffee increase c) Burning a match increase d) Ice forming on the surface of a lake decrease e) Cleaning your room decrease f) Warming up your lunch in the microwave increase g) Mixing hot and cold water increase Note: For these last 2 questions, answer them as best you can. If you aren t sure, make a guess. We will have time in class on Monday for you to debate your answers with your classmates.

9 ChemistryI Solutions&EmpiricalFormulas Name: 1. ManydifferentoxidesofironarefoundintheEarth scrust: wüstite FeO magnetite Fe3O4 hematite Fe2O3 Youobtainthesampleofironoxideshownatright.Youareableto performanexperimentthatdeterminesthesampleis69.9ironand 30.1oxygenbyweight.Whichoxideofirondoyouhave?Defendyour choice. 2. Yourexperimentcallsfora200.0mLof0.25Msolutionofmagnesiumphosphate. a. Howmanygramsofmagnesiumphosphateshouldyoumeasureoutbeforeaddingwater? b. Foralaterexperiment,youneed50.0mLof0.10Msolution howmuchofyour0.25mstock solutionshouldyoudilute?!!

10 ChemistryI Name: Solutions&EmpiricalFormulas 3. Circletrueorfalseforeachofthefollowingstatements.Explainyourthinkingasneededinthe spaceprovidedonlyifyouthinkpartofaquestionisambiguousorrequiresexplanation. truefalse Whenyouaddwatertoasolution,itsmolaritydecreases. truefalse Theformationofhydrationshellsarounddissolvedionsisanexothermic process. truefalse Tapwaterisabetterconductorofelectricitythandistilledwaterdueto thepresenceofelectricallychargedionsinsolution. truefalse Whencarbondioxideisdissolvedinwater(likeinacanofsoda),the covalentbondsbetweencarbonandoxygenatomsarebrokeninthe process. truefalse TheZn (s)reactswithi 2(aq)inacidifiedwater,thetransferofelectronsthat occursisanexampleofanendothermicreaction. truefalse Whenasugarcubedissolvesinwater,theentropyofthesolution increases. truefalse Inorderforareactiontoproceedonitsown,you dexpecteitherthe entropyofthesystemtoincrease,theenthalpyofthesystemtodecrease, orboth. truefalse Ifyoursolutioniscloudy&opaque,thesoluteislikelynotsolublegiven theamountofsolventyou veadded.