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Transcript
1. What does this quotient represent: gfm 1mol Conversion factor, ratio 2. For each blank in these equations, write the conversion factor needed to perform the calculation. Use cancel lines to indicate which units will cancel. a. moles _______________ = mass b. particles _______________ =moles c. volume of gas, L _____________ _____________ = representative particles d. mass _______________ _______________ =volume of gas The Mole The term mole is used to describe a certain amount of matter. That amount is 6.02*1023 of the smallest particles that make up that form of matter. Thus, one mole of an element contains 6.02*1023 atoms; one mole of a molecular compound contains 6.02*1023 molecules; and one mole of an ionic compound contains 6.02*1023 formula units. Chemistry Students must be able to convert from atoms, ions, molecules and formula units to moles, and vice versa. The following exercises will help you develop those skills. Example A How many molecules are there in 6.8 mol if carbon monoxide, CO, gas? SOLUTION The beginning point in problems of this kind should be the fundamental relationship 1 mole = 6.02*1023 representative particles (in this case, molecules). In this problem, however, you are asked to find the number of molecules in 6.8 mol. The problem should be set up so that units will cancel to give the correct units in the answer. GIVEN : 6.8 mol CO 6.8mol 6.02 *10 23 moleculesC O 40.936 *10 23 moleculesC O 1molCO Notice that the answer must have two significant figures since the original measurement (6.8mol) contained only two. Written in standard scientific notation the answer is 4.1*1024 molecules CO. You Try It 1. How many sodium ions are there in 0.482 mol of Sodium Sulfate? 𝟎. 𝟒𝟖𝟐 𝐦𝐨𝐥 𝐍𝐚𝟐𝐒𝐎𝟒 𝟔. 𝟎𝟐 × 𝟏𝟎𝟐𝟑 𝐟𝐨𝐫𝐦𝐮𝐥𝐚 𝐮𝐧𝐢𝐭𝐬 𝟐 𝐍𝐚 + 𝐢𝐨𝐧𝐬 | | 𝟏 𝐦𝐨𝐥 𝐍𝐚𝟐𝐒𝐎𝟒 𝟏 𝐟𝐨𝐫𝐦𝐮𝐥𝐚 𝐮𝐧𝐢𝐭 𝐍𝐚𝟐𝐒𝐎𝟒 Example B How many moles are there in 6.32*1024 molecules of chloroform, CHCl3? SOLUTION Begin with the fundamental relationship 1 mol CHCl3 = 6.02*1023 molecules CHCl3 Then multiply the given number of molecules of chloroform by a conversion factor that will give an answer expressed in moles. 1 mol CHCl 3 6.32 *10 24 molecules CHCl 3 1.05 *101 mol CHCl 3 23 6.02 *10 molecules CHCl 3 The answer should contain three significant figures and should probably be written in a simpler form 10.5 mol CHCl3. You Try it 2. A sample of silver metal contains 1.91*1021 atoms. How many moles of silver is this? 1.91 × 1021 𝑎𝑡𝑜𝑚𝑠 𝐴𝑔 1 𝑚𝑜𝑙 𝐴𝑔 | = 6.02 × 1023 𝑎𝑡𝑜𝑚𝑠 𝐴𝑔 Problems For You To Try 3. How many moles are there in 3.04*1023 molecules of hydrogen, H2, gas? 4. How many phosphorus atoms are there in 1.75 mol of calcium phosphate, Ca3(PO4)2? Gram Formula Mass Atomic, molecular, and formula masses are all relative numbers. That is, they contain no units. Saying that the atomic mass of magnesium is 24 means that its atoms are twice as heavy as those of some other element (carbon) with an atomic mass of 12. It is possible, however, to assign units to atomic, molecular, and formula masses. When the unit gram is used with an atomic mass, we define the gram atomic mass of an element. The gram atomic mass is the mass of one mole of an element. Similarly the gram molecular mass is the mass of one mole of a molecular compound, and the gram formula mass is the mass of one mole of an ionic compound. The term molar mass is a general expression that refers to the gram atomic, molecular, or formula mass of any substance. The exercises below will help you improve your ability to use these concepts. Example A What is the gram molecular mass of phosphoric acid, H3PO4? SOLUTION The formula for phosphoric acid, H3PO4 tells you that one mole of this compound consists of three moles of hydrogen atoms, one mole of phosphorus atoms, and four moles of oxygen atoms. The gram molecular mass of H3PO4 then can be found by first determining the molar mass of the three elements that make up the compound. 1.0 g 3molH 3.0 gH 1molH 31.0 gP 1molP 31.0 gP 1molP 16.0 gO 4molO 64.0 gO 1molO molar mass H3PO4 = 3.0g H + 31.0g P + 64.0g O = 98.0g Therefore the gram molecular mass of H3PO4 is 98.0g. You Try It 5. What is the gram molecular mass of aluminum hydroxide, Al(OH)3? Example B Find the molar mass of iron (III) sulfate, Fe2(SO4)3. SOLUTION Each mole of Fe2(SO4)3 contains two moles of iron atoms, three moles of sulfur atoms, and 12 moles of oxygen atoms. You can find the molar mass of Fe2(SO4)3 by adding the molar masses of each of the three atoms that make up the compound. 55.8 g 111.6 gFe 1molFe 32.1gS 3molS 96.3gs 1molS 16.0 gO 12molO 192.0 gO 1molO The molar mass of Fe2(SO4)3 = 11.6g Fe + 96.3g S + 192.0g O = 399.9g 2molFe You Try It 6. What is the molar mass of sucrose, C12H22O11? 12C = 12(12.01g) = 144.12g 22H = 22 (1.01g ) = 22.22g 11O = 11(16.00 g) =176.00g 342.34g/mol Problems For You To Try 7. Find the gram molecular mass of p-dichlorobenzene, C6H4Cl2. 8. What is the molar mass of calcium bisulfate, Ca(HSO4)2? Converting With Moles Chemists often need to know about the quantitative relationships among the elements and compounds involved in a chemical reaction. These relationships may involve masses and or volumes. For Example, a chemist might want to know what volume of gas is produced when a certain mass of a compound is heated. The concept of mole is the key idea that makes it possible for chemists to deal easily and efficiently with all mass and volume relationships in chemical reactions. At standard temperature and pressure, one mole of a gas has a volume of 22.4L. The following exercise illustrates methods of converting among moles, mass, and volume. Example A What is the mass in grams of 4.52 mol of barium chloride, BaCl2? SOLUTION The problem involves the relationship between the number of grams and the number of moles of a compound. Begin by expressing the conversion factor between these two quantities. 1mol BaCl2 = 208.3g BaCl2 Determine the molar mass first! This is your conversion factor Then write this relationship in such a way as to convert the amount of BaCl2 in moles to its equivalent in grams. 208.3g BaCl 2 4.52mol BaCl 2 941.52g BaCl 2 1 mol BaCl 2 The correct answer, expressed to three significant figures is 942g BaCl2. You Try It 9. In a chemical reaction, 0.397 mol of ethyl chloride (C2H5Cl) is produced. What is the mass in grams of this amount of C2H5Cl? Example B A chemist plans to use 435g of ammonium nitrate (NH4NO3) in a reaction. How many moles of NH4NO3 is this? SOLUTION 1 mol of NH4NO3 = 80.0 g NH4NO3 Use this conversion factor to convert a mass to moles. The problem is as follows. 1 mol NH 4 NO3 435 g NH 4 NO3 5.44 mol NH 4 NO3 80.0 g NH 4 NO3 You Try It 10. A small bottle in the chemistry stockroom contains 43.25g of nickel (II) carbonate, NiCO3. How many moles of NiCO3 is this? Example C What is the volume at STP of 2.66 mol of methane(CH4) gas? SOLUTIN Remember that for any gas at STP the following is true 1 mol = 22.4L To convert the volume of the gas in moles to its equivalent in liters, multiply as shown below. 22.4 L CH 4 2.66 mol CH 4 59.6 L CH 4 1 mol CH 4 Notice that you would obtain the same answer to this problem no matter what gas the problem mentioned. You Try It 11. What is the equivalent in moles of 135L of ammonia (NH3) gas? Problems For You To Try 12. A nurse has been asked to get 0.0465 mole of quinine (C20H24N2O2). What mass of quinine should the nurse obtain? 13. During an electroplating process, 5.8625g of silver is deposited in a steel bar. How many moles of silver is this? 14. A helium filled balloon has a total volume of 136500L at STP. How many moles of helium are in the balloon? 15. A chemist is studying one of five oxides of Nitrogen: N2O, NO, N2O3, NO2 or N2O5. He learns that 250 mL of the gas has a mass of 0.335g. Which oxide is he working with?