Chemical Formula s equations and reaction yields Weighing and counting molecules The laws of chemical combination assert that chemical reactions occur in such a way that the number of atoms of a given type is conserved in every chemical reaction except nuclear reactions It is usually impractical to count the numbers of atoms or molecules in laboratory or industrial scale reactions we must find a way to relate the masses of the reactants and products in those reactions to the numbers of atoms or molecules involved Chemists established a scale of relative atomic masses in the 19th century while developing the laws of chemical combination accuracy of that scale was grealy improved upon in the 20th century using mass spectrometry That relative atomic mass scale must be converted to a macroscopic scale that allows us to count atoms and molecules by weighing The concept and methods that allow us to do this are developed in this section NA Avogadro s number We must be able to relate the relative atomic mass scale to the macroscopic scales used in practice The link between the two scales is provided by Avogadro s number N A defined as the number of atoms in exactly 12 g of 12 C the currently accepted value of which is N A is 6 0221420 10 23 The Mole weighing and counting molecules Relative atomic masses relating them to macroscopic quantities Enables counting atoms and molecules by weighing Avogadro s number NA 6 0221420 x 1023 is the number of atoms in 12g 12C The Mole weighing and counting molecules Relative atomic masses relating them to macroscopic quantities Enables counting atoms and molecules by weighing Avogadro s number NA 6 0221420 x 1023 is the number of atoms in 12g 12C Masses of Avogadro s number of other elements Na relative atomic mass 22 9877 The mass of NA Na atoms is The mass in grams of N A atoms of any element is numerically equal to the relative atomic mass of that element The same conclusion applies to molecules From the relative molecular mass of water the mass of N A molecules of water is 18 0152 g Typical example calculating atomic mass in g NOTE UNITS IN g NOT SI UNIT OF kg The Mole mol Groups atoms molecules into laboratory counting units of number of moles mol Comes from moles Latin for heap or pile Must define moles of what I mole of oxygen ambiguous mole of O2 or mole of O atoms 1 mole of water molecules contain NA of the atomic or molecular species One mole of O2 molecules contains 2 moles of O atoms Molar Mass units g mol 1 g mol Molar mass atomic mass or atomic weight Is the mass of element 1 mole of the element in g Molar mass of 12C is 12g Molar mass molecular weight of molecules H2O 18 0152 g mole 1 2x1 00079g 15 9994g mole 1 Determining the number of moles from weight If a sample of iron weighs 8 232 g then What a mole looks like Fig 2 1 p 39 Using molar quantities If 0 2000 mol of water is needed in a chemical reaction then Moles of water x molar mass mass of water 0 2000 mol H2O x 18 015 g mol 1 3 603 g H2O Simply weigh of 3 603 g H2O But think of 0 2 x NA molecules of water Density rho and molecular size Eureka Eureka Greek I found it Density mass volume not SI units kg m3 Units g cm3 g ml 1cm3 1 ml 10 3 L 1 L 10 6 m3 Molar volume For example the density of liquid benzene C 6H6 is 0 8765 g cm 3 near room temperature Suppose that we wanted to find the mass of benzene contained in a volume that measured 0 2124 L We simply multiply the volume by the density as follows m V g cm 3 cm3 g The water expands until at 0 C it solidifies in a structure which is a very open crystalline structure Molar volumes of gases I mole of gas at room temperature and atmospheric pressure occupies 24 6000 cm3 mol 1 24 6 liters volume n number of moles R universal gas constant 8 3145 J mol K N number of molecules k Boltzmann constant 1 38066 x 10 23 J K k R NA NA Avogadro s number 6 0221 x 1023 mol In chemistry standard condition for temperature and pressure STP IUPAC s standard is a temperature of 0 C 273 15 K 32 F and an absolute pressure of 100 kPa 14 504 psi 0 986 atm Determining the volume occupied by a single molecule and the approximate size of molecule or atom The volume per molecule is the molar volume divided by Avogadro s number for ice this gives Cube root 3 of volume 3 2 x 10 8 cm or 0 32 nm 3 2 Empirical and Molecular Formulas Molecular formula specifies number of constituent atoms in a molecule CO2 C6H6 glucose C6H12O6 Applies to all gases and liquids solids with a well defined molecular structure Empirical formula simplest formula of relative number of atoms Glucose CH2O 1 2 1 benzene CH Cobalt II chloride empirical formula CoCl2 NaCl SiO2 Extended solids like salts called formula unit CoCl2 6H2O hexahydrate Six water molecules incorporated into the formula unit Period Used to separate well defined molecular unit in the solid CoCl2 6H2O hexahydrate CoCl2 2H2O dihydrate The hexahydrate is deep purple in color whereas the anhydrous form is sky blue Because of the ease of the hydration dehydration reaction and the resulting color change cobalt chloride is used as an indicator for water in desiccants Fig 2 2 p 41 Empirical formula and percentage composition The empirical formula H 2 O specifies that for every atom of oxygen in water there are two atoms of hydrogen Equivalently one mole of H 2 O contains two moles of hydrogen atoms and one mole of oxygen atoms The number of atoms and the number of moles of each element are present in the same ratio namely 2 1 The empirical formula for a substance is clearly related to the percentage composition by mass of that substance This connection can be used in various ways Determination of empirical Formula from Mass Composition Reverse the procedure just described and determine the empirical formula from the elemental analysis of a compound 1 1 2 Trichloroethane solvent no longer produced in US ozone O3 hole Empirical Formula determined from elemental analysis by Combustion Empirical Formula determined from Elemental Analysis by Combustion Hydrocarbon contains only C and H Lets determine its empirical formula Hydrocarbon O2 H2O CO2 Determine m in the two tubes in g by weighing before and after combustion experiment Fig 2 3 p 42 Note gas density in g liter 1 Avogadro ratio of molar masses of two gases is ratios of densities at STP Additional information given is the density of the unknown 1 06 g L 1 This information provides a way to calculate the molar mass of the unknown molecule Acetylene …
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