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Chemical formulas, Molar Mass, Molar Volume & Mr. Shields Regents Chemistry U04 L02 1 Writing Chemical Formulas Subscripts indicate the number of atoms of each element in a representative unit If there is no subscript, it is understood to be 1. (Ex. CaO – 1 Calcium & 1 Oxygen) Subscripts refer to the element immediately preceding them (Ex. H2S – 2 Hydrogen). Subscripts after parentheses refer to everything inside (Ex. Ca(OH)2 – 2 Oxygen & 2 Hydrogen) 2 Chemical Formulas What atoms and how many of each are present in the following compounds? CH4 ? Ba(CN)2 ? Cr2O3 ? Cr(OH)3 ? 3 Number of atoms in formulas Subscript outside ( ) x subscript inside ( ) = # of atoms How man atoms of each element are in Cr2(NO3)3 Cr = 2 N = 1 X 3 = 3 O = 3 X 3 = 9 Total = 14 4 Coefficients Numbers in front of formulas Multiply the total # of atoms for each element in the formula by the coefficient Example: 2H2O 2 X 2 = 4 H’s 2 X 1 = 2 O’s 5 Hydrates A group of salts that have water molecules stuffed in the empty spaces Formulas are distinctive Ex: CuSO45H2O “” means “is associated with” Not a true chemical bond between the water and the compound, but the structure is always the same and the number of water molecules is always the same 6 5CuSO45H2O How many atoms are there? (note the coefficient & the water of hydration) Coefficients x Subscripts 5 X Cu 5XS 20 X O 5x5x2 H 5x5 O Total # atoms = 5+5+20+50+25 = 105 7 CuSO45H2O What is the % water in CuSO4 5H20? First calculate the formula mass: 1 X Cu = 1XS = 4XO = 5x2 H = 5x1 O = 63.546 32.066 15.999 x 4 1.008 x 10 15.999 x 5 Second find the % water (90.075 / 249.687) x 100 = 36.08% water formula mass = 249.687amu 8 Ba(OH)28H2O What is the % water in this compound? Count the atoms first 1 x Ba 2x O 2XH 16 X H 8XO Molar Mass = 315.47 g/mol Then multiply # of atoms by gram atomic mass 137.33 g 32. 2.O1 16.13 128. Ba O H H O % H20 = (144.13/315.47) x 100 = 45.69% 9 10 A Real Mole No, we are not Talking about This kind of mole! 11 The Mole (Mol) is the SI unit for Counting Just like 1 Dozen = 12 of anything 1 Mole = 6.022 X 1023 of anything How much is a mole of pennies? A mole of pennies would Cover the entire Earth to a depth of 300 meters 12 Remember… 1 mole = 6.02 x 1023 atoms = an elements Gram atomic mass 13 The gram molecular mass is obtained by Multiplying the number of atoms of each element By it’s gram atomic mass. Let’s consider hexane… For example the Gram molecular mass of Hexane (C6H14) is 6C + 14H = 6(12.011) + 14(1.008) = 86.178g So … 1 mole = 86.178g = 3 moles = 258.534g = And 1/3 mole = ? 6.022 x1023 molecules 1.807 x 1024 molecules 14 MOLAR MASS Definition: Molar mass is the mass in grams of one mole (or 6.02 x 1023 molecules) of any element or chemical compound Therefore, the molar mass = gram molecular mass or gram formula mass or gram atomic mass of the element or compound What is the molar mass of Sr(C2H3O2)2 ? 87.62 + (4*12.01) + (6*1.01) + (4*16) = 205.72 g/mol 15 MOLAR MASS Problem: If the molar mass of Sr(C2H3O2)2 is 205.72g/mol then how many moles are in 134.75g? 134.75g / 205.72g/mol = 0.66 moles 16 17 Molar Volume # N2 100L = # CO2 100L Back to Avogadro … Avogadro proposed that equal volumes of gases at the same temp and pressure contained equal numbers of particles. This was known as AVOGADRO’S HYPOTHESIS For example: 100 liters of N2 at the same temperature and pressure has the same # of molecules as 100 liters of CO2 18 Molar Volume 22.4L So 1 mole of any gas occupies the same volume as 1 mole of any other gas at the same temp and pressure Which means 6.023 x 1023 molecules of any gas occupies the same volume for any gas The volume occupied by one Mole of any gas = 22.4 Liters This is called the MOLAR VOLUME (Abbreviated “ Vm “) 19 Molar Volume If equal volumes of a gas have An equal no. of molecules Molar mass of CO2 = 44g And … 22.4L equals one mole of any gas Then… 22.4L contain a molar mass of any gas AT STP ! 20 Molar Volume Problems PROBLEM 1: If one mole of a gas at STP occupies 22.4L, how many liters will two moles occupy? 22.4 L x 2 Moles = no. of liters = 44.8 Liters 1 mole Problem 2: How many moles are there in 14L of O2 at STP? 14 Liters / 22.4L 1 mole = # mole = 0.625mol PROBLEM 3: What is the mass of 1L of O2? (1L / 22.4L) x 32 = 1.43g 21