Dimensional Analysis, Sig Figs, and the Mole Unit VI Dimensional Analysis, Sig Figs, and the Mole

Obj. 1…Formula vs. Molecular Mass Formula mass = total mass of ionic compounds - starts with a metal Molecular mass = total mass of molecular compounds - starts with a non-metal To calculate… - find mass of each element on P.T. (whole #s) - multiply mass by subscript and/or coefficient - add all masses together for total mass

Obj. 1 cont… Al2(SO4)3 S3F6 96 + 114 = 210 amu 54 + 96 + 192 = 342 amu Practice… Al2(SO4)3 S3F6 27 x 2 = 54 16 x 12 = 192 32 x 3 = 96 19 x 6 = 114 32 x 3 = 96 96 + 114 = 210 amu 54 + 96 + 192 = 342 amu molecular mass formula mass

two compounds chemically bonded together… NOT multiply!!! Obj. 1 cont… two compounds chemically bonded together… NOT multiply!!! more practice… CuSO4  5H2O 3CaC2O4 64 32 64 10 80 40 x 3 = 120 16 x 12 = 192 OR 16 x 4 = 64 40 x 1 = 40 64 + 32 + 64 + 10 + 80 = 250 amu 12 x 6 = 72 12 x 2 = 24 120 + 72 + 192 = 384 amu 40 + 24 + 64 = 128 x 3 = 384 amu

1 mole = 6.02 x 1023 units = Avagadro’s # Obj. 2…The Mole a pair = 2 (shoes) a dozen = 12 (eggs) a score = 20 (years) A mole describes a chemical quantity…it’s just a number!!! 1 mole = 6.02 x 1023 units = Avagadro’s # you can have a ‘mole’ of anything…shoes, cars, people etc…

Obj. 2 cont… recall that one p+ / n0 = 1.67 x 10-24 grams therefore…one mole of nucleons = 1 gram ** 1.67E -24 x 6.02E23 = 1 amu = g/mole atomic mass molar mass one mole = ~ 6.02 x 1023 atoms, molecules or ions in a compound ~ molar mass of compound / element ~ 22.4 liters of any gas at STP (standard temp. and pressure) 760 mmHg 0°C 101.3 kPa 273K 1 atm

Obj. 3-7…Mole Conversions use ‘mole map’ to solve mole conversions… ** going away from moles = x ** going toward moles = ÷ X by 22.4  by molar mass Liters Mole Grams  by 22.4 X by molar mass  by 6.02 x 1023 X by 6.02 x 1023 # of atoms/ molecules

Obj. 3-7 cont… Practice, practice, practice… 1.2 x 1024 2.0 moles He = ____________ atoms He 1.2 x 1024 2.0 moles x 6.02E23 = 2 16 = 18 2.0 moles H2O = ____________ grams H2O 36 2.0 moles x 18 = 67.2 liters O2 = ____________ moles O2 3.00 67.2 liters ÷ 22.4 = 2 16 = 18 27 grams H2O = ____________ moles H2O 1.5 27 grams ÷ 18 =

Obj. 3-7 cont… more practice… 6.00 3.61 x 1024 molecules NaCl = ____________ moles NaCl 6.00 3.61 x 1024 molecules ÷ 6.02 x 1023 = 5.0 moles CO2 = ____________ liters CO2 110 5.0 moles x 22.4 = 12 32 = 44 ** 3.61 x 1024 molecules CO2 = ____________ grams CO2 ** 264 3.61 x 1024 molecules ÷ 6.02 x 1023 = 6 moles x 44 grams =

Obj. 8-9…% Composition % composition is the % mass that one particular element contributes to an entire compound. to calculate… (% = part / whole) ~ find total mass of one molecule ~ ÷ mass of element (part) by total mass (whole) ~ x by 100 practice… 14 4 14 48 = 80 amu what is the % of oxygen in NH4NO3? % oxygen = 48 / 80 = 0.6 x 100 = 60% % nitrogen = 28 / 80 = 0.35 x 100 = 35%

Obj. 8-9 cont… hydrated compounds…(chemically bonded to water) ~ calculate total mass of molecule (including water) ~ ÷ mass of water by total mass 64 32 10 80 = 250 amu 64 what is the % of water in CuSO4  5H2O % water = 90 / 250 = 0.36 x 100 = 36%

Obj. 10-11…Empirical Formulas empirical formulas are the lowest whole # ratio of a compound. i.e. the empirical formula for glucose (C6H12O6) would be… CH2O to calculate from % composition… 1. ÷ given % by atomic mass (to find moles) 2. ÷ each mole in step 1 by lowest mole value (to find ratio) 3. values from step 2 = subscripts ~ subscripts MUST be a whole # (multiply if they are not!)

Obj. 10-11 cont… Na SO *** practice… 1. What is the empirical formula of a compound that contains 36.5% sodium, 25.4% sulfur and 38.1% oxygen? Na = 36.5% = 1.59 moles = 2 23 0.794 S = 25.4% = 0.794 moles *** = 1 Na SO 32 subscripts 0.794 2 3 O = 38.1% = 2.38 moles = 3 16 0.794

Obj. 10-11 cont… x 3 = 3 x 3 = 4 Fe O *** more practice… 2. What is the empirical formula of a compound that contains 72.4% iron (Fe) and 27.6% oxygen? Fe = 72.4% = 1.293 moles *** = 1 x 3 = 3 56 1.293 O = 27.6% = 1.725 moles = 1.33 x 3 = 4 16 1.293 MUST be a whole #!!! Fe O 3 4

Obj. 10-11 cont… calculate hydrated compounds (bonded to water) same way… ~ use ‘’ to separate water from crystal ~ since water is a compound, you are finding the coefficient… NOT the subscript! practice… What is the formula of a compound that contains 18.28% Ca, 32.36% Cl and 49.36% H2O? Ca = 18.28% = 0.457 moles *** = 1 40 0.457 Cl = 32.36% = 0.925 moles CaCl  H2O 6 = 2 2 35 0.457 H2O = 49.36% = 2.742 moles = 6 18 0.457