Ch 7 Mole & chemical composition

Avogadro’s number & mole conversion Size of atoms, ions, and molecules very small Use the mole to convert a large # of these particles. Mole: SI base unit used to measure the amount of a substance whose # of particles is the same as the # of atoms in 12.0 g of C-12. Avogadro’s number: 6.022 x 1023, which is the # of particles in 1.000mol Used to count any kind of particle. 1.0 mol = 6.022 x 1023

Mole conversion Mol <--> # particles Avogadro's number Molar/formula mass (relate moles to mass) The mass in grams of one mole of a substance Equal to the sum of all the atomic weights in cpd. Units g/mol. Mass <---------> moles <-----> # particles Molar mass Avogadro’s number Convert the following: 6.12 x 1014 formula units ReO2 to mass. 9925g NO2- to # of ions.

Relative atomic mass & chemical formulas. Average atomic mass Weighted average of the masses of all naturally occurring isotopes of an element. Know mass of each isotope and % occurrence = (mass istopeA x %) + (mass isotopeB x %) + … % in decimal form Determine the average atomic mass of chlorine with the following data: Cl-35 mass = 34.969 amu & 75.8%, and the rest Cl-37 mass = 36.996 amu. The relative average atomic can also predict which isotope has the higher relative %. Example: Carbons has 3 isotopes, C-12, C-13 & C-14, with an average atomic mass of 12.01 amu. Which isotope is in highest %?

Chemical formula Gives what element and ratio of elements in a cpd or polyatomic ions From formula can calculate the molar mass of cpd. Units g/mol Determine the number of atoms of each element in the cpd Al2(SO4)3 Determine the molar mass of the following: Na2SO4, Ca3(PO4)2

Formula and % composition. The % by mass of each element in a cpd. Used % composition to determine chemical formula. Determine the % by mass of each element in Mg(NO3)2. Determining empirical formula: Empirical formula: lowest (simplest) ratio of atoms in a cpd. 1. Convert % to mass each element (direct) 2. Convert mass to moles of each (atomic wt.) 3. Divide smallest into all mole amount -> lowest whole # 4. If end in .2 then x5, .25 then x 4, .33 then x 3, .5 then x2 All values.

Determine formula cont. Determine the empirical formula for the following 63.52% Fe, 36.48% S 26.58% K, 35.35% Cr, 38.07% O 29.15% N, 8.333% H, 12.50% C, 50.00% O

Molecular formula Whole # multiple (n) of the empirical formula. Not necessarily the smallest whole number. n determine by dividing formula mass into the molar mass of cpd. Determine molecular formula from each: Molar mass 232.41 g/mol and empirical formula OCNCl Molar mass 120.12 g/mol and empirical formula CH2O