Chapter 8: Chemical composition Chemistry 1020: Interpretive chemistry Andy Aspaas, Instructor

Atomic masses Atomic mass unit, amu: a very small unit of mass in which masses of atoms and molecules are given A single carbon-12 atom has mass of 12 amu But, since there are different naturally occurring isotopes of carbon (carbon-12, carbon-13, carbon-14), the average mass of a carbon atom is larger than 12 amu Average atomic mass (atomic weight) of carbon is 12.01 amu The weighted average of all isotopes in a natural sample

The mole 2 objects = 1 pair, 12 objects = 1 dozen Grouping can allow for large quantities to be more easily counted Laboratory-sized samples of chemicals contain a very, very large number of atoms or molecules The “mole” is a unit chemists use to represent very large numbers of particles (atoms, molecules, ions) Could be used for anything though, just like pair, dozen

What is a mole? 1 mole (mol) defined as number of atoms in exactly 12 g of carbon-12 Avogadro’s number, NA = 6.022 x 1023 particles/mole Just another quantity like Pair = 2 objects Dozen = 12 objects Gross = 144 objects Ream = 500 objects Mole = 6.02 x 1023 objects

How big is Avogadro’s number? 1 mole of chemistry textbooks would cover the surface of the earth to a depth of 300 km If you won 1 mole of dollars when you were born and spent a billion dollars per second, 99.999% would still be left at 90 years old A mole of pennies placed side by side would stretch more than a million light years

Converting between particles and moles Avogadro’s number is the same no matter the type of particles Use 1 mol = 6.022 x 1023 particles as a conversion factor Number of particles in 5.000 mol carbon? (Same as 5.00 mol Fe, H2O, or anything!) Number of moles in 5.21 x 1024 Al atoms?

Converting between moles and mass Since 1 mol = number of atoms in 12 g of carbon-12, 1 mol of natural carbon atoms has a mass of 12.01 g The mass of 1 mol of any element is equal to its atomic weight (from the periodic table) in grams! 1 mol Fe = 55.85 g, 1 mol Na = 22.99 g, etc Use values like this from the periodic table as conversion factors

Molar mass Idea extends past just atoms Once can calculate mass of 1 mol of any molecule too! Just add up atomic weights of all atoms in the molecule, and you get the molar mass of that compound Molar mass of H2O = 2(1.008) + 16.00 = 18.016 g/mol

Mass percent Sometimes useful to know composition of a compound in terms of the masses of elements involved Mass percent: mass of 1 element in 1 mol of the compound divided by mass of 1 mol of the entire compound, times 100% Percents are just fractions multiplied by 100 Practice: mass percentages of each element of ethanol, C2H5OH

Empirical and molecular formulas Empirical formula: formula that describes the simplest ratio of elements in a compound Molecular formula: formula that describes the actual number of atoms of different elements in a single molecule Ex. Butane Molecular formula: C4H10 (actual number of atoms in molecule) Empirical formula: C2H5 (simplest whole-number ratio)

Calculating empirical formulas If given masses of elements in a compound, convert each mass to moles Then divide all mole values by the smallest one to get the ratios of atoms in the compound If all the ratio values are integers, they become subscripts in the empirical formula If there are non-integer values, multiply them all by the smallest integer to make them all integers

Empirical formula calculation practice Determine the empirical formula of the following: A sample of phosphoric acid contains 0.3086 g hydrogen, 3.161 g phosphorus, and 6.531 g oxygen A sample of para-dichlorobenzene contains 5.657 g carbon, 0.3165 g hydrogen, and 5.566 g chlorine.

Empirical formula from percent composition If you’re given percent composition, assume you have a 100-g sample, and convert percentages directly to grams Ex. 52.5% carbon becomes 52.5 g carbon All should add up to 100 g since all percentages must add up to 100% Then, work as before by converting to moles and finding ratios

Calculation of molecular formulas If you know the molar mass, you can convert an empirical formula into a molecular formula Calculate empirical formula mass, and divide by molar mass (molar mass) / (empirical formula mass) = n Multiply n by subscripts in empirical formula to get molecular formula Ex. empirical formula = CH, molar mass = 78