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