Atomic Mass/Percent Composition

Average of Isotopes

Every element typically exists in several isotope forms

Isotopes are atoms of the same element with different numbers of neutrons

The mass recorded on the periodic table is a weighted average of all the naturally occurring isotopes of the element

Mass Spectometer

Instrument used to measure the mass of components of a substance

Example

Chlorine is found in two isotope forms. One has a mass of 34.97 amu, the other a mass of 36.97 amu. The first makes up 75.5% of the sample. What is the average atomic mass of the chlorine?

Percent Composition

In a compound, each element has constant percent of each element by mass (law of definite composition)

What is the percent composition of each element in the compound C10H14O?

Example

What is the mass of carbon in 2.86g of CO2?

What is the mass of hydrogen in 1.85g of water?