Stoichiometry Worksheet printable pdf download

Name ______________________

AP CHEM

___/___/___

Chapter 3 Outline

Stoichiometry

Atomic Masses



The modern system of atomic masses, instituted in 1961, is based on carbon-12.



Carbon-12 is assigned the mass of exactly 12 atomic mass units (amu)



The most accurate method currently available for comparing the masses of atoms involves the use of the

mass spectrometer. (LO 1.14)



Atoms or molecules are passed through a beam of high speed electrons. This knocks electrons off the

substance being analyzed and changes it to a positive ion. The electric field accelerates the positive ions

into a magnetic field. Since an accelerated ion creates its own magnetic field, an interaction with the

applied magnetic field causes a change in the path of the ion. The amount of deflection is a function of a

substance’s mass. The most massive ions are deflected the smallest amount, which causes the ions to

separate. A comparison of the positions where the ions hit the detector plate gives very accurate values of

their relative masses.



For example: When isotopes carbon-12 and carbon-13 are analyzed in a mass spectrometer, the ratio of

their masses is found to be: 1:1.0836129 To calculate the mass of carbon-13: (1.0836129)(12 amu) =

13.003355 amu.



Since the modern system is based on carbon-12, it may seem surprising that carbon’s atomic weight is

12.01 instead of 12.0. This is due to the fact that there are three carbon isotopes(

C &

C) and the

atomic weight listed on the periodic table is an average value representing the mass and abundance of each

isotope.

Average Atomic Weight



The atomic weight of an element is the weighted average of the masses of the isotopes of that element.

The weighted average is determined using the abundance and mass of each isotope. Most elements have

more than one naturally occurring isotope.



For example, there are two naturally occurring isotopes of copper, copper-63 (62.93 amu) and copper-65.

(64.93 amu). The natural abundances of the isotopes are 69.17% and 30.83% respectively.



To determine the atomic weight:

Step 1: Multiply the mass number and the relative abundance (as a decimal). The mass of the electron is

insignificant in this calculation and is not used.

isotope name

atomic mass

abundance (as a decimal)

result

copper-63

62.93 amu

0.6917

43.53

copper-65

64.93 amu

0.3083

20.02



Step 2: Add up your results.

Atomic Weight

63.55 amu

Stoichiometry Worksheet

Related Articles

Related forms

Related Categories