Calculations in Chemistry Average Atomic Mass Isotopes An

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Calculations in Chemistry: Average Atomic Mass

Calculations in Chemistry: Average Atomic Mass

Isotopes � An element can exist in a number of forms, called isotopes. Isotopes

Isotopes � An element can exist in a number of forms, called isotopes. Isotopes are forms of the same atom that vary in mass as a result of a different number of neutrons.

Isotopes of Copper � There are two naturally occurring isotopes of copper. � One

Isotopes of Copper � There are two naturally occurring isotopes of copper. � One isotope weighs in at 62. 93 amu, the other has a mass of 64. 94 amu. (atomic mass units) � These two isotopes have different proportions in a natural sample of copper. � The lighter isotope is more common with an abundance of 69. 09% of all naturally occurring copper. � What is the percent abundance of the other isotope?

� The remainder of the atoms, 30. 91 %, have a mass of 64.

� The remainder of the atoms, 30. 91 %, have a mass of 64. 94 amu.

Average Atomic Mass � The calculation of the average atomic mass is a WEIGHTED

Average Atomic Mass � The calculation of the average atomic mass is a WEIGHTED AVERAGE. � To find the AVERAGE ATOMIC MASS of an atom, we take into account all of the isotopes that exist and the percentage of each type.

Calculation of Average Mass Average atomic mass = Σ (mass of isotope × relative

Calculation of Average Mass Average atomic mass = Σ (mass of isotope × relative abundance) Σ = sum or to add them all together

Calculation of Average Mass � Since there are two isotopes for copper, we will

Calculation of Average Mass � Since there are two isotopes for copper, we will be adding the contributions of 2 isotopes. (That’s where the Σ sign comes in) � The relative abundance is simply the percentage of the isotope, but in decimal form. � 69. 09% corresponds to a relative abundance of 0. 6909.

Calculation of Average Mass Average atomic mass of copper = (62. 93 amu ×

Calculation of Average Mass Average atomic mass of copper = (62. 93 amu × 0. 6909) [the mass and abundance of isotope #1] + (64. 94 amu × 0. 3091) [the mass and abundance of isotope #2] = 63. 55

Calculation of Average Mass � An AVERAGE atom of copper has a mass of

Calculation of Average Mass � An AVERAGE atom of copper has a mass of 63. 55 amu. � Notice that in this problem, we could predict that the average is closer to the weight of the lighter isotope. Why? � The lighter form of copper is more abundant.

Practice Isotope name Isotope mass (amu) Percent abundance Silver-107 106. 90509 51. 86 Silver-109

Practice Isotope name Isotope mass (amu) Percent abundance Silver-107 106. 90509 51. 86 Silver-109 108. 90470 remainder Isotope name Isotope mass (amu) Relative abundance Silicon – 28 27. 98 92. 21% Silicon – 29 28. 98 4. 70% Silicon – 30 29. 97 3. 09% Isotope name Isotope mass (amu) percentage Iron – 54 53. 94 5. 90% Iron – 56 55. 93 91. 72% Iron – 57 56. 94 2. 10% Iron – 58 57. 93 0. 280%

Check Your Answers � Silver = 107. 87 amu � Silicon = 28. 09

Check Your Answers � Silver = 107. 87 amu � Silicon = 28. 09 amu � Iron = 55. 84 amu

Homework � Read section 1. 4 in the text to support what we did

Homework � Read section 1. 4 in the text to support what we did in class today. � pg 27 #1 � Pg 29 #1 -5, 9

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