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Calculations in Chemistry: Calculations in Chemistry:

Calculations in Chemistry: - PowerPoint Presentation

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Calculations in Chemistry: - PPT Presentation

Average Atomic Mass 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 ID: 741511

isotope mass amu average mass isotope average amu abundance isotopes copper atomic iron relative silicon calculation silver atom percentage

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Presentation Transcript

Slide1

Calculations in Chemistry:

Average Atomic MassSlide2

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.

 

IsotopesSlide3

T

here

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?

Isotopes of CopperSlide4

The remainder of the atoms, 30.91 %, have a mass of 64.94 amu

. Slide5

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

.

Average Atomic MassSlide6

Average atomic mass

=

 Σ (mass of isotope × relative abundance)

Σ = sum or to add them all together

Calculation of Average MassSlide7

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 MassSlide8

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]=63.55Calculation of Average MassSlide9

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.Calculation of Average MassSlide10

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%

Practice

Isotope name

Isotope mass (amu)

Percent abundance

Silver-107

106.90509

51.86

Silver-109

108.90470remainder

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%Slide11

Silver = 107.87 amu

Silicon = 28.09

amuIron = 55.84

amu

Check Your AnswersSlide12

Read section 1.4 in the text to support what we did in class today.

pg

27 #1Pg 29 #1-5,9

Homework