Quantitative Chemistry

Atomic Number

Tells the number of protons and electrons the element contains

Atomic Mass

The average of all the mass numbers of all the

isotopes of that element. The unit is

amu, stands for atomic mass units

Mass Number

The number of protons plus the number of

neutrons found in that element

Isotope

Atoms of the same element with different numbers of neutrons

LecturePLUS Timberlake 7

Calculating Atomic Mass

Percent(%) abundance of isotopes

Mass of each isotope of that element

Weighted average =

mass isotope1(%) + mass isotope2(%) + …

100 100

LecturePLUS Timberlake 8

Atomic Mass of Magnesium Isotopes Mass of Isotope Abundance 24Mg = 24.0 amu 78.70%

25Mg = 25.0 amu 10.13%

26Mg = 26.0 amu 11.17%

Atomic mass (average mass) Mg = 24.3 amu

Mg24.3

LecturePLUS Timberlake 9

Learning Check AT7

Gallium is a metallic element found in small lasers used in compact disc players. In a sample of gallium, there is 60.2% of gallium-69 (68.9 amu) atoms and 39.8% of gallium-71 (70.9 amu) atoms. What is the atomic mass of gallium?

LecturePLUS Timberlake 10

Solution AT7

Ga-69

68.9 amu x 60.2 = 41.5 amu for 69Ga

100Ga-71 (%/100)

70.9 amu x 39.8 = 28.2 amu for 71Ga 100

Atomic mass Ga = 69.7 amu

LecturePLUS Timberlake 11

Finding An Isotopic Mass

A sample of boron consists of 10B (mass 10.0 amu) and 11B (mass 11.0 amu). If the average atomic mass of B is 10.8 amu, what is the % abundance of each boron isotope?

LecturePLUS Timberlake 12

Assign X and Y values:X = % 10B Y = % 11B

Determine Y in terms of XX + Y = 100Y = 100 - X

Solve for X:X (10.0) + (100 - X )(11.0) = 10.8

100 100

Multiply through by 10010.0 X + 1100 - 11.0X = 1080

LecturePLUS Timberlake 13

Collect X terms 10.0 X - 11.0 X = 1080 - 1100

- 1.0 X = -20

X = -20 = 20 % 10B - 1.0Y = 100 - X

% 11B = 100 - 20% = 80% 11B

LecturePLUS Timberlake 14

Learning Check AT8

Copper has two isotopes 63Cu (62.9 amu) and 65Cu (64.9 amu). What is the % abundance of each isotope? (Hint: Check periodic table for atomic mass)

1) 30% 2) 70% 3) 100%

LecturePLUS Timberlake 15

Solution AT8

2) 70%

Solution

62.9X + 6490 - 64.9X = 6350-2.0 X = -140

X = 70%

Ratio

Is the mathematical relationship between two quantities in the form of a fraction or

percentage

Ratio Analysisis essentially concerned with

the calculation of relationships

Law of Conservation of Mass

States that in any reaction, the same amount of mass must be found on both sides of the equation.

Significant Figures

When making measurements, it is

the size of the measurement needed

to be precise.

Sig. Fig. Rule

1) Non-zero numbers are always significant

2) Zeros between non-zero numbers are always significant.

3) All final zeros to the right of the decimal place are significant.

Sig. Fig. Rule4) Zeros that act as placeholders are

not significant.5) All numbers in calculations should

have same sig. fig. go with the smallest sig. fig. number

6) Counting numbers and defined constants do not change.

What’s the Sig. Fig.• 857• 950• 1800• 45.2• 45.0• 92000.0

• 3• 2• 2• 3• 3• 6

•0.00243•505•0.0012300•3.22X105

•1200.0060•0023

•3•3•5•3•8•2

Scientific NotationExpresses numbers as a multiple of two factors-a

number between 1 and 10, and 10 raised to a power or exponent; makes it easier to write a really big number or

really small number.

Round to 3 sig. fig.• 15624• 17853• 19278• 17750• 17650• 24503• 276453

• 15600• 17900• 19300• 17800• 17600• 24500• 276000

Rounding Rules• If digit after sig # is above 5, then

round up• If digit after sig # is below 5 then

leave as is• If digit after sig # is 5 and # after 5 is

not zero, then round up• If digit after sig # is 5 and # after 5 is

zero; if sig # is odd round up and if even then leave as is.

• 1018 exa E• 1015 peta P• 1012 tera T• 109 giga G• 106 mega M• 103 kilo k• 102 hecto h• 101 deka da

• 10-1 deci d• 10-2 centi c• 10-3 milli m• 10-6 micro µ• 10-9 nano n• 10-12 pico p• 10-15 femto f• 10-18 atto a

Dimensional AnalysisA problem

solving method that focuses on the units that are used to

describe matter.

System International Units (SI Units)

Units are a name tag that lets you know what measurement

the number is for. The SI Units are the standard units use throughout the world.

SI Units• Time= second

(s)• Length=

meter (m)• Mass=

kilogram (kg)

• Temperature= Kelvin (K)

• Volume= liter (L) or cubic centimeter (cm3)

• Density= g/cm3