quantitative chemistry. atomic number tells the number of protons and electrons the element contains
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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
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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?
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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?
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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
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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%
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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