# Atomic Masses How do you measure something too small to be seen? A scale is needed – what would be a good standard? 12 C 6 1 atomic mass unit (AMU) = 1/12.

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Atomic Masses How do you measure something too small to be seen? A scale is needed – what would be a good standard? 12 C 6 1 atomic mass unit (AMU) = 1/12 mass of Carbon-12

What would be the atomic mass of an element that was twice as heavy as Carbon-12? 24 6 What about one that is half as heavy? The average atomic mass is NOT the mass number…

The Mass Spectrometer - Gas state atoms are ionized at low pressure - The ions are accelerated toward a magnet - Magnet deflects them from straight line - Deflection is inverse of atomic mass - Atoms can be compared by weight

For example: What is the mass of Fluorine? 1.583 X 12.00amu = 19.00amu It takes a magnet 1.583 times as strong to deflect a stream of Fluorine atoms as it does a stream of Carbon-12 atoms

Fluorine is unique All Fluorine atoms have 9 protons and 10 neutrons All other atoms have several isotopes In order to determine the atomic mass – The relative amounts of each isotope must be accounted for

When a pure sample of Chlorine is passed through the device – the following graph is obtained

What does this tell us? Cl-3534.97amu75.53% Cl-3736.97amu24.47% Atomic massAbundance

Atomic mass - includes the fact that not all samples of elements are like Fluorine - not all abundances are like Chlorine - the periodic table value is the average mass of all of those elements in the universe - the units are amu’s Q: How is the periodic table value calculated?

What the?... Y 1 and Y 2 are the masses of each isotope %Y 1 and %Y 2 are the abundances of each isotope

Bromine is a red-orange liquid that has an average atomic mass of 79.90amu. It’s name is derived from the Greek “bromos” which means “stench”. It has two naturally occurring isotopes; Br-79 (78.92 amu) and Br-81 (80.92 amu). What is the abundance of the heavier isotope? Strategy: the two isotopes have to add up to 100%. If Br-81 = X, then Br-79 must equal?... 100 - X

The heavier isotope has an abundance of 49%

Q: If 75% of the Chlorine in the universe is Cl-35 and the rest is Cl-37; should the table entry for Chlorine be closer to 35 or 37? Q: What is Chlorine’s atomic mass? 35 35.453 amu

Masses of atoms and Avagadro’s number Consider H and He Helium is about 4 times as heavy as Hydrogen A sample of 100 Helium atoms should weigh about 4 times as much as 100 Hydrogen atoms A sample of a million Helium atoms should weight about 4 time as much as a million Hydrogen atoms

By that thinking: 4 grams of Helium should contain the same number of atoms as 1 gram of Hydrogen Specifically: 4.003 grams of Helium contains the same number of atoms as 1.008 grams of Hydrogen ** A sample of any element with a mass in grams equal to its atomic mass contains the same number of atoms (N A ), regardless of the identity of the element **

The numerical value of N A is 6.022x10 23 and is known a “Avagadro’s Number” It was obtained by experiments comparing the weights of various gases whose relative weights were already known Q: How many atoms are in a ten gram sample of Titanium?

10.0 g Ti x 48 g Ti 1 mole Ti x 6.022 x 10 23 atoms = 1.26 x 10 23 atoms Q: How many atoms are in a ten gram sample of Titanium?

Avagadro’s Number worth of anything is called a “mole” The mass of one mole of a compound is numerically equal to the sum of all of the atoms in the formula Sugar is C 12 H 22 O 11 Baking Soda is NaHCO 3 Copper nails are just Cu

Mole – Mass Conversions m = MM x n One of the most important equations used in chemistry!!! (m) is the mass in grams (MM) is the molar mass (n) is the number of moles

m = MM x n Aspirin is a compound called acetylsalicylic acid (ASA). Its formula is C 9 H 8 O 4 What is the mass of 0.509 moles of ASA? MM = 180 g/mol C = 12 x 9 = 108 H = 1 x 8 = 8 O = 16 x 4 = 64 m = MM x n = (180 g/mole) x (0.509 moles) = 91.6 g

Same problem done MY way O.509 moles C 9 H 8 O 4 x 1 mole C 9 H 8 O 4 180 g C 9 H 8 O 4 =91.6 g

Some problems are harder than others… A 1.00 g sample of aspirin contains 75.2% by mass of C 9 H 8 O 4 How many moles of ASA are in the sample? Mass of ASA? 0.752 x 1.00g = 0.752 g 0.752 g C 9 H 8 O 4 x 180 g C 9 H 8 O 4 1 mole C 9 H 8 O 4 = 0.00418 moles

How many Carbon atoms are present in a 12.0 g sample of C 9 H 8 O 4 ? 12.0 g C 9 H 8 O 4 x 180 g C 9 H 8 O 4 1 mole C 9 H 8 O 4 x 6.02 x 10 23 molecules x 1 molecule 9 C atoms 3.61 x 10 23 Carbon atoms There is only ONE problem in chemistry!!!

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