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Mass Relationships in Chemical Reactions Chapter 3 Handout 1
Atomic Mass and The Mole (The start of stoichiometry!!!!)
Atomic Mass Units: The atomic mass (atomic weight) of an element is the average of the isotopic masses, weighted according to the naturally occurring abundances of the isotopes of the element. How does that work? Let’s Try: Naturally occurring chlorine is a mixture of two isotopes. In every sample of this element, 75.77% of the atoms are 35Cl and 24.23% are atoms of 37Cl. The accurately measured atomic mass (using a mass spectrometer of 35Cl is 34.9689 u that of 37Cl is 36.9659 u. From these data, calculate the average atomic mass of chlorine. The Mass Spectrometer:
Here is what the mass spectrometer would give us:
What is the atomic mass of neon found on the periodic table? Does the mass spectrometer data match?
The Mole Concept Consider the following approximate atomic masses of some elements. These numbers represent the relative mass of each element. The units are atomic mass units (abbreviated amu, and given the symbol u), a very small unit of mass. H 1.0 u He 4.0 u C 12.0 u O 16.0 u Cu 63.5 u Pb 207 u U 238 u
This scale tells us relative masses, with carbon12 being defined as exactly 12 u. A helium atom has a mass 4 times that of hydrogen. A carbon atom has a mass 3 times that of helium (ratio of 12/4). A uranium atom is almost 20 times heavier than a carbon atom (ratio of 238/12). Now suppose we have a number of atoms (called an amount) that has a mass equal to the atomic masses in grams. Let’s call this amount N atoms. N atoms of hydrogen weigh 1.0 grams What would be the mass of N atoms of helium? What would be the mass of N atoms of oxygen? This reasoning should be clear: The same N atoms (the same amount) of each element would have a mass equal to its atomic mass in grams. Mass N atoms of hydrogen 1.0 g N atoms of helium 4.0 g N atoms of carbon 12.0 g N atoms of oxygen 16.0 g N atoms of copper 63.5 g N atoms of lead 207 g N atoms of uranium 238 g This amount, the number of atoms that has a mass equal to the atomic mass in grams, is called a mole. A mole of an element always has the same amount of atoms, so far designated by N. But a mole does not have to refer only to atoms. We can have compounds in which a molecule (a group of atoms) is the smallest unit. On the atomic mass scale: Mass 1 molecule H2O 18.0 u 1 molecule CO2 44.0 u 1 molecule C6H12O6 180 u Here we find the mass of each molecule (molar mass) by just adding up the atomic masses in the formula.
By the same reasoning illustrated above: Mass N atoms of carbon 12.0 g N molecules of H2O 18.0 g N molecules of CO2 44.0 g N molecules of C6H12O6 180 g N represents the number of elementary units in a mole. What’s the molar mass of ammonium nitrate? Definition of A Mole The mole is the SI unit of amount. An atom of carbon12 has a mass of exactly 12 u. (this defines the amu scale) A mole of carbon12 is the amount of carbon12 having a mass of exactly 12 grams. A mole of any element contains the same number of atoms as is contained in exactly 12 grams of carbon12. A mole of any substance contains the same number of elementary entities as is contained in exactly 12 grams of carbon12. “Elementary entities” may be atoms, molecules, ions, electrons, or any other appropriate unit. Working definition: A mole of any substance contains the number of elementary units (atoms or molecules) which has a mass equal to its atomic or molecular mass in grams. So far, we have designated that amount by the symbol N. The value of N has been determined experimentally. It is known as Avogadro’s number. N = 6.022 x 1023 particles (four significant figures) This is a very large number. To get some idea of its magnitude, consider this example.
If we had a mole of dollars and divided it equally among all the people in the world, how much would each person get? Rounded to 1 significant figure, there are 6 billion people in the world. Obviously, we don’t have a mole of dollars to distribute, but a mole of carbon, 12.0 g, could be held easily in your hand. What does this say about the size of atoms? Mole Relationships Standard Base Units (SBU) 6.022 x 1023 atoms/ molecules = 1 mole= 22.4 L (of a gas)= molar mass (grams) What is the mass of 3.00 moles of aluminum? How many atoms are in 3.00 moles of aluminum? What is the mass of 7.50 x 1020 molecules of methane, CH4?