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Chapter 3: Atoms: The Building Blocks of Matter
Section 3-3: Counting Atoms
THE MOLE CONCEPT
Q: What is a mole:
counting value, like a dozen or a pain
Q: How many items are in a mole?
6.022 x 10^23 items in a mole like 12 items in a dozen and 2 in a pair
Q: Why do we use such a large number?
b/c atoms are so small, we need a lot of them to count them
Q: Does this number have a name?
Yes, Avagadro's number, named after the guy who discovered it, Amedeo Avagadro. It's N(subscript A)
Q: How do we use it? iTS A CONVERSION FACTOR SO pAY ATTENTION TO THE UNITS Atomic mass a.k.a. molar mass (g « mol)
Turns out that atomic mass is the most useful as MOLAR MASS in units of g/mol (2-d unit)
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LI
6.941 <------ 6.941 g/mol can be used to convert from MOL TO Li to g of Li or vise versa
Avogadro’s number (atoms « mol)
N(subscript A) is used for any element! to convert atoms to moles or vise versa
The conversion factor is 6.022 x 10^23 atoms or 1 mole ______________________________ 1 mole or 6.022 x 10^23 atoms
g « atom conversions (two step conversion)
Given same number of grams of Li x 1 mol Li x 6.022 x 10^23 atoms = atoms of Li
6.941 g Li x 1 mole
Sample Problems: 1. How many moles of Ca are in 5.0 g of Ca?
2. What is the mass, in grams, of 3.50 mol of Cu?
1. How many moles of Ag are in 3.01 x 1023 atoms of Ag?
2. How many atoms of Al are in 2.75 mol of Al?
1. What is the mass, in grams, of 7.5 x 1015 atoms of Ni?
2. How many atoms of S are in 4.0 g of S?
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