Chemistry 1: Chapter 3: Moles
Day 26
To identify an atom, use the symbol and the mass number
C-14 and Co-60 identify specific forms of atoms.
If the mass number is not included with the atom symbol, round the atomic mass to
the nearest whole number to represent the mass number.
Ba, having an atomic mass of 137.33, would have a mass number of 137, Ba-137
Ba-140 would be an isotope because it is the same atom but has a different number
of neutrons.
To count atoms, use the mole
Three definitions
1. Atomic mass of an atom in grams (gram atomic mass) also called mole mass
2. Avogadro’s number of particles
3. 22.4 L of any gas at STP (standard temp and pressure-273 K and 101.3 kPa)
Mole is the amount of substance in 12.000000 g of Carbon 12
It is a chemistry dozen because 1 mole = 6.02 x 10 23 particles (NA)
Just like a dozen always containing 12, 1 mole always contains NA particles
So 12 g of C contains NA C atoms while 55.8 g of Fe contains NA atoms of Fe
Since the atoms have different makeups of sub atomic particles, the mole mass will
be different but the number of particles will be the same.
General Mole formula
g of substance = # of moles x mole mass
Show other forms
Given grams x 1mole/mole mass = moles
Given moles x mole mass/ 1 mole = grams
Given moles x NA of particles / 1 mole = # of atoms
Given # of atoms x 1 mole/ NA particles = # of moles
Given grams x NA particles /mole mass = number of atoms
Solving of number of atoms, requires the use of Avogadro’s number
so # of atoms = # of moles x NA