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Units, Constants and Useful Formulas

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Avogadro's Number
-----------------

One Mol is the number of atoms in 12 g of Carbon-12.
This number is called Avogadro's Number, NA and is
equal to:

6.022 x 1023

Atomic Mass Unit(amu)
---------------------

One amu (u) is defined as 1/12 of the mass of an
unbound neutral atom of Carbon-12 in its nuclear
and electronic ground state.

u = 1.660538921(73)×10-24 grams

Note: u is simply the inverse of Avogadro's number
expressed in grams.

= 1/(6.022 x 1023) grams

Molar Mass
----------

The molar mass of any molecular is the sum of the
atomic masses of the atoms in the molecules.

Atomic Weight
-------------

The atomic weight of an element is the weighted
average of the atomic masses of the different
isotopes of an element.

Formulas
--------

# of Mols = Mass (g)/Molar Mass (g/M)

# of molecules/atoms = # of Mols x 6.022 x 1023

Examples
--------

1.  What is the molar mass of Carbon Tetrachloride,
CF4?

MM of C = 12
MM of F = 19

12 + 4(19) = 88 g/mol

2.  Which is the formula for the compound whose
percent composition 47.51% O, 38.62% K and
13.87% N?

# of Mols = Mass/Molar Mass

MM of O  = 16
MM of K  = 39
MM of N  = 14

O: 47.51/16 = 2.97 Mols
K: 38.62/39 = 0.93 Mols
N: 13.87/14 = 0.99 Mols

Compute the ratios:

N:  0.99/0.99 = 1
K:  0.93/0.99 ~ 1
O:  2.97/0.99 ~ 3

Therefore,

K2NO2

3.  What is the formula for a compound that
contains 64.75g Nitrogen and 185.25g Oxygen?

# of Mols = Mass/Molar Mass
64.75/14 = 4.6 Mols of Nitrogen
185.25/16 = 11.575 Mols of Oxygen
The ratio 11.575/4.6 ~ 2.5 (= 5/2)

Therefore, N2O5 is the correct formula.

4.  What is the mass of 8 moles of Sodium Chloride?

Mass = # of Mols x Molar Mass
= 8 x 58.44
= 467.52g

5.  How many moles of atoms are in one mole of
AlPO4?

Mols of P  = 1
Mol2 of O  = 4
Mols of Al = 1
---
6

5.  How many molecules are in 3.6 grams of NaCl?

# of molecules = Mass/Molar Mass
= 3.6/58.44
= 0.062 Mols

# of molecules = Mols x 6.022 x 1023
= 3.7 x 1022 molecules

6.  How many grams are in 1.946 moles of NaCl?

Mass = Mols x Molar Mass
= 1.946 x 58.44
= 113.8g

7.  What is the percent composition of Phosphorus
in Zn3(PO4)2?

MM of P  = 30.97
MM of O  = 16
MM of Zn = 65.38

% = 2(30.97)/[3(65.38) + 2(30.97 + 4(16))] x 100
= [61.94/(197.44 + 189.94) = 61.94/387.38] x 100
= 16%

8.  Calculate the number of molecules in 4.0 Mols of
H2O.

# of molecules = # of Mols x 6.022 x 1023
= 4 x 6.022 x 1023
= 24.088 x 1023 molecules

9.  What is the mass in grams of 1.02 x 1024 atoms
of Manganese (Mn)?

# of Mols = # of atoms/6.022 x 1023
= 1.02 x 1024/6.022 x 1023
= 1.69 Mols

Mass = # of Mols x Molar Mass
= 1.69 x 54.94
= 93g