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Units, Constants and Useful Formulas
Avogadro's Number
-----------------
One Mol is the number of atoms in 12 g of carbon-12. This number
is called Avogadro's Number, N_{A} and is equal to:
6.022 x 10^{23}
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 10^{23}) 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 10^{23}
Examples
--------
1. What is the molar mass of Carbon Tetrachloride, CF_{4}?
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,
K_{2}NO_{2}
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, N_{2}O_{5} 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 AlPO_{4}?
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 10^{23}
= 3.7 x 10^{22} 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 Zn_{3}(PO_{4})_{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 mol H_{2}O.
# of molecules = # of Mols x 6.022 x 10^{23}
= 4 x 6.022 x 10^{23}
= 24.088 x 10^{23} molecules
9. What is the mass in grams of 1.02 x 10^{24} atoms manganese (Mn)?
# of Mols = # of atoms/6.022 x 10^{23}
= 1.02 x 10^{24}/6.022 x 10^{23}
= 1.69 Mols
Mass = # of Mols x Molar Mass
= 1.69 x 54.94
= 93g