Molar Mass
The molar mass of a compound
is the total of the molar masses of its constituent atoms. The molar mass
of an element is the mass given in the periodic table expressed in grams.
This is the average atomic mass and takes into account the mass of each
elements’ isotopes and their abundance.
The molar mass of NaCl is 58.44 g/mol because 22.99 g/mol (Na) + 35.45
g/mol (Ca) = 58.44 g/mol.
CaCl2: Ca, 40.08 g/mol; Cl, 35.45 g/mol
(1 × 40.08 g/mol) + (2 × 35.45 g/mol) = 110.98 g/mol
When a compound contains a polyatomic ion or a repeated unit, multiply
the subscripts inside the parentheses by the subscript outside the
parentheses:
Ca3(PO4)2: Ca, 40.08 g/mol; P,
30.97 g/mol; O, 15.99 g/mol
(3 × 40.08 g/mol) + (2 × 1 × 30.97 g/mol) + (2 ×
4 × 15.99 g/mol) =
CH3(CH2)3Cl: C, 12.01 g/mol; H,
1.01 g/mol; Cl, 35.45 g/mol
(1 × 12.01 g/mol) + (3 × 1.01 g/mol) + (3 × 12.01
g/mol) + (3 × 2 × 12.01 g/mol) + (1 × 35.45 g/mol) =
92.58 g/mol
Others to try: XeF
2 (169.29 g/mol), SnCl
4 (260.51
g/mol), SnCl
2 (189.61 g/mol), CO
2 (44.01 g/mol),
NH
3 (17.04 g/mol)
Stoichiometry
The key to
stoichiometry is to always convert everything to moles before you even
begin. When you have found the answer you can always convert to mass or
volume.
Here are some sequences with which to work in order to solve different
types of problems:
1. Starting with the number of moles of one of the substances in the balanced
chemical equation you can easily find the number of moles of one of the other
substances using the relevant
molar ratio. Make
sure to put the substance you know on the bottom of the ratio and the substance
you need to find on the top of the ratio before you multiply.
2. Starting with a mass requires a further step: convert that mass into a number
of moles using the
molar mass
of the substance. Then simply follow the same steps used above.
3. If the answer required is a mass, simply convert the number of moles
you find to the mass using the molar mass of the substance in
question.
4. If you are given a volume and a density, simply find the mass using D
= m/V and then calculate moles and complete the problem. Or simply use
this five-step method:
-
Write correct formulas for all reactants and products, and balance
the resulting equation.
-
Convert the quantities of some or all given or known substances
(usually reactants) into moles.
-
Use the coefficients in the balanced equation to calculate the number
of moles of the sought or unknown quantities (usually products) in
the problem.
-
Using the calculated numbers of moles and the molar masses, convert
the unknown quantities are required (usually grams).
-
Check that your answer is reasonable in physical terms.
C
3H
8 + 5O
2 → 3CO
2 +
4H
2O
Moles Example: You are told that a bottle contains 40 mol
C
3H
8 (propane) and asked how many moles of
O
2 you would need to burn it all.
The molar ratio is 5 mol O
2 to 1 mol
C
3H
8.
40 mol C
3H
8 × 5 mol O
2/1 mol
C
3H
8 = 200 mol O
2
Mass Example: Given 32 g of O
2 and an excess of
C
3H
8 how much water would the reaction produce, in
grams?
Convert mass to moles: 32 g O
2 × 1 mol/16 g = 2 mol
O
2
Use molar ratio (4 mol H
2O to 5 mol O
2) to find
moles of water: 2 mol O
2 × 4 mol H
2O/5 mol
O
2 = 1.6 mol H
2O
Convert moles back to mass: 1.6 mol H
2O × 18.02 g/1 mol
= 28.83 g
Others to try: S + 6HNO
3 → H
2SO
4 +
6NO
2 2H
2O
starting w/14 g S; how many grams of HNO
3 are needed? (165.07
g)
3 mol H
2O produced; how many grams of S reacted? (48.098
g)
10.0 L NO
2 produced (D = 2.05 g/L); how many moles of S
reacted? (3.74 ×10
-1 mol)