- Diman Regional Voc-Tech
- Pre-AP/Extra Credit Pages
- Pre-AP Cram Sheet #2: Moles and Formulas
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Textbook Reading:
Holt "Chemistry" Ch. 7 S. 3 p 241-245
Silberberg "Chemistry" Ch.3 S.2A Molecular Formula Is
- The actual number of atoms in a molecule.
- The molecular formula of propane is C3H8.
- One mole of propane contains 3 moles of carbon atoms and 8 moles of hydrogen atoms.
- One molecule of propane is made of 3 carbon atoms and 8 hydrogen atoms covalently bonded together.
- Able to be used to calculate the molar mass of a molecule by adding up the masses of the atoms in the formula.
- The molar mass of propane is (3 x 12.011) + (8 x 1.0079) = 44.096 g/mol.
- One mole of propane has a mass of 44.096 g.
- Not as informative as a full structure.
- The full structure of propane shows how the atoms bond together
- Sometimes one molecular formula can describe multiple compounds. These are called isomers.
- There are 3 isomers of C5H12, known as 'pentanes'.
An Empirical Formula is
- A formula giving the lowest ratio of atoms in a molecule.
- The molecular formula of water is H2O. The empirical formula is also H2O.
- The molecular formula of hydrogen peroxide is H2O2, but the empirical formula is simply HO.
- The molecular formula of ethane is C2H6, but the empirical formula is CH3.
- A formula giving the lowest ratio of moles of atoms in a compound.
- Water has 2 moles of H for each mole of O.
- Ethane has 3 moles of H for each mole of C.
- Not as informative as a molecular formula.
- Can be readily determined from the masses or percentages of different elements in a compound.
- This can be found by actual experiment in many cases.
- Empirical means 'by experiment'.
Use the Molar Mass to Convert from Empirical Formulas to Molecular Formulas
A compound has the empirical formula of CHO2 and a molar mass of 90.0 g/mol. What is the molecular formula?
(1 x 12.011) + (1 x 1.008) + (2 x 15.9994) = 45.018 g/mol
90.0 g/mol = 2
45.018 g/mol
Molecular formula is 2 x (CHO2) = C2H2O4Find the Simplest Integer Ratio of Moles to Determine an Empirical Formula
A compound has molecular formula of C12H10. What is the empirical formula?
Both 12 and 10 have a common factor of 5.
The empirical formula is C6H5
A sample of an unknown compound contains 0.21 mol Zn, 0.14 mol P, 0.56 mol O. What is the empirical formula?
Of the 3 amounts, the smallest is 0.14 mol. Divide all the amounts by 0.14 mol.
0.21 mol Zn / 0.14 mol = 1.5 Zn
0.14 mol P / 0.14 mol = 1 P
0.56 mol O / 0.14 mol = 4 O
We can't use decimals in formulas!
We notice that 0.5 = 1/2, so we multiply everything by 2Zn(1.5)P(1)O(4) x 2 = Zn3P2O8
The empirical formula is Zn(1.5)P(1)O(4) x 2 = Zn3P2O8
A sample of an unknown compound contains 2.82 g Na, 4.35 g Cl, 7.83 g O. What is the empirical formula?
Formulas relate moles, so convert grams to moles.
HINT: Keep extra decimal places, round at the end!
This avoids calculation errors.2.82 g Na x 1 mol Na = 0.12266 mol Na
22.99 g Na4.35 g Cl x 1 mol Cl = 0.12270 mol Cl
35.45 g Cl7.83 g O x 1 mol O = 0.48938 mol O
16.00 g O
Smallest is 0.12266 mol so divide:
0.12266 mol Na / 0.12266 mol = 1.00 Na
0.12270 mol Cl / 0.12266 mol = 1.00 Cl
0.48938 mol O / 0.12266 mol = 3.99 O
We rounded to 3 sigfigs because that's what we originally had.
Error in the last place is not important (rounding!):
Empirical formula is NaClO4An unknown hydrocarbon contains 6.291% hydrogen by mass and has molar mass of about 128 g/mol. What is the molecular formula?
HINT: We can pretend our sample is 100 grams just to make it simple.
The sample size doesn't affect the formula: a drop of water and a
swimming pool are both H2O!6.291 g H x 1 mol H = 6.2417 mol H
1.0079 g H
We know percentages must add up to 100% so 93.709% carbon:93.709 g Cl x 1 mol C = 7.8019 mol C
12.011 g C
6.2417 mol H / 6.2417 mol = 1.00 H
7.8019 mol C / 6.2417 mol = 1.25 C
We notice that 0.25 is 1/4, so multiply everything by 4.
Empirical formula is C5H4
Molar mass of C5H4 is (5 x 12.011) + (4 x 1.0079) = 64.08 g/mol
128 / 64 = 2
Molecular formula is C10H8Check for understanding! Take the test here! As before, this is worth extra credit in your TEST category.
- The actual number of atoms in a molecule.