WRITING FORMULAS: MOLAR MASS, %COMPOSITION, EMPIRICAL AND MOLECULAR FORMULAS
REVIEW: Polyatomic ions, writing names from formulas, oxidation number rules
I. WRITING FORMULAS FROM NAMES:
A. Rules:
1. Know the polyatomic ion formulas and their total charge.
2. Write the symbols of elements or polyatomic ions in the name given. Place oxidation numbers
above the symbols.
3. If the sum of the charges add up to zero, then the compound is balanced. Do no more. If not,
move to step 4.
4. If the sum of the charges do not equal zero, the compound is not balanced. Use the criss-cross
method, adding subscripts until the sum of the charges equal zero.
5. Polyatomic ions must have parentheses around them if they need a subscript.
6. NOTE: Do NOT use oxidation numbers with two nonmetals. The prefix gives the number of
atoms and becomes the subscript to the element to which it is attached.
B.
In the following examples, first write each ion with charges, then write the balanced equations with
appropriate subscripts.
1. sodium chloride
Na+ Cl
NaCl
2. calcium sulfate
3. iron (III) oxide
4. calcium chloride
5. aluminum bromide
6. lead (II) oxide
7. lead (IV) phosphate
8. carbon tetrachloride
9. magnesium hydroxide
10. tin (IV) acetate
11. copper (I) phosphate
12. tin (II) chlorate
13. ammonium sulfite
14. lead (II) nitrite
15. sodium sulfide
16. sodium sulfite
17. manganese (III) iodide
18. diphosphorus pentoxide
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19. nitrogen monoxide
20. boron trifluoride
II. MOLAR MASS (a.k.a. molecular weight or gram formula weight). The sum of all the atomic weights of all
the atoms represented in a formula. In other words, the mass of one mole of a compound.
A. EXAMPLES:
1. H2O
H2O
 
2 atoms H(1.008 g) + 1 atom O (15.99g) = 18.01 g/mol (4 sig figs)
2. C12H22O11
3. Ca(OH)2
4. oxygen molecule O2
5. What is the mass of one mole of aluminum sulfate?
6. What is the molar mass of Na2CO3 10 H2O?
III. PERCENT COMPOSITION:
A. To find the percent of each element in a compound, use the following:
atomic weight of the element x the number of atoms of the element
total molar mass of the compound
B.
X 100
EXAMPLES:
1. What is the percent of each element in the compound iron (III) oxide?
2. What is the percent of each element in the compound, Ca(OH)2?
3. What is the percent of water in the hydrate, Na2CO3 10 H2O?
IV. DETERMINING THE EMPIRICAL FORMULA FROM GRAMS OR PERCENTAGES:
A. Procedure:
1. Convert given grams to moles of each element. If value given is in percents, assume percents
are equal to grams.
2. Convert moles to simplest whole number by dividing each mole calculated by the smallest
number of moles determined from doing step number 1.
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3. If the number comes out to be a fraction of ½, multiply everything by 2 to get a whole number.
The rule applies to fractions of ¼, multiply by 4.
4. The resulting number for each element is the number of atoms (subscript) of each element in the
compound.
B.
EXAMPLES:
1. A compound is found to contain 75.0 grams of carbon and 25.0 grams of hydrogen. What is the
empirical formula?
2. A typical charcoal briquette that is used in making a barbeque fire is composed of 43.2 grams of
carbon. When the charcoal lump is burned, it combines with oxygen and the resulting
compound has a mass of 159.0 grams. What is the formula of the compound?
3. What is the empirical formula of a compound that contains 70.0% iron and 30.0% oxygen?
C.
Determining molecular formulas (true formulas):
1. Determine the empirical formula.
2. Determine the molar mass of the empirical formula.
3. Divide the molecular mass given by the problem by the calculated molar mass of the empirical
formula.
4. Multiply the subscripts of the empirical formula by the number resulting from # 3 above.
D.
EXAMPLES:
1. What is the molecular formula of a compound that contains 40.0% carbon, 6.7% hydrogen,
53.3% oxygen. The molar mass of the compound is 180.0 grams/mole.
2. The sweetener, Saccharin, is 45.90% carbon, 2.75% hydrogen, 26.2% oxygen, 17.5% sulfur and
7.65% nitrogen. The molecular weight of Saccharin equals 183.18 grams/mole. What is the
molecular formula of Saccharin?
3. Cholesterol is composed of 83.9% carbon, 12.0% hydrogen and 4.1% oxygen. The molecular
mass of the cholesterol is 386 grams/mole. What is the molecular formula of cholesterol?
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E.
Hydrated Compounds
1. Contain water loosely bonded as part of its structure, Na2CO3 10 H2O.
2. The  means to add the mass of all the waters to the mass of the anhydrous salt, Na2CO3, [the
part of the compound or residue without the water].
3. When trying to solve for amount of water, use empirical formula method.
F.
EXAMPLES: Find the percent water without masses given:
1. What percent of water is found in BaCl2 2 H2O?
2. Find the percent of anhydrous salt in Sr(NO3)2 4 H2O
G.
EXAMPLES: Find the percent water with masses given.
1. A 5.00 gram sample of the hydrate Na2CO3 was heated and only 1.85 grams of the anhydrate
remains. What percentage of water is in the hydrate?
2. A 2.5 gram sample of a hydrate, Na2S 9 H2O was heated to constant mass. How much
anhydrous salt remains?
3. A 7.5 gram sample of CaSO4 is heated until dry. What percent of compound remains if the mass
of the residue weighs 4.7 grams.
H.
Find the value of “n”.
1. A 3.0 gram sample of the hydrate ZnSO4nH2O is heated to constant mass. 1.68 grams of the
anhydrate remains. What is the value of n?
2. A student weighed out 7.5 grams of Na2CO3nH2O and heated until 2.77 grams of anhydrous
salt remained. Find the value of “n”.
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3. A 5.00 gram sample of CuSO4nH2O is heated to constant mass leaving 3.20 grams of residue.
Find the value of “n”.
V. MOLE PROBLEMS WITH COMPOUNDS:
A. Work these problems the same way you do with elements, but use molar mass for the compound
instead of atomic weight of the element.
B. EXAMPLES:
1. How many grams are in 6.00 moles of water?
How many moles are in 0.50 grams of calcium hydroxide?
2. How many grams are in 1.9 moles of lithium chloride?
C.
Overall review:
1. What is the formula of ammonium phosphate?
2. What is the molar mass of ammonium phosphate?
3. What is the percent of each element in ammonium phosphate?
4. If you have 3.8 moles of ammonium phosphate, how many grams are there?
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Name_____________________________________Date_________.
WRITING FORMULAS, MOLAR MASS, AND PERCENT
COMPOSITION ASSIGNMENT- UNIT 4
A. Write ions with charges and balanced formulas for the following compounds:
EXAMPLE: barium chloride
Ba 2+ Cl 1
BaCl2
1. calcium oxide
2. aluminum sulfide
3. magnesium fluoride
4. sodium iodide
5. hydrogen sulfide
6. ammonium nitrate
7. potassium bicarbonate
8. calcium chromate
9. potassium permanganate
10.ammonium phosphate
11.strontium sulfite
12.zinc carbonate
13.lithium dichromate
14.aluminum hydroxide
15.hydrogen cyanide
16.copper (II) acetate
17.lead (II) phosphate
18.iron (III) sulfate
19.nickel (II) chlorate
20.copper (I) oxide
21.iron (III) bromide
22.chromium (III) sulfite
23.dinitrogen pentoxide
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24.sulfur trioxide
25.phosphorus tribromide
26.nitrogen monoxide
27.carbon tetrabromide
28.sodium oxide
29.sodium peroxide
30.mercury (I) chloride
B. MOLAR MASS PROBLEMS. Show all your work including units.
31. What is the molar mass of aspirin (acetylsalicylic acid) C9H8O4?
9 C = 9 ( 12.01 g) = 108.09 g
8 H = 8 (1.008 g) = 8.06 g
4 O = 4 (15.99 g) = 63.96 g
180.11 g/mol
32. Sulfuric acid (H2SO4) is an excellent dehydrator and decaffeinator. What
is the molar mass?
33. Calcium phosphate has the formula Ca3(PO4)2. Determine the molar mass.
34. Magnesium sulfate (Epsom salts) has the formula, MgSO47H2O. What
is the molar mass?
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C. PERCENTAGE COMPOSITION PROBLEMS. Show all work including units.
35. Vinegar contains acetic acid, HC2H3O2. Find the percent composition of each
element.
4 H = 4 (1.008 g) = 4.032 g
% H = 4.032 g x 100 = 6.72 %
%O = 53.17 %
2 C = 2 (12.01 g) = 24.02 g
60.03 g
2 O = 2 (15.99 g) = 31.98 g
% C = 24.02 g x 100 = 40.11 %
60.03 g
60.03 g
36. Mechloroethamine hydrochloride (Mustargen®)is used in the treatment of
Hodgkin’s disease and lymphsarcoma. Calculate the percent of carbon,
nitrogen, chlorine and hydrogen in this drug:
37. What is the percentage composition of each element in Ca(OH) 2?
38. Calculate the percent water in CuSO45H2O.
39. Calculate the percent of hydrate found in Na2CO3H2O.
40. Determine the percent composition of each element in (NH4)3PO4.
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D. EMPIRICAL AND MOLECULAR FORMULA PROBLEMS: Show all work.
41. What is the empirical formula of a compound with 32.4% sodium, 22.5% sulfur,
and 45.1% oxygen?
32.4 g Na x 1 mol = 1.41 mol = 2 22.5 g S x 1 mol = 0.70 mol = 1
22.99 g 0.70 mol
32.06 g 0.70 mol
45.1 g O x 1 mol = 2.82 mol = 4
15.99 g
0.70 mol
Na2SO4
42. Write the empirical formula for a hydrated compound that is 25.3% copper,
12.9% sulfur, 25.7% oxygen, and 36.1% water.
43. One compound of platinum and chlorine is known to consist of 42.1% chlorine.
What is the empirical formula? Assume 100 % = 100 g.
44. The empirical formula of a compound is NO2. Its molar mass is 92.0 gram/mol.
What is the molecular formula of this compound?
45. A compound is 54.5% carbon, 9.1% hydrogen and 36.4% oxygen. Its molecular
mass is 88.0 grams/mol. What is the molecular formula?
46. A 2.5 gram sample of a hydrate of BaCl2 is heated and 2.15 gram of the
anhydrous salt remained. What is the percent of water in the hydrate?
47. A 5.0 gram sample of Ca(NO3)2 was heated and 3.4 grams of the anhydrous salt
remained. What percent of water was in the hydrate?
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48. A 5.0 gram sample of Cu(NO3)2n H2O is heated and 3.9 grams of the anhydrous
salt remains. What is the value of n?
49. Analysis of a compound shows it to be 43.6% phosphorus and 56.33% oxygen.
What is the empirical formula?
50. Myleran® is a useful anticancer agent with a very short duration of action.
Calculate the percentage of sulfur in this drug.
O
O
‫׀‬
‫׀‬
CH3OSO CH2CH2CH2CH2OSOCH3
‫׀‬
‫׀‬
O
O
51. A student heated 5.00 grams of CuSO45H2O until all water of hydration was
expelled. How much weight (water) will the sample lose?
52. A sample of hydrate lost 14% of its original weight during heating. Determine
the number of moles of hydration per mole of anhydrous substance if the
molecular weight of the anhydrate is 208 grams/mole.
53. A student heated 2.48 grams of CaSO42H2O until all the water had been
expelled. What was the mass of the anhydrate?
54. Calculate the percent water of hydration in Na2CO33H2O.
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