EMPIRICAL AND MOLECULAR FORMULAS NOTE: The text offers an alternative method for determining molecular
formula.
TITLE: ___________________________________
NAME: ________________________
1. Create a spreadsheet of the data below.
Samples of 1-butene were decomposed. The mass of each element was then recorded.
Mass of
carbon
(g)
.221
.973
.608
1.17
.778
Mass of
hydrogen
(g)
.037
.164
.101
.196
.131
2. Create a graph of these data.
3. Describe what you see:
4. How would you interpret this?
5. This is an example of which law?
6. Now, add two columns and compute the MOLES of each element. Here, show those calculations
for ONE ROW of data. Use this to check your spreadsheet equations.
7. Now, add another column, and compute the ratio of moles of hydrogen per mole of carbon.
8. What do you suspect is the chemical formula of 1-butene?
9. Are there other possibilities? What would they be?
Percent Composition
Background: It is important to understand that there are two
different types of percent composition:
Theoretical: Uses the chemical formula, covered in the reading.
Experimental: Based on laboratory data.
1. Complete the extensive reading assignment on &quot;Percent
Composition.&quot; (see the index to find…)
2. Calculate the percent composition of the following compounds:
A. CaF2
B. BaSO4
C. Cu(OH)2
D.Ca3(PO4)2
3. Calculate the experimental percent composition of glucose if
.683g of glucose is decomposed, resulting in .271g carbon, .361 g
oxygen, and the remainder hydrogen.
4. Using the formula, calculate the theoretical percent composition
of glucose:
5. Using 3 and 4 above, calculate the experimental percent error in
the “percent hydrogen” answer in #3.
6. What mass of sodium hydroxide (NaOH) would contain 22.3g oxygen?
7. What is the percent composition of a compound if 3.41 g
decomposes into hydrogen and 3.22g oxygen?
8. What mass of oxygen is in 7.31g of sodium chlorite (NaClO3)?
9.Calculate the percent composition of CH3COOH (acetic acid).
10. Calculate the percent composition of the following compounds:
A. C2 H4
B. C3H6
C. C10H20
12. Can you explain why?
13. Circle the compounds that have the same percent composition:
C2H4O
C7H14O
C10H20O5
CH2O
C4H8O2
EMPIRICAL AND MOLECULAR FORMULAS – PAGE 1
1. A 2.20g sample of a hydrocarbon (composed of ONLY hydrogen and carbon) was broken
down, yielding 1.79g carbon. Determine the empirical formula of this compound.
2. Determine the empirical formula of a compound that is 60% O and 40% S.
3. 7.75g of rust (an iron oxide) was decomposed, yielding 5.43g iron. Determine the empirical
formula of this compound.
4. What is the molecular formula of a compound if:
Emp. Form
Molecular mass (g/mol)
CH2
56
CH2
140
CH2O
180
NH2
32
5. Determine the molecular formula of a compound that is 40% C, 53%O and 6.7% H. The
molecular mass is 90 g/mol.
6. Determine the molecular formula of a compound that is 56.3% O and 43.7% P and has a
molecular mass of 284 g/mol.
7. 2.16g of a carbon chloride was decomposed, releasing 1.85g chlorine. Determine the
empirical and molecular formulas, if the formula mass is known to be 166 g/mol.
8. A sample of an organochloride was found to have a molecular mass of 181.4 g/mol and a
sample was shown to contain 1.01g C, .042g H, and 1.49g Cl. Determine the empirical and molecular
formulas of this compound.
9. A compound was determined to have a molecular mass of 88 g/mol and to be composed of
54.5%C, 31.8%N, and 13.6%H. Determine the empirical and molecular formulas.
10. Determine the empirical formula of a compound, if a 5.00 gram sample was broken down
into 3.00g C, .65g H, and oxygen.
11. What is the molecular formula of the compound in the previous problem, given that the
molecular mass is 240 g/mol?
12. Given the molecular formula shown, indicate the empirical formula::
A. C6H12O3
B. C12H24
A. C10H20O3
A. C7H14O5
13. Determine the empirical and molecular formulas of hydrogen peroxide, given that the
formula mass is 34 g/mol and the compound is 5.88% H and 94.1% O.
14. A sample of a compound was broken down and found to contain 4.14g Na, 2.16g C, and
5.76g O. Determine the empirical and ACTUAL (NOT “molecular”) formulas of this compound,
given that the formula mass is 134 g/mol. Can you determine the name of this compound? Hint: it
includes a polyatomic ion we have not previously seen.
EMPIRICAL AND MOLECULAR FORMULAS -- PAGE 2
1. 4.27g of a mercury chloride was decomposed, releasing .64g chlorine. Determine the empirical
and &quot;molecular&quot; (it's a little more complex in this case…) formulas, if the formula mass is known to be
472 g/mol.
2. A sample of trinitrobenzene (closely related to TNT) was found to have a molecular mass of 213
g/mol and a composition of 33.8%C, 1.41%H, 19.7%N, and 45.1%O. Determine the empirical and
molecular formulas.
3. 2.14g aluminum chloride was decomposed, leaving behind .433g Al. Determine the empirical
formula of aluminum chloride.
4. Determine the empirical formula of a compound that is 60.0%C, 13.4%H, and 26.6%O
5. Determine the empirical and molecular formulas of a compound that has a molecular mass of 78
g/mol, if a 17.96g sample is decomposed to yield 16.57g C, and hydrogen.
6. Given the molecular formula shown, indicate the empirical formula:
A. C6H12O6
B. C16H24
A. C10H20O5
A. C6H14O5
7. Determine the empirical and molecular formulas of a compound that is 85.7%C and 14.3%H, and
has a molecular mass of 280 g/mol.
8. Determine the empirical formula of a compound that was decomposed to yield .696gC, .116gH, and
.406gN.
9. Determine the molecular formula of the compound in the previous problem, given that the molecular
mass is 168 g/mol.
10. Determine the percent water in each of the following hydrates:
A. Al2(SO4)3•18H2O
B. NiCl2•6H2O
C. Cr2(SO4)3•15H2O
11. Determine the empirical and molecular formulas of a compound
that is 54.5%C, 9.09%H, and 36.4%O. The molecular mass is 176
g/mol.
12. A sample of a hydrocarbon was combusted (with excess oxygen), resulting in 2.19g water and
10.73g carbon dioxide. Determine the empirical formula. Given that the molecular mass is 78 g/mol,
also find the molecular mass.
HYDRATE PROBLEMS
1. Compute the mass of water in 3.55g of Al2(SO4)3•18H2O.
2. Compute the mass of anhydrous nickel(II) chloride that should
result when 5.08g of NiCl2•6H2O is heated.
3. Compute the percent water in the hydrate Cr2(SO4)3•15H2O.
4. 6.31g of hydrated barium hydroxide was (gently) heated to form
3.43g of anhydrous barium hydroxide. Determine the formula of
hydrated barium hydroxide.
5. A chemist needs to add 3.29g of sodium carbonate to a solution.
The only form available is the decahydrate: Na2CO3•10H2O. What mass
of this material should be added to the solution.
6. Compute the mass of water driven off when 12.05g of sodium
acetate trihydrate is heated.
7. 6.27g of hydrated lithium hydroxide was heated, yielding 3.61
grams of anhydrous lithium hydroxide. Calculate the formula of the
hydrate.
HYDRATE PROBLEMS
1. Compute the mass of water in 3.55g of Al2(SO4)3•18H2O.
2. Compute the mass of anhydrous nickel(II) chloride that should
result when 5.08g of NiCl2•6H2O is heated.
3. Compute the percent water in the hydrate Cr2(SO4)3•15H2O.
4. 6.31g of hydrated barium hydroxide was (gently) heated to form
3.43g of anhydrous barium hydroxide. Determine the formula of
hydrated barium hydroxide.
5. A chemist needs to add 3.29g of sodium carbonate to a solution.
The only form available is the decahydrate: Na2CO3•10H2O. What mass
of this material should be added to the solution.
6. Compute the mass of water driven off when 12.05g of sodium
acetate trihydrate is heated.
7. 6.27g of hydrated lithium hydroxide was heated, yielding 3.61
grams of anhydrous lithium hydroxide. Calculate the formula of the
hydrate.
REVIEW: percent composition, empirical formulas, molecular
formulas, and hydrate problems
1. Determine the percent composition for each of the following
compounds:
A. NaOH
B. Ag2SO4
C. C7H14O3
2. Determine the empirical formula of a compound that was found to
be 35.93% Al and 64.07% S.
3. 1.962g of a compound was broken down, yielding .040g H, .641g S
and the remainder O. Determine the empirical formula of this
compound, which is the most widely produced chemical in the world.
4. A compound was isolated from the natural fermentation of fruit
juice. A sample of this compound was found to contain .36g C,
.0599g H, and .479g O. The compound was also determined to have a
molecular weight of 60 g/mol. Find the empirical and molecular
formulas. Could this compound be the familiar acid in vinegar,
acetic acid (CH3COOH, also called hydrogen acetate)?
5. Given the actual molecular formula shown, indicate the empirical
formula:
C20H14O4
Phenolphthalein, an acid-base indicator we will use
C6H10OS2
Allicin, the natural antibiotic found in garlic
C6H16N2
Hexamethylenediamine, used to manufacture nylon
6. A compound was isolated from the root of the madder plant and
used in ancient Egypt as a dye. Its name is alizarin, and it is
69.98% C, 3.36% H, and 26.64% O. The compound also has a molecular
weight of 240.2 g/mol. Find the empirical and molecular formulas of
alizarin.
7. If 4.88g of SnCl4•5H2O is heated, what mass of the anhydrous compound
will remain?
8. What mass of Na2CO3•10H2O should be used to &quot;deliver&quot; 1.22g of anhydrous
sodium carbonate to a particular reaction?
9. 1.56g of hydrated barium hydroxide was heated, leaving behind .86g of
Ba(OH)2. Determine the formula of the hydrate.
10. A chemist obtained a .541g sample of an organic hydrate known as
phloroglucinol. When gently heated this sample released .119g of water.
The remaining compound had a molecular weight of 126 g/mol and was
decomposed to yield .241g C, .0199g H, and .160g O. Determine the formula
of the original compound.
PERCENT OF WATER IN HYDRATED COPPER SULFATE
DATA ANALYSIS
This activity will give us a chance to process a large quantity of
importance with the upcoming spreadsheet test.
group
mass of
empty
crucible
mass of
cruc. and
hydrate
mass
after
heat.
mass
hydrated
CuSO4
mass
H2O
exper.
percent
water
theor.
percent
water
percent
error
2. From your classmates, acquire data for the first four columns and
fill in. Acquire ten rows of data. (Extra available below…)
3. Enter formulas in the spreadheet to calculate the other
quantities. (Theoretical percent water is a constant 36.07)
4. Add a final row and compute average values for experimental
percent water and percent error.
5. Graph mass of hydrate versus mass of water. This should be a
straight line -- as the mass of hydrate increases the mass of water
does, always in the same proportion (theoretically 36.07%)
Extra data:
group
A1
A2
A3
A4
A5
A6
Use as needed to achieve ten rows of data.
mass of
crucible
(g)
mass cruc.
and hydrate
(g)
Mass
After
heat (g)
22.21
18.56
17.81
20.12
19.02
20.77
28.21
24.12
22.33
27.12
22.98
25.15
25.98
22.12
20.54
24.68
21.45
23.42