1. Chapter 3Topics Molecular Compounds The Mole
Molecular Formulas
Ionic Compounds
Ions and charges
Naming ionic compounds
Molecular Compounds
The Mole
Mole to mass; mass to mole
Describing Compound Formulas
Mass percent – percent composition
Empirical and Molecular Formulas
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Kull Chem 105 Chapter 2

2. Molecular Formulas
Molecule: smallest identifiable unit of pure substance; still maintains composition and chemical properties
Formula (molecular): description of the composition C3H6O2
Condensed formula
CH3COCH2OH CH3OCH2CHO
Structural formula
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3. Ionic Compounds
Ions and charges
Naming ionic compounds
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4. Predicting Charges on Monatomic Ions
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5. 3.4 Molecular Compounds Nonmetal to nonmetal
Use prefixes – mono, di, tri, etc.
Second component add –ide
Only one element in cation spot, mono not required
Phosporus triiodide
N2F4
Dioxygen difluoride
P4O10
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6. 3.5 The Mole Mole to mass; mass to mole
Citric acid C6H8O7
MgCO3
MgSO4· 7H2O
Formula mass
Mass percent
Moles to grams
Grams to moles to molecules
% composition
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7. 3.6 Describing compound formulas: Using percent composition Empirical and molecular formulas
A pure compound always consists of the same elements combined in the same proportions by weight.
Therefore, we can express molecular composition as PERCENT BY WEIGHT
Ethanol, C2H6O
52.13% C
13.15% H
34.72% O
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8. Percent Composition Consider NO2, Molar mass = ?
What is the weight percent of N and of O?
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9. Calculating a formula
In chemical analysis we determine the % by weight of each element in a given amount of pure compound and derive the EMPIRICAL or SIMPLEST formula.
PROBLEM: A compound of B and H is 81.10% B. What is its empirical formula?
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10. A compound of B and H is 81.10% B. What is its empirical formula?
Because it contains only B and H, it must contain 18.90% H.
In g of the compound there are g of B and g of H.
Calculate the number of moles of each constituent.
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11. A compound of B and H is 81.10% B. What is its empirical formula?
Calculate the number of moles of each element in g of sample.
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12. A compound of B and H is 81.10% B. What is its empirical formula?
Now, recognize that atoms combine in the ratio of small whole numbers.
1 atom B + 3 atoms H --> 1 molecule BH3 or
1 mol B atoms + 3 mol H atoms ---> mol BH3 molecules
Find the ratio of moles of elements in the compound.
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13. A compound of B and H is 81.10% B. What is its empirical formula?
Take the ratio of moles of B and H. Always
divide by the smaller number.
But we need a whole number ratio.
2.5 mol H/1.0 mol B = 5 mol H to 2 mol B
EMPIRICAL FORMULA = B2H5
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14. Is the molecular formula B2H5, B4H10, B6H15, B8H20, etc.?
A compound of B and H is 81.10% B. Its empirical formula is B2H5. What is its molecular formula?
Is the molecular formula B2H5, B4H10, B6H15, B8H20, etc.?
B2H6 is one example of this class of compounds.
B2H6
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15. A compound of B and H is 81. 10% B. Its empirical formula is B2H5
A compound of B and H is 81.10% B. Its empirical formula is B2H5. What is its molecular formula?
We need to do an EXPERIMENT to find the MOLAR MASS.
Here experiment gives 53.3 g/mol
Compare with the mass of B2H5
= g/unit
Find the ratio of these masses.
Molecular formula = B4H10
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16. Data to Determine the formula of a Sn—I Compound
Reaction of Sn and I2 is done using excess Sn.
Mass of Sn in the beginning = g
Mass of Sn remaining (recovered) = g
Mass of iodine (I2) used = g
(See p. 125)
Convert these masses to moles
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17. Tin and Iodine Compound
Reaction of Sn and I2 is done using excess Sn.
Mass of iodine (I2) used = g
Mass of Sn initially = g
Mass of Sn recovered = g
Mass of Sn used = g
Find the mass of Sn that combined with g I2.
Find moles of Sn used:
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18. Tin and Iodine Compound
Now find the number of moles of I2 that combined
with 3.83 x 10-3 mol Sn. Mass of I2 used was 1.947g.
How many mol of iodine atoms?
= x 10-2 mol I atoms
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19. Tin and Iodine Compound
Now find the ratio of number of moles of moles of I and Sn that combined.
Empirical formula is SnI4
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20. Naming Ionic and Covalent Compounds
3.86. Name each of the following compounds, and tell which ones are best described as ionic:
(c) SrSO4 (h) Al2S3
(d) Ca(NO3)2 (i) PCl3
(e) XeF4 ( j) K3PO4
3.86 (a) chlorine trifluoride (f) oxygen difluoride (b) nitrogen trichloride (g) potassium iodide, ionic (c) strontium sulfate, ionic (h) aluminum sulfide, ionic (d) calcium nitrate, ionic (i) phosphorus trichloride (e) xenon tetrafluoride (j) potassium phosphate, ionic
3.87. Write the formula for each of the following compounds, and tell which ones are best described as ionic:
(b) boron triiodide
(c) aluminum perchlorate
(e) potassium permanganate
(f ) ammonium sulfite
(g) potassium dihydrogen phosphate
(h) disulfur dichloride
(i) chlorine trifluoride
3.87 (a) NaOCl, ionic (f) (NH4)2SO3, ionic (b) BI3 (g) KH2PO4, ionic (c) Al(ClO4)3, ionic (h) S2Cl2 (d) Ca(CH3CO2)2, ionic (i) ClF3 (e) KMnO4, ionic (j) PF3
3.88. Complete the table by placing symbols, formulas, and names in the blanks.
Cation Anion Name Formula
______ ______ ammonium bromide ______
Ba2 ______ __________________ BaS
______ Cl iron(II) chloride ______
______ F __________________ PbF2
Al3 CO3 __________________ ______
3.88 Cation Anion Name Formula NH4+ Br– ammonium bromide NH4Br Ba2+ S2– barium sulfide BaS Fe2+ Cl– iron(II) chloride FeCl2 Pb2+ F– lead(II) fluoride PbF2 Al3+ CO32– aluminum carbonate Al2(CO3)3 Fe3+ O2– iron(III) oxide Fe2O3
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21. Molecules, Compounds, & the Mole
Which of the following statements about 57.1 g of octane, C8H18, is (are) not true?
(a) 57.1 g is mol of octane.
(b) The compound is 84.1% C by weight.
(c) The empirical formula of the compound is C4H9.
(d) 57.1 g of octane contains 28.0 g of hydrogen atoms.
3.102 (a) True mol C8H18 · = 57.1 g C8H18
True · 100% = 84.1% C
True.
False g C8H18 · = 9.07 g H
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22. Percent Composition
3.104 A metal M forms a compound with the formula MCl4. If the compound is 74.75% chlorine, what is the identity of M?
Molar mass MCl4 = g
Atomic weight M = g
MCl4 – (4)(35.453) g Cl = 47.9 g M M is Ti, titanium
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23. Empirical and Molecular Formulas
3.56 Nicotine, a poisonous compound found in tobacco leaves, is 74.0% C, 8.65% H, and 17.35% N. Its molar mass is 162 g/mol. What are the empirical and molecular formulas of nicotine?
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24. Determining Formulas from Mass Data
3.61 Zinc metal (2.50 g) combines with 9.70 g of iodine to produce zinc iodide, ZnxIy. What is the formula of this ionic compound?
g Zn · = mol Zn
9.70 g I · = mol I
The empirical formula is ZnI2
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25. 3.63. Write formulas for all of the compounds that can be made by combining the cations NH4 and Ni2 with the anions CO3 and SO4.
3.63 (NH4)2CO3
(NH4)2SO4
NiCO3
NiSO4
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