1. III. Moles/Stoichiometry

2. In all chemical reactions there is a conservation of mass, energy, and charge.
Matter cannot be created nor destroyed, only changed from one form to another
Energy cannot be created nor destroyed, only changed from one form to another
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3. Types of chemical reactions include synthesis, decomposition, single replacement, and double replacement.
Synthesis – one product forms from several reactants
A + B  AB
Na(s) + Cl2(g)  2NaCl(s)
Decomposition – several products form from one reactant: AB  A + B
2KClO3(s)  2KCl(s) + 3O2(g)
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4. Precipitate – not soluble in water
Single replacement – an element higher on table J replaces another element in a compound that is lower on table J
AB + C AC + B
Zn(s) + CuSO4(aq) ZnSO4(aq) + Cu(s)
Double replacement – two elements or groups of elements switch
AB + CD AD + BC
AgNO3(aq) + KCl(aq) KNO3(aq) + AgCl(s)
Check Table F for insoluble compounds that form precipitates when solutions are mixed.
Precipitate – not soluble in water
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5. A single replacement reaction will take place when:
A more active metal replaces a less active metal from its compound
A more active nonmetal replaces a less active nonmetal from its compound
Criss Cross to find the new formula
Check Table J for the activity of metals and nonmetals.
Zn +PbCl ZnCl2 + Pb
Pb + ZnCl No Reaction
Zinc is more active than lead
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6. A more active metal will replace a less active metal from its compound during a single replacement reaction.
Zn + CuSO4 Cu + ZnSO4
Zinc replaces copper because zinc is more active than copper.
Cu + ZnSO No Reaction
Copper cannot replace zinc
Hydrogen is on Table J to show which metals will react with acids (HCl, H2SO4, HNO3) to produce hydrogen.
Zn + 2HCl ZnCl2 + H2
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7. A double replacement reaction will take place when:
The reactants are in solution and one of the products is not soluble in water
Check Table F for solubility rules
A B + C D AD + CB
A combines with D and B combines with C
Criss Cross to find the new formulas
Notice: the metal comes first
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8. Polyatomic ions are found on Reference Table E
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9. AgCl is a precipitate because it is insoluble in water.
Check Table F to see if there is a precipitate during a double replacement reaction.
AgCl is a precipitate because it is insoluble in water.
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10. A balanced chemical equation represents conservation of atoms
A balanced chemical equation represents conservation of atoms. The coefficients in a balanced chemical equation can be used to determine mole ratios in the reaction.
A mole of molecules is made up of 6.02 x 1023 molecules
A chemical equation is balanced to ensure the conservation of matter. The types and number of atoms on each side of the equation (before and after the reaction) must be equal.
The total mass before the reaction must equal the total mass after the reaction has taken place.
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11. Chemical equations must be balanced so that mass can be conserved.
Word equation: hydrogen + oxygen water
Chemical equation: 2H2 + O H2O
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12. How many on the left? How many on the right? What do I do?
Balance the reaction Fe + O Fe2O3
There are 2 oxygen atoms on the left and 3 on the right. To get them equal I need to multiply the left by 3 and the right by 2. These multiples are called coefficients and are placed in front of the formula. The yield sign and the plus separate the formulas.
Fe + 3O Fe2O3
Now the number of iron atoms has to be balanced. There is one iron on the left and four on the right. Use a coefficient of 4 in front of the Fe on the left.
4Fe + 3O Fe2O3
Now it is correctly balanced
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13. Regents Question: 01/03 #46
According to Reference Table J, which of these metals will react most readily with 1.0 M HCl to produce H2(g)? Ca K Mg Zn þ
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14. The oxidation states of the elements are available on the Periodic Table of the Elements
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15. To find the formula of a compound, use the criss-cross method
Write the symbol and oxidation state of the metal (lower electronegativiy if both are nonmetals) in the upper right hand corner of the symbol. Positive oxidation state is written first.
Repeat for nonmetal (-) C4+ O2-
Drop the sign C4 O2
Criss Cross C2 O4
Reduce if possible CO2
Although Carbon and Oxygen are both nonmetals, Carbon has a lower electronegativity and therefore uses a positive oxidation number
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16. When using the criss-cross method with polyatomic ions, use parenthesis ( )
Write the symbol and oxidation state of the metal (+ ion) in the upper right hand corner of the symbol.
Repeat for nonmetal (- ion) (NH4) 1+ (PO4) 3-
Drop the sign (NH4) (PO4) 3
Criss Cross (NH4) (PO4) 1
Reduce if possible (numbers outside parenthesis only)
If there is no number following the parenthesis, drop the parenthesis.
(NH4)3 PO4
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17. Compounds are electrically neutral.
A compound is a substance composed of two or more different elements that are chemically combined in a fixed proportion. A chemical compound can be broken down by chemical means (not physical means.) A chemical compound can be represented by a specific chemical formula and assigned a name based on the IUPAC system.
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Compounds are electrically neutral.

18. The subscript of 1 is never written
A chemical formula is both qualitative and quantitative. It tells which elements are in the compound with symbols and how many of each with subscripts
The formula for sulfuric acid is H2SO4
Element Number of atoms H S O
The subscript of 1 is never written
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19. Compounds have more than one capital letter in the formula
Binary Compounds – Composed of just two elements (2 capital letters)
Water H2O elements 3 atoms
Methane CH elements 5 atoms
Ammonia NH elements 4 atoms
Aluminum oxide Al2O elements 5 atoms
Other Compounds -more than 2 elements (more than 2 capital letters)
Potassium hydroxide KOH elements 3 atoms
Calcium sulfate CaSO elements 6 atoms
Ammonium phosphate (NH4)3PO4 4 elements 20 atoms
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20. Regents Question: 08/02 #6
Which species represents a chemical compound?
(1) N2
(2) Na
(3) NH4+
(4) NaHCO3
A compound is made up of 2 or more different elements.
A compound is electrically neutral. þ
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21. Regents Question: 08/02 #9
If an equation is balanced properly, both sides of the equation must have the same number of
(1) atoms
(2) coefficients
(3) molecules
(4) moles of molecules þ
2H2 + O H2O
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22. Regents Question: 06/03 #59
Given the reaction between two different elements in the gaseous state
Box A below represents a mixture of the two reactants before the reaction occurs.
The product of this reaction is a gas. Draw the system after the reaction has gone to completion, based on the Law of Conservation of Matter.
One reactant is in excess.
Box A – System before reaction
Box B – System after reaction
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23. Regents Question: 08/03 #47 þ Given the unbalanced equation:
_Al + _CuSO _Al2(SO4)3 + _Cu
When the equation is balanced using the smallest whole-number coefficients, what is the coefficient of Al? 1
(2) 2
(3) 3
(4) 4 þ
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24. Regents Question: 01/03 #48 þ Given the unbalanced equation:
_Fe2O3 + _CO _Fe + _CO2
When the equation is correctly balanced using the smallest whole-number coefficients, what is the coefficient of CO? 1 2 3 4
Try a 3 in front of CO2 þ
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25. Regents Question: 06/03 #9
Which substance can be decomposed by a chemical change?
(1) Co
(2) CO
(3) Cr
(4) Cu þ
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26. The formula mass of a substance is the sum of the atomic masses of its atoms. The molar mass (gram formula mass) of a substance equals one mole of that substance.
Formula mass of H2SO4 H 2 x 1 = 2
S 1 x 32 = 32
O 4 x 16 = 64
98 amu
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27. The formula mass represents the mass of one molecule of a substance while the gram formula mass represents the mass of a mole (6.02 x 1023 molecules) of that substance.
The calculation for formula mass and gram formula mass (GFM) are the same, the difference is in the units.
Formula Mass of H2SO4 = 98 amu
Gram Formula Mass of H2SO4 = 98 g
If they tell you to use the mass rounded to the nearest tenth then use tenths for each mass as well as your final answer.
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28. Regents Question: 06/02 #41 þ The gram formula mass of NH4Cl is
22.4 g/mole
28.0 g/mole
53.5 g/mole
95.5 g/mole
N 1 x 14.0 =
H 4 x =
Cl 1 x 35.5 = _______
g/mole þ
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29. Regents Question: 06/02 #34
A compound has a gram formula mass of 56 grams per mole. What is the molecular formula for this compound?
CH2
C2H4
C3H6
C4H8 þ
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30. Grams/GFM = Moles To convert grams into moles, divide the grams by the gram formula mass To convert moles into grams, multiply by the moles by the gram formula mass The unit for gram formula mass is grams/mole
How many moles in 49 grams of H2SO4?
49 g = X moles X= 0.50 moles
98 g/mol
How many grams are contained in 2.00 moles of H2SO4?
X = moles X = 196 grams
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31. Types of chemical formulas include empirical, molecular, and structural.
Molecular Formula
Empirical Formula
Structural Formula
Which elements and how many of each in a molecule
Which elements and the ratio of how many of each
Shows the bonds and how the atoms are arranged
C2H6
CH3
H H
H C C H
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32. The empirical formula of a compound is the simplest whole-number ratio of atoms of the elements in a compound. It may be different from the molecular formula, which is the actual ratio of atoms in a molecule of that compound.
Hydrogen peroxide Sugar molecular formula: H2O C6H12O empirical formula: HO CH2O
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33. Regents Question: 06/03 #8 þ Which is an empirical formula? (1) P2O5
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34. The molecular formula must be a whole number multiple of the empirical formula
Molecular Empirical Multiple
Formula Formula
H2O HO
C3H CH
C6H12O CH2O
Hg2O HgO
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35. The formula mass must be a whole number multiple of the empirical mass
Molecular Empirical Multiple
Formula Mass Formula Mass
H2O HO
C3H CH
C6H12O CH2O
Hg2O HgO
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36. Regents Question: 06/03 #19
Which formula correctly represents the compo-sition
of iron (III) oxide?
(1) FeO3
(2) Fe2O3
(3) Fe3O
(4) Fe3O2 þ
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37. The percent composition by mass of each element in a compound can be calculated mathematically.
Calculate the formula mass and then divide the component of the element you are looking for by the total mass of the formula then multiply by 100%.
Formula mass of H2SO4 % of Oxygen in H2SO4
H 2 x 1 = /98 x 100% = 65% S 1 x 32 = O 4 x 16 =
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38. Regents Question: 01/03 #8
What is the percent by mass of oxygen in H2SO4? [ formula mass = 98]
16%
33%
65%
98%
64/98 x 100 þ
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39. Regents Question: 06/02 #7
The percent by mass of hydrogen in NH3 is equal to
17 x 100 1
(2) 17 x 100 3
1 x 100 17
(4) x 100 þ
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40. Regents Question: 06/03 #10
The percent by mass of calcium in the compound calcium sulfate (CaSO4 ) is approximately
15%
(2) 29%
(3) 34%
(4) 47% þ
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41. A hydrate is a compound which has water trapped in its crystal structure. We can determine the percentage of water in a hydrate.
CuSO45H2O – for every copper(II) sulfate there are 5 trapped water molecules
Find the formula mass including the water
Cu 1 x 64 = 64
S 1 x 32 = 32
O 4 x 16 = 64
H x 1 = 10 Water contributes 90 to
O 5 x 16 = 80 the 250
/250 x 100% = 36%
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42. Regents Question: 06/02 #36
What is the total number of oxygen atoms in the formula MgSO4•7H2O? [The • represents seven units of H2O attached to one unit of MgSO4] 11 7 5 4 þ
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43. Use the coefficients to predict the amount of reactant consumed or product formed (Mole-Mole problems)
The ratio of the coefficients is a ratio of the moles taking part in a reaction.
Reactants (on the left of the arrow) are consumed
Products (on the right of the arrow) are formed
Given the number of moles of any substance in a reaction, you can use the coefficients to find the number of moles of any other substance.
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44. Place the moles given over the coefficient for that substance and set up a proportion with the coefficients.
How many moles of oxygen are produced when 4 moles of KClO3 react according to the equation: 2 KClO KCl + 3 O2 X
2 KClO KCl + 3 O2
4 = X
cross multiply to get 2X = 12
X=6 moles
unknown
given
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45. Regents Question: 01/03 #42 þ Given the equation:
2C2H2(g) + 5O2(g) CO2(g) + 2H2O(g)
How many moles of oxygen are required to react completely with 1.0 mole of C2H2?
2.5
2.0
5.0 10 þ
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46. Regents Question: 06/02 #37 þ Given the reaction:
6CO2 +6H2O C6H12O6 + 6O2
what is the total number of moles of water needed to make 2.5 moles of C6H12O6 ?
2.5
6.0 12 15 þ
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47. Regents Question: 06/03 #20 þ Given the reaction:
PbCl2 (aq) + Na2CrO4 (aq) PbCrO4 (s) + 2NaCl(aq)
What is the total number of moles of NaCl formed when 2 moles of Na2CrO4 react completely?
(1) 1 mole
(2) 2 moles
(3) 3 moles
(4) 4 moles þ
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