WRITING FORMULAS

                                                          Does the second word end with -ide?
                                                               yes                               no
                                                            /                                         \
                Is the second word "hydroxide"?                                          \
                    no                         yes                                                    \
                     |                              \                                                     \
 Is the first word "ammonium"?         \                                                     |
      no                        yes ---------------------------------------------->  TERTIARY COMPOUND
      
|
BINARY COMPOUND                                                                      Is the first word the name of a
                                                                                                     metal or "ammonium"?
Is the first word the name of a                                                       /                         |
nonmetal or a metal? --------->    General Rules for Writing Formulas                    1. Write "NH4+1"
       |                                     1. Write the symbol for the metal with                  2. Write symbol for the second
1. Write symbol for first                its oxidation number.                                         element with its oxidation
    element.                             2. Write the symbol for the second                          number or formula for poly-
2. Prefix of first word tells            element with its oxidation number                        atomic ion with its oxidation
    subscript of first symbol.          or formula for polyatomic ion with                        number
3. Write symbol for second          its oxidation number                                         3. Cross out the "+" and "-".
    element.                             3. Cross out the "+" and "-".                                 4. Reduce (simplify) the
4. Prefix of second word           4. Reduce (simplify) the numbers if                            numbers if possible.
    tells subscript of second           possible.                                                         5. "Criss-cross" the numbers.
    symbol.                               5. "Criss-cross" the numbers.                                    (The oxidation number of
                                                 (The oxidation number of the first                           ammonium becomes the
                                                  part becomes the subscript of the                       subscript of the second part.
                                                  second part.  The oxidation number                    The oxidation number of the
                                                  of the second part becomes the                              second part becomes the
                                                  subscript of the first part.)                                    subscript of the first part.)

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EXAMPLES:
1.) calcium oxide

2.) carbon tetrachloride

3.) ammonium chromate

4.) lead (II) nitrate

5.) aluminum hydroxide

6.) iron (III) bromide

 


Alternate notes for Writing Formulas:

1.)  Does the name end with “-ide”?

          à  If yes, go to # 2

à     If not, go to # 4

 

2.)  Is the second word “hydroxide”?

          à  If yes, go to # 4

à     If no, go to # 3

 

3.)  Is the first word “ammonium”?

          à  If yes, go to # 4

à     If no, go to # 5

 

4.)  The compound is tertiary (or ternary – compound has more than 2 elements).

      Is the first word the name of a metal or “ammonium”?

          à  If first word is name of a metal, go to # 7

          à  If first word is “ammonium”, go to # 8

 

5.)  The compound is binary (has only two elements). 
      Is the first word the name of a nonmetal or a metal?

          à  If first word is name of a nonmetal, go to # 6

à     If first word is name of a metal, go to # 7

 

6.)  To write formula for compound:

          ~ Write the symbol for the first element.
          ~ Prefix of first word tells subscript of first element.

          ~ Write symbol for second element.

          ~ Prefix of second word tell subscript of second element.

 

7.)  To write formula for compound:

          ~ Write the symbol for the element with its oxidation number.

          ~ Write the symbol for the second element with its oxidation number (if binary
             cmpd)     OR     Write the formula for the polyatomic ion with its oxidation
             number (if tertiary cmpd) 

          ~ Cross out the “+” and “-“.

          ~ Reduce (simplify) the numbers if possible.
          ~ “Criss-cross” the numbers.  (The oxidation number of the first part becomes
             the subscript of the second part.  The oxidation number of the second part
             becomes the subscript of the first part.)

 

8.)  To write formula for compound:

          ~ Write “NH4+1

          ~ Write the symbol for the second element with its oxidation number  OR

            Write the formula for the polyatomic ion with its oxidation number 

          ~ Cross out the “+” and “-“.

          ~ Reduce (simplify) the numbers if possible.
          ~ “Criss-cross” the numbers.  (The oxidation number of ammonium becomes the
             subscript of the second part.  The oxidation number of the second part becomes
             the subscript of ammonium.)

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EXAMPLES:
1.) calcium oxide

2.) carbon tetrachloride

3.) ammonium chromate

4.) lead (II) nitrate

5.) aluminum hydroxide

6.) iron (III) bromide
NAMING COMPOUNDS

                                                                  How many elements does
                                                                    the compound contain?
        BINARY CMPD. <------------------------ only 2    |     more than 2 ------------------------> TERTIARY CMPD.

Is the first element a
nonmetal or a metal? ------------------> Is the metal in Group 1, 2, or                                  Is the first element a
     |                                                   Al, Zn, or Ag?                       <-------------------------metal or a nonmetal?
     V                                                  yes                             no                                                              |
1. name of first                                    |                                  \                                                             V
    element with                                   V                                   V                                        1. first word =
    prefix telling                    1. name of first                 1. name of first                                        "ammonium"
    subscript of                         element                           element                                         2. second word =
    first element*                 2. name of second             2. (Roman numeral                               name of second
2. prefix telling                       element with -ide               telling oxidation                                element with
    subscript of                        ending or name of              number of first                                -ide ending or
    second element                  polyatomic ion                    element)                                         name of poly-
    with name of                                                          3. name of second                                   atomic ion
    second element                                                          element with -ide           
    with -ide ending                                                         ending or name of
                                                                                    polyatomic ion
* no prefix on first word
   if there is no subscript

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EXAMPLES:
1.) P2O5

2.) MgSO4

3.) CuCl2

4.) (NH4)3PO4

5.) FeCO3

6.) K2O

 

 

 

 

 

 

 

Alternate Notes for Naming Compounds:

1.)  How many elements does the compound contain?

            à  If compound contains only two elements, go to # 2

à      If compound contains more than two elements, go to # 3

 

2.)  The compound is binary.  Is the first element a nonmetal or a metal?
           
à  If the first element is a nonmetal, go to # 4

            à  If the first element is a metal, go to # 5

 

3.)  The compound is tertiary (or ternary).  Is the first element a nonmetal or a metal?

            à  If the first element is a nonmetal, go to #6

            à  If the first element is a metal, go to # 5

 

4.)  To name the compound: 

            ~  1st word = name of 1st element with prefix telling the # of the subscript*

            ~  2nd word = prefix telling subscript, then name of 2nd element with “-ide” ending

*no prefix on first word if there is no subscript

 

5.)  Is the metal in Group 1, 2, or Al, Zn, or Ag?
           
à  If metal is in Group 1, 2, or Al, Zn, or Ag, go to # 7

            à  If metal is NOT in Group 1, 2, Al, Zn, or Ag, go to # 8

 

6.)  To name the compound:

            ~ 1st word = “ammonium”

            ~ 2nd word = name of polyatomic ion or name of 2nd element with “-ide” ending

 

7.)  To name compound:

            ~ 1st word = name of metal

            ~ 2nd word = name of 2nd element with “-ide” ending (if binary)  OR
                                     name of polyatomic ion (if tertiary)

 

8.)  To name compound:

            ~ 1st word = name of metal

            (Roman numeral indicating the oxidation number of the metal)

            ~ 2nd word = name of 2nd element with “-ide” ending (if binary)  OR
                                     name of polyatomic ion (if tertiary)

 

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EXAMPLES:
1.) P2O5

2.) MgSO4

3.) CuCl2

4.) (NH4)3PO4

5.) FeCO3

6.) K2O


Oxidation Numbers

  • Any uncombined element (element not in a compound) has an oxidation number of 0.
  • Fluorine always has an oxidation number of -1 in a compound.
  • Oxygen has an oxidation number of -2 in all compounds except when it is part of a binary compound with a halogen.
  • Hydrogen has an oxidation number of +1 except when it is with metals.
  • The algebraic sum of the oxidation numbers in a compound is zero.

EXAMPLE:  Find the oxidation number of carbon (C) in Na2CO3.

            ~ Let x = oxidation # of carbon
            ~ Na’s oxid. # is +1
            ~ O’s oxid. # is -2
            ~ sum of (# of each element in cmpd. multiplied by its oxid. #) = 0
            ~ 2 (+1) + 1 (x) + 3 (-2) = 0
                Na            C          O
            ~ 2 + x – 6 = 0
            ~ x – 4 = 0
            ~ x = +4 (Carbon’s oxidation number in Na2CO3 is +4.)

Find the oxidation number of the underlined element in each compound.

1.  KMnO4

2.  LiNO3

3.  NaClO

4.  MnO2

5.  Ca(NO2)2

6.  Ba(ClO4)2

 


Percent Composition

PERCENT COMPOSITION:  the percentage by mass of each element in a compound

FORMULA FOR % COMPOSITION:
% composition = mass of element in compound x 100
                                   
molar mass of compound

EXAMPLES:
Q1.  Find the % composition of copper (I) sulfide, Cu2S.
A1.  Finding % composition means that you have to find the % of each element in the cmpd.                    
            molar mass of Cu2S =
            Cu:  __ x _____ = 
            S:    __ x _____ = 
              MM of Cu2S = 
                % Cu =    ______ x 100 =  
  
                             
                % S =    ______  x 100 = 
                                    

A1 = Cu:  _____ % and S: _____ %
            * For % comp. problems, you can round to 3 SF's. *

Q2.  Find the percent of oxygen in calcium phosphate, Ca3(PO4)2.
A2.  molar mass of Ca3(PO4)2 =
        Ca:  __ x _____ = 
        P:    __ x _____ = 
        O:   __ x _____ = 
        MM of Ca3(PO4)2
        % O =   ______  x 100 =  
                            
A2 = _____ %

 


Empirical Formulas

  • opposite of percent composition
  • use % to find formula for compound

EMPIRICAL FORMULA:  simplest formula;  #'s of subscripts are reduced to lowest terms

MOLECULAR FORMULA:  subscripts are multiples of empirical formula subscripts

MOLECULAR FORMULA                EMPIRICAL FORMULA
            C6H6               
           C6H12O6
          C12H16O4N8
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TO SOLVE EMPIRICAL FORMULA PROBLEMS:
A sample of a compound is found to contain 36.0 % calcium and 64.0 % chlorine.  Calculate the empirical formula.

Step 1:  Rewrite % as grams.
36.0 g Ca                64.0 g Cl

Step 2:  Find moles of each element. 
Ca:  36.0 g Ca | 1 mole Ca = 0.898 moles Ca            Cl64.0 g Cl | 1 mole Cl = 1.80 moles Cl
                     | 40.1 g Ca                                                           | 35.5 g Cl
                    
Step 3:  Find mole ratio.  (Divide by smallest number of moles.)
Ca:  0.898 moles = 1        Cl1.80 moles = 2
        0.898                            0.898
    * These whole numbers are subscripts in formula.*

Step 4:  Write the formula.
Ca1Cl2  ====>   CaCl2

Example 2:  A sample of a compound contains 66.0 % calcium and 34.0 % phosphorus.  What is the empirical formula?
Ca:  66.0 g Ca | 1 mole Ca = 1.65 moles Ca        P:  34.0 g P | 1 mole P = 1.10 moles P            
                      | 40.1 g Ca                                                     | 31.0 g P

Ca:  1.65 = 1.5                P:  1.10 = 1
       1.10                              1.10
Q: So, what happens now?  I can't write Ca1.5P1.  And 1.5 is not close enough to round to 2.

A: The easiest way to get 1.5 to a whole # is to multiply by 2.  Remember to multiply both #'s by 2 to get your answer. 
            Ca:  1.5 x 2 = 3                P:  1 x 2 = 2                So, formula is Ca3P2

PRACTICE - A compound contains 43.4 % sodium, 11.3 % carbon, and 45.3 % oxygen.  What is the empirical formula for this compound?

 


MOLECULAR FORMULAS

To find the molecular formula, one more piece of information must be given - the molar mass (also called molecular mass or formula mass).

EX. 1-  An organic compound is found to contain 92.25% carbon and 7.75% hydrogen.  If the molecular mass is 78, what is the molecular formula?

STEP 1:  Find the empirical formula.
C:    92.25 g C | 1 mole C = 7.69 moles C                        H:    7.75 g H | 1 mole H = 7.75 moles H
                      | 12 g C                                                                      | 1 g H

7.69 moles C   = 1             7.75 moles H = 1 
7.69                                  7.69

So...    empirical formula is CH.

STEP 2:  Find molar mass of the empirical formula.
C:    1 x 12.0 = 12.0
H:    1 x 1.0   = 1.0
            MM =   13.0

STEP 3:  Find "multiple" number.
                                    MM of molecular formula = multiple #                  78 = 6
                                    MM of empirical formula                                     13

STEP 4:  Write molecular formula.
Multiply "multiple" # by all subscripts in the empirical formula.
So...      molecular formula is C6H6.

PRACTICE - An oxide of nitrogen contains 30.4 % nitrogen and 69.6 % oxygen.  If the molar mass of this compound is 92 g/mole, what is the molecular formula?


HYDRATES

 

Hydrates are compounds with a certain number of water molecules attached to them.  Their formulas look the same except that there is a “. # H2O” after it.                                  Example:  MgSO4 . 7 H2O

 

When determining the empirical formula for a hydrate, generally you will be determining the number in front of the H2O in the formula.  In order to determine this number, you will need to find the “mole ratio” between the moles of the compound and moles of water.

 

EXAMPLE:

A hydrated sample of sodium carbonate has a mass of 29.00 grams.  The sample is then heated and all water is removed.  The anhydrous salt that remains has a mass of 10.75 grams.  What is the empirical formula for the hydrated sodium carbonate?

STEP 1:  Determine the formula for sodium carbonate.  Na+1  CO3-2 à         Na2CO3

 

STEP 2:  Find moles of sodium carbonate.

10.75 g Na2CO3 | 1 mole Na2CO3 = 0.1014 moles Na2CO3    Na: 2 x 23.0 = 46.0

                       | 106 g Na2CO3                                                    C:   1 x 12.0 = 12.0

                                                                                                O:   3 x 16.0 = 48.0 +
                                                                                                                       106

 

STEP 3:  Find moles of water.

First, find grams of water…         hydrated sample = 29.00 grams

                                                Anhydrous sample = 10.75 grams

                                                Mass of water = 18.25 grams

18.25 g H2O | 1 mole H2O = 1.014 moles H2O

                  | 18 g H2O

 

STEP 4:  Find mole ratio of water to sodium carbonate.

1.014 moles   = 10                                 So  empirical formula for this hydrate is Na2CO3 . 10 H2O

0.1014 moles

 

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