See book pages 343 – 351 Name: ______Empirical and Molecular Formulas INFORMATION (p. 348 – 349): An is a “lowest common denominator” molecular formula for covalent molecules. It represents the ratio in which atoms (or MOLES of atoms) combine to form compounds, but not the actual numbers of atoms in the compound. Multiple compounds can have the same empirical formula.

Glucose, C6H12O6, contains , , and . The ratio of carbon to hydrogen to oxygen is 1:2:1. CH2O is the empirical formula for glucose.

Since the percent composition of each element in an empirical formula represents the ratio in which they occur, the percent composition can be used, in conjunction with the molar masses of the elements, to determine the empirical formula of a compound. It is generally prudent to assume a 100.0 gram sample of the compound for the purpose of simplifying the calculations.

Example 1: Caffeine has the following percent composition: carbon, 49.48%; hydrogen 5.19%; oxygen, 16.48%; and , 28.85%. What is its empirical formula? (see page 349)

INFORMATION (p. 350 – 351): A molecular formula is a formula that represents the actual number of each atom in a compound. Whereas CH2O is the empirical formula for glucose, C6H12O6 is the molecular formula. An actual molecule of glucose contains six carbon atoms, twelve hydrogen atoms, and six oxygen atoms. (Or six moles of carbon atoms, 12 moles of hydrogen atoms, and six moles of oxygen atoms).

Since an empirical formula indicates the ratios of the elements in the compound, it can be used, along with the of the compound, to determine the molecular formula. As with empirical formulas, assume a 100.0 gram sample to simplify the calculations.

Example 2: The molar mass of caffeine is 194.19 g/mol. What is its molecular formula? (See page 351)

Example 3: What is the empirical formula for a compound that contains 0.0134 grams of , 0.00769 grams of , and 0.0115 grams of oxygen? (Two steps: (1) See page 344 AND (2) See Example 1 OR page 349)

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Hint for determining EF (Empirical Formula): % to mass, mass to mole, divide by small, multiply till whole Here are the steps to follow to work Empirical Formula problems: 1. Take either the GRAMS of each element given or the PERCENTAGE of each element given and divide it by the atomic mass of that element. Leave the result of each division in AT LEAST 4 SIG FIGS. 2. Compare all the results from Step 1 by selecting the SMALLEST value and dividing ALL values by this smallest one. 3. The results of Step 2 will either be VERY close to whole numbers or will be recognizable mixed number fractions (decimals such as 1.5, 2.333, 6.67, etc). a. If any result from Step 3 is a mixed number, you must multiply ALL values by some number to make it a whole number. Ex: 1.33 x 3, 2.25 x 4, 2.50 x 2, etc. 4. Use these whole number results as SUBSCRIPTS and write the empirical formula, listing the elements in the order they are given in the problem. (HINT: don’t be surprised if the subscripts in some formulas are VERY large-many organic molecules are huge) ______Hint for determining MF (Molecular Formula): MF=(EF)n n=MF molar mass (given in problem) EF molar mass How to find the molecular formula: 1. Calculate the mass of the empirical formula (which you have already found or it will be given to you ) 2. Divide the known molar mass by the mass of the empirical formula. 3. Multiply that number by the subscripts of the empirical formula to get the subscripts for the molecular formula. ______How to find the percent composition of a compound: 1. Write a correct formula for the compound 2. Find the molar mass of the compound 3. Divide the total atomic mass of EACH ELEMENT by the molar mass 4. Multiply by 100 to convert your results to a percent 5. Since you have no significant figures to go by, express your answer to TWO decimal places with the % sign. ______Empirical or Molecular Formula Problems: Be sure to show ALL work!!! 1. Find the empirical formula for a compound that contains 32.8% and 67.2% . (See Example 1 OR page 349)

2 2. What is the empirical formula for a compound that contains 67.1% and the rest oxygen? (See Example 1 OR page 349)

3. The percent composition of a binary compound is 92.3% carbon and 7.7% hydrogen. If the actual molar mass is 78.0grams/mol, find the molecular formula? (Two steps: (1) See Example 1 OR page 349 AND (2) See Example 2 OR page 351)

4. What is the molecular formula of dichloracetic acid if the empirical formula is CHOCl and the actual molar mass is 129.0 grams/mol? (See Example 2 OR page 351)

5. An empirical formula is found to contain three atoms of carbon, four atoms of hydrogen, and two atoms of oxygen. The actual molar mass is 288.0 grams/mol. Find the molecular formula.

6. Find the empirical formula of a compound that contains l5.8 g of Al, 28.1 g of S, and 56.1 g of oxygen.

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7. The empirical formula for a hydrocarbon is CH2. Its actual molar mass is 294 g/mol. What is its molecular formula?

8. 500. grams of a compound are found to contain 133 grams of , l76.5 grams of chromium and the rest is oxygen. What is the empirical formula for the compound?

______NOMENCLATURE REVIEW: (SEE PAGES 281 – 302, 308 – 310)

SEE PAGES 308 - 310

Determining the formula for Fluoride? 1. Identify the charges = Mg2+ F1 2+ 1 2. Cross the Charges, Mg F = Mg1F2 3. If the subscript is a 1 it does not need to be written.

4. If there is a common subscript such as 2 as in Mg2O2, reduce it to Mg1O1 which is also MgO.

4 9. Write the formulas for the following ionic (p. 288 – 289, 292-293, or Acids – p. 301) compounds: a. chloride i. sulfate 1+ 1- Na 1Cl 1 = NaCl b. phosphide j. phosphate

c. Iron (III) bromide k. Magnesium nitrate

d. Aluminum sulfide l. Potassium chlorate

e. (II) iodide m. Hydrobromic acid

f. n. Hydrochloric acid

g. arsenide o. Nitric acid

h. (IV) nitride p. Nitrous acid

10. Write the names for the following ionic (BIC, TIC, or Acids) compounds (p. 290-291, 294): q. KBr w. CuO cc. FeCO3

r. Fe3N2 x. Ba3P2 dd. CuSO4

s. SrBr2 y. Ca3(PO4)2 ee. H2SO4

t. AlCl3 z. LiNO3 ff. H2SO3

u. Sn3P2 aa. MgSO4

v. CaI2 bb. Al2(CO3)3

11. Write the names or formulas for the following covalent (BMC) compounds (p. 296 – 298): If only two elements are combining, the compound is said to be binary. The element which is less electronegative (closer to the metals in the periodic table) is named first, followed by the other element’s name with an –ide suffix. Number 1 2 3 4 5 6 7 8 9 10 Prefix mono di tri tetra penta hexa hepta octa nona deca

a. Dicarbon Tetrahydride i. CH4

b. Carbon Tetrafluoride j. C3H8

c. Nitrogen Trichloride k. CI4

d. l. NF3

e. m. PF3

f. Sulfur Dichloride n. SeO3

g. Heptacarbon heptahydride o. OF2

h. p. SO3

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More Practice with Naming & Percent Composition: 12______SrO 23______potassium sulfide

13______KNO2 24______tin (II) oxide 14______CuF2 25______sulfuric acid 15______HClO4 26______lead (IV) oxide

16______Al2(CO3)3 27 ______calcium perchlorate 17______H3PO4 28 ______aluminum bromide 18______HCl 29 ______copper (II) permanganate

19______Li3N 30 ______strontium phosphide 20______PbSO3 31 ______nitric acid

21______BeBr2 32 ______iron (III) bromide 22______HF 33 ______ammonium sulfite

34. SO3 35. CO 36. OF2 37. N2O3 38. NO3

39. PI3 40. N2S5 41. phosporus nonabromide

42. carbon tetrachloride 43. disulfur dioxide 44. nitrogen trioxide

45. Determine the percent oxygen in potassium sulfate. In a 125.0 gram sample of potassium sulfate what mass is oxygen?

46. Determine the percent oxygen in potassium sulfite. In a 125.0 gram sample of potassium sulfite what mass is oxygen?

47. What is the percentage composition by mass of iron (II) nitrite?

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