Empirical or Molecular Formula Problems and Nomenclature Review

Empirical and Molecular Formulas

INFORMATION

An empirical formula 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.

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.

Key Question

  1. Caffeine has the following percent composition: carbon, 49.48%; hydrogen 5.19%; oxygen, 16.48%; and nitrogen, 28.85%. What is its empirical formula?

INFORMATION

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 ratiosof the elements in the compound, it can be used, along with the molar mass of the compound, to determine the molecular formula. As with empirical formulas, assume a 100.0 gram sample to simplify the calculations.

Key Question

  1. The molar mass of caffeine is 194.19 g/mol. What is its molecular formula?

Hint for determining EF (Empirical Formula):

% to mass, mass to mole, divide by small, multiply till whole

Hint for determining MF (Molecular Formula):

MF=(EF)n n=MF molar mass (given in problem)

EF molar mass

  1. A compound is found to have 46.67% nitrogen, 6.70% hydrogen, 19.98% carbon and 26.65% oxygen. What is its empirical formula?
  1. Benzoic acid contains 68.8% carbon, 4.95% hydrogen and 26.25% oxygen. Find the empirical formula.
  1. A compound consists of 75.0% magnesium and 25.0% oxygen by mass. What is the empirical formula?
  1. A compound contains 50.0% magnesium, 24.0% carbon, 16.0% oxygen, and 10.0% hydrogen. What is the empirical formula?
  1. Freons are gaseous compounds used in refrigeration. A particular Freon contains 9.93% carbon, 58.64% chlorine, and 31.43% fluorine. What is the empirical formula?
  1. A compound consists of 85.0% silver and 15.0% fluorine by mass. What is the empirical formula?
  1. A compound consists of 40.0% calcium, 12.0% carbon, and 48.0% oxygen by mass. What is the empirical formula by mass?
  1. A compound with an empirical formula if CH is found to have a molar mass of 26.04 g/mol. What is its molecular formula?
  1. A compound has an empirical formula of ClCH2, and its molar weight is 98.96 g/mol. What is the molecular formula of this compound?
  1. A compound contains 75.46% carbon, 4.43% hydrogen, and 20.11% oxygen by mass. It has a molar weight of 318.31 g/mol. What is the molecular formula?
  1. Write the formulas for the following ionic (BIC, TIC, or Acids) compounds:

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  1. Sodium chloride
  1. Strontium phosphide
  1. Iron (III) bromide
  1. Aluminum sulfide
  1. Copper (II) iodide
  1. Indium oxide
  1. Rubidium arsenide
  1. Lead (IV) nitride
  1. Calcium sulfate
  1. Lithium phosphate
  1. Magnesium nitrate
  1. Potassium chlorate
  1. Hydrobromic acid
  1. Hydrochloric acid
  1. Nitric acid
  1. Nitrous acid

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  1. Write the names for the following ionic (BIC, TIC, or Acids) compounds:

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  1. KBr
  1. Fe3N2
  1. SrBr2
  1. AlCl3
  1. Sn3P2
  1. CaI2
  2. CuO
  1. Ba3P2
  1. Ca3(PO4)2
  1. LiNO3
  1. MgSO4
  1. Al2(CO3)3
  2. FeCO3
  1. CuSO4
  1. H2SO4
  1. H2SO3

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  1. Write the names or formulas for the following covalent (BMC) compounds:

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  1. DicarbonTetrahydride
  1. Carbon Tetrafluoride
  1. Nitrogen Trichloride
  1. Carbon Monoxide
  1. Carbon Dioxide
  1. Sulfur Dichloride
  1. Heptacarbonheptahydride
  1. Nitrogen Dioxide
  1. CH4
  1. C3H8
  1. CI4
  1. NF3
  1. PF3
  1. SeO3
  1. OF2
  1. SO3

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Nomenclature Test Review

Naming and Writing Formulas for BIC, TIC, and Acids

6______SrO 22______potassium sulfide

7______KNO2 23______tin (II) oxide

8______CuF2 24______sulfuric acid

9______HClO4 25______lead (IV) oxide

10______Al2(CO3)3 26 ______calcium perchlorate

11______H3PO4 27 ______aluminum bromide

12______HCl 28 ______copper (II) permanganate

13______Li3N 29 ______strontium phosphide

14______PbSO3 30 ______nitric acid

15______BeBr2 31 ______iron (III) bromide

16______HF 32 ______ammonium sulfite

17______Pb(CO3)2 33______iron (II) oxalate

18______(NH4)2Cr2O7 34______silver dichromate

19______CuO 35______aluminum permanganate

20______Ca3(PO3)236______barium sulfide

21______Cu2CrO4 37______sodium bromide

a. SO3b. COc. OF2d. N2O3e. NO3

f. PI3g. N2S5h. phosporus nonabromide

i. carbon tetrachloridej. disulfur dioxidek. nitrogen trioxide

l. sulfur dioxidem. dichlorince monoxiden. oxygenpentanitride

For credit, you MUST show all work. Answers are in parentheses at the end of some problems.

38. What is the empirical formula for a compound containing only carbon and hydrogen if it is known to contain 84.2% carbon? (Answer: C4H9)

39. 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. (Answer: Al2S3O12 or Al2(SO4)3)

40. The empirical formula for a hydrocarbon is CH2. Its actual molar mass is 294 g/mol. What is its molecular formula? (Answer: C21H42)

41. 500. grams of a compound are found to contain 133 grams of potassium, l76.5 grams of chromium and the rest is oxygen. What is the empirical formula for the compound? (Answer: K2Cr2O7)

42. Determine the percent oxygen in potassium sulfate.

In a 125.0 gram sample of potassium sulfate what mass is oxygen?

43. Determine the percent oxygen in potassium sulfite.

In a 125.0 gram sample of potassium sulfite what mass is oxygen?

44. What is the percentage composition by mass of iron (II) nitrite? (Answer: 37.84% Fe, l8.92%N, 43.24% oxygen)

45. What is the empirical formula for a compound if 300.grams of it are known to contain 82.4624 grams of Mo, 45.741 g of Cl, and the rest is bromine? (Answer: Mo2Cl3Br5)

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