Worksheet % empirical formula

Empirical and Molecular formulas Page 21. Determine the Empirical Formula of the compounds with the following % compositions.example: 52.2 % C, 13.0 % H, 34.8 % O52.2 g C x 1 mol_ = 4.35 mol C 13 g H x 1 mol = 13.0 mol H 34.8 g O x 1 mol = 2.18 mol O12 g C 1 g H 16 g Odivide by smallest (2.18) 2 mol C 6 mol H 1 mol O so Empirical Formula = C2H6Oa. 85.6 % C, 14.4 % H (CH2) b. 62.1 % C, 10.3 % H, 27.6 % O (C3H6O)Check list of special fractions for c and dc. 81.8 % C, 18.2 % H (C3H8) d. 47.4% C, 10.6 % H, 42.0% O (C3H8O2)e. 20.00 g of a compound contains 17.8 g Cu and 2.2 g O . Determine the empirical formula andname the compound. (need a Roman numeral) ___________________(Cu2O)f. 10.0 g of an ionic compound contains 4.48 g K, 1.84 g S, and 3.68 g O. What is the Empiricalformula and the name of the compound? (hint: check list of polyatomic ions) (K2SO4)2. Determine the molecular formula of the following molecular compounds.ex. Empirical formula CH2, experimental formula weight = 56 g/molCH2 = 14 ––> _56 g_ = 4 so the molecular formula is (CH2)4 or C4H814 ga. empirical formula C2H5, experimental formula weight = 58.0 g/mol (C4H10)b. empirical formula C2H3O, experimental formula weight = 172 g/mol (C8H12O4)3. You are called in to investigate a murder. You analyze the substance found at the site and findits percent composition is 40.0% C, 6.6% H, 53.4% O. You then find its experimental formulaweight = 180 g/mol. What is the molecular formula for this compound?(C6H12O6)

Name _____________________________________ Page 3Density, Formula weight : Formula weight = Density x 22.4 L or _g_ = g x 22.4 Lof gases at STP of a gas of Gas mol mol L mol( ) ( )1. What is the density ( in g/L) of these gases at STP? (divide FW by 22.4 L) . The density of air is1.19 g/L. Would each gas "float" in the air or "sink"?a. CH4 (0.714 g/L) b. SO3 (3.57 g/L)2. 1.25 L of a gas (at STP) has a mass of 2.46 g. a. Calculate the Density of the gas in g/L.b. Calculate the formula weight of the gas (in g/mol). Might this gas be SO3, NO, or N2O ?(1.97 g/L, 44 .1 g/mol)3. 500 mL of a gas (at STP) has a mass of 0.625 g. What is the formula weight of the gas (ing/mol)? Could this gas be carbon monoxide or carbon dioxide?(28 g/mol)Lab Application Review problem:4. The mass of an empty crucible is 13.400 g. You add0.600 g of Iron metal. You heat the crucible and theIron reacts with Oxygen in the air. The mass of theproduct + crucible is 14.257 g. (answers on back)a. Change the mass of Iron you started with to moles ofIron. (3 sig figs, 4 decimal places)b. What is the mass of product in the experiment?EmptyCrucibleIronCrucible +productProductOxygen inproductMass, grams MolesXXXXXXXXXXXXXXXXXXXXXc. What is the mass of oxygen in the product? Then Change the mass of Oxygen to moles ofoxygen. (3 sig figs, 4 decimal places)d. Determine the empirical formula for the product. Then Name the product (Roman numeral!)e. Calculate the experimental % Iron by mass in the product.f. How many grams of Iron are in 42.7 g of this Iron Oxide?

(Ans for #4a. 0.0107 mol, b. 0.857 g, c. 0.257 g , 0.0160 mol, d. Fe2O3 , e. 70.0 % Fe, f. 29.9 g Fe) Page 4Review problems: Moles, % Composition, empirical formulas5. a. You analyze a gas and find it is 82.8 % carbon and 17.2 % hydrogen. What is its empiricalformula?(C2H5)b. At STP, 1.29 g of this gas occupies a volume of 0.500 L. What are the density andexperimental formula weight of the gas?(2.59 g/L, 57.8 g/mol)c. What is the molecular formula for the gas? (C4H10)6. a. What is the percent composition by mass of NaHCO3? (27.4 % Na, 1.1 %H, 14.3 %C, 57.1 %O)b. An Alka seltzer tablet is about 38% NaHCO3. If a tablet has a mass of 5000 mg, how manymg of sodium do you get from "drinking" 2 of these antacid tablets? (It is recommended thatwe keep our sodium intake below about 3000 mg a day.)(1041 mg)7. At a local drug bust, you find 30.00 g of some green crystals. You analyze this substance andfind out that 8.70 g is Nickel, 7.11 g is Sulfur and 14.19 g is Oxygen.a. What is the percent composition by mass of this compound? (29.0 %Ni, 23.7 %S, 47.3%O)b. What is the empirical formula and name of this compound? (hint: think polyatomic ions, addsome parentheses).(Ni2S3O12)8. A gas has an empirical formula of CH2. At STP, 150 mL of the gas has a mass of 0.281 g.a. Calculate the density and formula weight of the gas. b. Determine the molecular formulafor the gas.(1.87 g/L, 42.0 g/mol, C3H6)9. You heat up 0.500 g of Manganese and it reacts with oxygen. The mass of the product is0.791 g. What is the formula and name (roman numeral) of the product? (MnO2)

Extra Review problems: Moles, % Composition, empirical formulas p 51. You analyze a compound and find this per cent composition: 55.8% C, 7.0% H, 37.2% O. Thenyou find its actual formula weight is 172 g/mol. What are the empirical and molecularformulas of the compound? (C2H3O, C8H12O4 )2. You have some orange crystals which are 26.5 % K, 35.4 % Cr and 38.1 % O. What is theempirical formula and name of this compound?(K2Cr2O7)3. 800 mL of a gas at STP has a mass of 2.54 g. What is its formula weight? If this gas is adiatomic element, which element is it?(71 g/mol)4. You analyze some Copper ore for your mining company. 3.98 g of a copper sulfide contains3.18 g of Copper. a. What is the % composition by mass of this copper sulfide? b. What is itsempirical formula and name? (Roman numeral for the copper)(79.9% Cu, 20.1 % S, Cu2S)b. What mass of Copper can be obtained from 150 g of this Copper sulfide? (120 kg)c. You have some Copper Ore that contains 37.5 % this Copper sulfide. How many grams ofCopper are in 65.3 kg of this Copper Ore?(19.6 kg)5. The empirical formula of a compound is C2HCl and its actual formula weight is 181.5 g/mol.What is its molecular formula?(C6H3Cl3)6. On page 4, problem 6b, you found the mass of Sodium in Alka selzter. How many milligramsof Sodium are in 400 mg of milk of magnesia antacid, Mg(OH)2? Which one is better forpeople on "low sodium" diets? (milk of magnesia or Alka seltzer)7. You analyze some Nickel ore for your mining company. You recover 35.5 g of Nickel from50.0 g of a Nickel oxide. What is the empirical formula and name for this Nickel oxide? (what’sits Roman numeral?)(Ni2O3)

14. You find a powder on a murder suspects sleeve. You analyze it and find its percentcomposition is 46.9% Na, 24.5% C, and 28.6% N. What is the empirical formula and the namefor this compound? Should you arrest this person? Why? (Hint: check your list of names ofpolyatomic ions).(NaCN)a. 42.9% C, 57.1% O , name the compound ______________________ (CO)b. empirical formula CH2, actual formula weight = 42 g/mol (C3H6)