Deterining Cheical Forulas Joseph Proust proposed the law of constant coposition. This law states that a copound contains eleents in certain fixed proportions (ratios) and in no other cobinations, regardless of how the copound is prepared or where it is found in nature. Ex: one olecule of ethane, CH 4(g), always contains one ato of carbon and four atos of oxygen. Percentage Coposition is the percentage, by ass, of each eleent in a copound. %H H 100% sa p le C %C 100% sa p le Epirical forula a forula that gives the lowest ratio of atos in a copound Deterining the Epirical Forula Ex: The percentage coposition of a copound is 69.9% iron and 30.1% oxygen. What is the epirical forula of the copound? Step 1: List Given Values Fe = 69.9% O = 30.1% Step 2: Calculate Mass () of Each Eleent in 100-g Saple 69. 9 Fe 100 g 100 Fe = 69.9 g 301. O 100 g 100 O = 30.1 g Step 3: Convert Mass () into Aount (n) 1 ol Fe nfe 69.9 g Fe 55.85 g Fe n Fe = 1.25 ol Fe
n = 30.1 g O O n O = 1.88 ol O 1 ol O 16.00 g O Step 4: State Aount Ratio n Fe : n O = 1.25 : 1.88 Step 5: Calculate Lowest Whole-Nuber Aount Ratio Divide all the aounts by the sallest aount. Here the sallest aount is 1.25 ol Fe. n Fe :n O = 1.00 : 1.50 Notice that this ratio has one nuber that is not a whole nuber. To obtain a whole nuber, ultiply both nubers by 2. n Fe :n O = 2 : 3 Therefore, the epirical forula of the copound is Fe 2 O 3. Ex 2: The percentage coposition of a copound is 21.6% sodiu, 33.3% chlorine, and 45.1% oxygen. What is the epirical forula of the copound?
Molecular forula a forula that indicates the actual nubers of atos in one olecule of a copound. Deterining the Molecular Forula The epirical forula of a copound is CH 3 O, and its olar ass is 93.12g/ol. What is the olecular forula of the copound? Step 1: List given values Epirical forula of copound = CH 3 O M copound = 93.12 g/ol Step 2: Deterine Molar Mass of Epirical Forula MCH3O = 1(12.01 g/ol) + 3(1.01 g/ol) + 1(16.00 g/ol) MCH3O = 31.04 g/ol Step 3: Deterine Ratio of Molar Mass of Copound to Molar Mass of Epirical Forula The Mc o p o und Mepirical foula ratio gives the factor by which the olar ass of the copound is greater than the olar ass of the epirical forula. Mc o p o und 93.12 g/ol M C H 3O 31.04 g/ol M M c o p o und C H 3O 3 Step 4: Calculate Molecular Forula Use the ratio deterined in step 3 to calculate the olecular forula. The olar ass of the copound is three ties greater than the olar ass of its epirical forula. Multiply the subscripts of the epirical forula by 3 to obtain the subscripts of the olecular forula. olecular forula = 3(epirical forula) = 3(CH 3 O) olecular forula = C 3 H 9 O 3 The olecular forula of the copound is C 3 H 9 O 3.
Ex2: The percentage coposition is 40.03% carbon, 6.67% hydrogen and 53.30% oxygen. The olar ass of the copound is found to be 180.18 g/ol. What is the olecular forula of the copound?
Calculating Percentage Coposition by Mass Calculate the percentage coposition of carbon dioxide, CO 2(g). Step 1: Calculate Total Mass of Each Eleent in Copound C = 12.01 u O = 2(16.00 u) O = 32.00 u Step 2: Calculate Molecular Mass (or Foula Unit Mass) of Copound CO2 = C + O = 12.01 u + 32.00 u CO2 = 44.01 u Step 3: Calculate Percentage Coposition by Mass of Copound % C = C CO2 % C = 12.01 u 44.01 u % C = 27.29% % O = O CO2 % O = 32.00 u 44.01 u % O = 72.71% The percentage coposition by ass of carbon dioxide is 27.29% carbon and 72.71% oxygen. Ex2: Calculate the percentage coposition by ass of potassiu sulfate, K 2 SO 4(s).
Worksheet 2.3: Deterining Cheical Forulas 1. Calculate the epirical forula of a copound that is found to contain 2.2% hydrogen, 26.7% carbon, and 71.1% oxygen. 2. The percentage coposition of a copound is 35.9% aluinu and 64.1% sulfur. What is the epirical forula of the copound? 3. A fat that is used to ake soap contains 76.5% carbon, 12.2% hydrogen, and 11.3% oxygen by ass. Deterine the olecular forula of the fat if its olar ass is 706.3 g/ol. 4. The percentage coposition of nicotine is 74.0% carbon, 8.7% hydrogen, and 17.3% nitrogen. Its olar ass is 162.23 g/ol. What is the olecular forula of nicotine? 5. Calculate the percentage coposition by ass of each of the following copounds: a. C 6 H 8 O 6(s) b. Al 2 O 3(s) c. Zn(NO 3 ) 2(s)