Empirical and Molecular Formula of a Compound General Chemistry 1 rst Qua ANALYN D. DALANG Developer Department of Education •Cordillera Administrative Region 1 Republic of the Philippines Department of Education Cordillera Administrative Region Schools Division of Benguet Wangal, La Trinidad, Benguet Published by: Curriculum Implementation Division– Learning Resource Management Section COPYRIGHT NOTICE 2021 Section 9 of Presidential Decree No. 49 provides: “No copyright shall subsist in any work of the Government of the Philippines. However, prior approval of the government agency of office wherein the work is created shall be necessary for exploitation of such work for profit.” This material has been developed for the implementation of K-12 Curriculum through the Curriculum Implementation Division (CID)—Learning Resource Management and Development System (LRMDS). It can be reproduced for educational purposes and the source must be acknowledged. Derivatives of the work including creating an edited version, an enhancement or a supplementary work are permitted provided all original work is acknowledged and the copyright is attributed. No work may be derived from this material for commercial purposes and profit. 2 What I Know Pre-Assessment: The following activity aims to assess your prior knowledge in finding the empirical and molecular formulas of a compound. What to do before (Pre-Test) Take the pretest before proceeding to the lessons. Check your answers against the answer key at the end of the module. Multiple Choice: Write the best letter of your answer in your answer sheet. Use capital letters only. 1. Which of the following refers to a formula with the lowest whole number ratio of elements in a compound? A. Chemical Formula C. Empirical Formula B. Covalent Formula D. Molecular Formula 2. Which of the following refers to a chemical formula that shows the actual number and kinds of atoms present in one molecule of a compound? A. Covalent Formula C. Ionic Formula B. Molecular Formula D. Empirical Formula 3. Which of the following is an empirical formula? A. N2O C. C2H4 B. P4O10 D. H2O2 4. All of the following are empirical formulas, which is the exception? A. Na2SO4 C. C3H8 B. N2O4 D. Al3 (SO4)2 5. Which of the following is the correct empirical formula of butane? A. C8H20 C. CH2.5 B. C4H10 D. C2H5 6. A substance has a molecular formula of C8H10N4O2. What is the empirical formula of the compound? A. C4H5N2O C. CHNO B. C9H7N3O D. C2H6N2O 7. What is the empirical formula of a compound with a percentage composition of 25.9% nitrogen and 74.1% oxygen? A. N2O5 C. NO B. N2O4 D. N4O6 2 8. What is the empirical formula of a compound with a percentage composition of 87.1% Ag and 12.9% S? A. Ag2S C. AgS2 B. Ag4S2 D. Ag3S5 9. The empirical formula of a substance is CH2O with a molar mass of 180g/mole. What is the molecular formula of the compound? A. C8H16O8 C. C2H4O2 B. C6H12O6 D. C4H8O4 10. Epinephrine (adrenaline) is a hormone secreted into the bloodstream in times of stress. What is the molecular formula of the compound if it contains 59.0%C, 7.15%H, 26.20% O, and 7.65% N and has a molar mass of 183 g/mole? A. C8H12NO2 C. C9H13NO3 B. C5H11N3O2 D. C7H9N2O 11. Which of the following does NOT have the empirical formula CH? A. Ethyne C. C6H6 B. C100H100 D. C3H4 12. An unknown compound is found to contain 37.47% carbon, 12.61% hydrogen, and 49.92% oxygen by mass. What is the empirical formula for this compound? A. C2H6O C. CH4O B. C2H4O D. CH2O 13. What is the empirical formula for a sample that is 75% carbon by weight and 25% hydrogen by weight? A. C12.5H50 C. C3H 25 B. C6.25H D. CH4 14. If element Y formed a metal oxide, what would the molecular formula be? A. YO2 C. Y2O3 B. YO3 D. Y3O2 15. What is the empirical formula for a compound that is composed of 22.6% carbon, 1.9% hydrogen, and 75.5% oxygen? A. C2H2O5 C. CH2O3 B. CHO D. C2H2O4 3 What’s In Let’s Review! Direction: Find the percent composition of all of the elements in the following compounds. Write your answer in your answer sheet. --------------------------------------------------------------------------------------------------------------------1. CuBr2 %Cu: _______________ %Br : _______________ -------------------------------------------------------------------------------------------------------------------2. NaOH % Na: ______________ % O : _______________ % H : _______________ --------------------------------------------------------------------------------------------------------------------3. (NH4)2S % N : _______________ % H : _______________ % S : _______________ --------------------------------------------------------------------------------------------------------------------4. N2S2 % N: ________________ % S: ________________ ---------------------------------------------------------------------------------------------------------------------- What’s New Let’s get started! Direction: Identify whether the following compound is an empirical or molecular formula. Write EF for “Empirical Formula” and MF for molecular formula. Write your answer in your answer sheet. 1. NO2 6. C6H12O6 4 2. N2O4 7. HO 3. CH 8. H2O2 4. C2H2 9. ClO3 5. CH2O 10. Cl2O6 What is it Let’s Discuss! Discovering a new compound does not just stop there, the chemist analyzed the compound quantitatively to determine its percent composition. The chemist will then determine the empirical formula and molecular formula from its percent composition. But what is the difference between empirical and molecular formulas? Empirical formula consists of the symbols of all the elements present in the compound, with the subscripts showing the smallest whole-number ratio of the atoms in the compound. On the other hand, the molecular formula gives the actual number of each kind of atom present in a molecule of the compound. For example, hydrogen peroxide has the molecular formula H 2O2. The actual number of hydrogen atom and oxygen atom in one molecule of H 2O2, as shown by the subscripts in the chemical formula, are both 2. The lowest ratio, however, of hydrogen to oxygen in hydrogen peroxide is 1:1. Therefore, the empirical formula of hydrogen peroxide is HO. Table 1: Empirical and Molecular Formulas of some Compounds Formula (Name) Classification of Formula CH Empirical C2H2 (ethyne) Molecular C6H6 (benzene) Molecular CH2O ( methanol or formaldehyde) Empirical and Molecular C2H4O2 (ethanoic or acetic acid) Molecular C6H12O6 (glucose) Molecular Problem 1: Imagine yourself as an analytical chemist who analyzed a sample of compound that is found to contain 40.05% sulfur and 59.95% oxygen. What is the empirical formula of the compound? Step 1: Since you are given the percentage composition of the compound, you can 5 assume 100.0g of the compound. Mass of S = 40.05 g Mass of O = 59.95 g Step 2: the mass of each element in the compound is used to find the number of moles of sulfur and oxygen. The mass of each element is multiplied by its molar mass. Moles of S = 40.05g S x 1 mol S_ = 1.249 mol S 32.07g S Moles of O = 59.95 g O x 1 mol O = 3.747 mol O 16.00gO Step 3: Convert the number of moles to the whole number ratio. Find which has the smaller whole number subscript between sulfur and oxygen. Since sulfur has the smaller number of moles, it will have the smallest subscript possible, 1. Thus, you can divide the mole values of sulfur and oxygen by the same number (1.249) Whole number ratio of S = 1.249 mol S = 1 mol S 1.249 Whole number ration of O = 3.747 mol O = 3 mol O 1.249 Therefore, the simplest whole number mole ratio of S atoms to O atoms is 1:3 Step 4: Use the whole number ratio as subscript of the elements to come up with the empirical formula of the compound. Therefore, the empirical formula of the compound is SO3 Problem 2: What is the molecular formula of a compound with a given molar mass of 60.0g/mol and an empirical formula of CH4N? Step 1: Calculate the molar mass of CH4N. Mass C = 1 x 12.01g = 12.01 g Mass H = 4 x 1.01 g = 4.01 g Mass N = 1 x 14.01g = 14.01 g Empirical Molar mass of CH4N = 30.0g/mol Molar mass of the compound is 60.0 g/mol 6 Step 2: Divide the given molar mass by the empirical formula molar mass to determine n which is the number of times the empirical formula is found in the molecular formula. n= 60.0 g/mol 30.0 g mol =2 Step 3: The subscripts of the symbols in the empirical formula are multiplied by the value of n to determine the actual subscripts in the molecular formula. Multiplying the subscripts of CH4N by 2, the molecular formula is C2H8N2. What’s More Let’s Try! Activity 1: Calculating the empirical formula from the percentage composition of a compound Direction: Calculate the empirical formula of the compound given the percentage composition of each element. Write your complete solution in your answer sheet. 1. What’s the empirical formula of a molecule containing 65.5% Carbon, 5.5% hydrogen and 29.0% oxygen? 2. What’s the empirical formula of a molecule containing 18.7% lithium, 16.3% carbon and 65.0% oxygen? 3. Determine the empirical formula for each compound whose percentage composition is shown below: a. 43% C and 57% O : ______________ b. 26.7% Cr, 40.3% K, and 33.0% O : ______________ c. 32.0% C, 18.7% N, 42.6% O, and the remainder H: _______________ d. 31.9% K, 28.9% Cl, and the remainder O : ________________ Assessment 1 Direction: From the previous activity, answer the following questions briefly. Write your answer in your answer sheet. 1. What information can you obtain from an empirical formula? 2. How do you calculate the empirical formula of a compound from the percentage composition of each element? 7 Activity 2: Calculating the molecular formula from the molar mass of a compound Direction: Calculate the molecular formula of the following compound given its molar mass. Write your complete solution in your answer sheet. 1. A compound with an empirical formula of C2OH4 and a molar mass of 88g/mole. What is the molecular formula of this compound? 2. A compound with an empirical formula of C4H4O and a molar mass of 136g/mole. What is the molecular formula of this compound? Assessment 2 Direction: From the previous activity, answer the following questions briefly. Write your answer in your answer sheet. 1. What information can you obtain from a molecular formula? 2. How do you calculate the molecular formula from the molar mass of a compound? Activity 3: Empirical and Molecular Formula Direction: Calculate the empirical and molecular formula of the following. Write your complete solution in your answer sheet. 1. An unknown compound was found to have a percent composition as follows: 47.0% potassium, 14.5% carbon and 38.5% oxygen. What is the empirical formula? If the true molar mass of the compound is 166.22g/mol, what is its molecular formula? 2. An unknown sample was analyzed and found to contain 3.60 g carbon, 0.61g hydrogen and 38.07g Iodine. What is the empirical formula of the compound if its molecular mass is 281.86 amu, what is the molecular formula of the compound? Assessment 3 Direction: Arrange the following procedures in calculating the empirical and molecular formula of a compound. Use numbers 1 to 6, 1 as the first procedure and 6 as the last. Write your answer in your answer sheet. a. Find the molecular formula. b. Find the number of empirical units in the molecular formula. c. Find the molecular weight of the empirical formula. d. Find the empirical formula. e. Find the ratios between the numbers of moles of each element. f. Find the number of moles of each element in a sample of the molecule. 8 What I Have Learned Direction: Summarize your understanding about empirical and molecular formula by writing a sentence using each letter of the concept. E- _____________________________ M-___________________________ M- _____________________________ O- ___________________________ P- ______________________________ L- ___________________________ I- _______________________________ E- ___________________________ R- ______________________________ C- ___________________________ I- _______________________________ U- ___________________________ C- ______________________________ L- ___________________________ A- _______________________________ A- ___________________________ L- _______________________________ R- ___________________________ What Can I Do Let’s Apply! Direction: Identify at least five to ten foods available in your kitchen then find their empirical and molecular formulas. Apply your knowledge in technology to come up with a five to ten minutes video presentation (using any applications you’re familiar with) on how you were able to find the empirical and molecular formulas of those foods. Submit your output via messenger / google classroom or other forms. Your output will be rated based on the following rubric Criteria Excellent Good Needs Improvement Understanding of Topic The student clearly understood the topic in-depth and presented his/her information convincingly. The student seemed to understand the main points of the topic and presented those with ease. The student clearly understood most aspects of the topic and presented his/her information with ease. 9 Content and Organization Lay-out and design Programs shows a continuous progression of ideas and tells a complete, easily followed story. Well organized. Excellent, well thought out explanation shows superior effort. Organization of presentation is excellent. Transitions add to the viewer's understanding of the topic. Titles are added to enhance understanding. Storyboard shows progression. Well organized. Format is easy to follow. Good explanation shows good effort. Portions may poorly organized. Hard to follow the progression of the story. Shows some effort. Sequence of project components is clear and evident. Transitions provide easy movement from one scene to another. Titles are used and add to the video's flow. Storyboard shows general outline. Adequate preparation and sequence is shown. Transitions are adequate. Titles are present. If storyboard is present, it shows the project in a broad outline. Post Assessment Part A: Multiple Choice: Choose the letter of the best answer then write in your answer sheet. Use capital letters only. 1. What is the molecular formula for a compound with the empirical formula: K2SO4 and a molecular mass of 696g? A. K2SO4 C. K8S4O8 B. K8SO16 D. K8S4O16 2. The ratio of carbon atoms to hydrogen atoms to oxygen atoms in a molecule of dicyclohexyl maleate is 4 to 6 to 1. What is its molecular formula if its molar mass is 280g? A. C16H24O4 C. C4H6O B. C8H12O2 D. C2H3O 3. What is the empirical formula of a compound containing (by mass) 36.5% sulfur and 63.5% iron? A. FeS C. 2FeS B. Fe2S D. Fe 2S2 4. Which of the following calculations is NOT needed to determine the empirical formula? A. Percentage composition C. Number of Moles B. Mass Composition D. Molar Mass 5. A compound is found to have 46.67% nitrogen, 6.70% hydrogen, 19.98% carbon and 26.65% oxygen. What is the empirical formula of the compound? A. N4H2CO C. NH2CO4 B. N2H4CO D. N2H2CO 10 6. Benzoic acid contains 68.8% carbon, 4.95% hydrogen and 26.25% oxygen. Find the empirical formula of the compound. A. C7H6O2 C. C3H6O2 B. C6H7O D. CH2O 7. A compound with an empirical formula of CH is found to have a molar mass of 26.04g/mol. What is its molecular formula? A. C2H2 C. C2H B. CH2 D. CH4 8. 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? A. Cl2C2H C. Cl4C4H2 B. Cl2C2H4 D. Cl4C4H4 9. What is the empirical formula by mass of the compound if it contains 40.0% calcium, 12.0% carbon and 48.0% oxygen by mass? A. CaCO2 C. CaCO4 B. CaCO3 D. Ca4CO2 10. 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? A. C10H7O2 C. C14H20O2 B. C5H6O D. C20H14O4 11. Epinephrine (adrenaline) is a hormone secreted into the bloodstream in times of stress. What is the molecular formula of the compound if it contains 59.0%C, 7.15%H, 26.20% O, and 7.65% N and has a molar mass of 183 g/mole? A. C8H12NO2 C. C9H13NO3 B. C5H11N3O2 D. C7H9N2O 12. An unknown compound is found to contain 37.47% carbon, 12.61% hydrogen, and 49.92% oxygen by mass. What is the empirical formula for this compound? A. C2H6O C. CH4O B. C2H4O D. CH2O 13. What is the empirical formula for a sample that is 75% carbon by weight and 25% hydrogen by weight? A. C12.5H50 C. C3H 25 B. C6.25H D. CH4 11 14. Which of the following is an empirical formula? A. N2O C. C2H4 B. P4O10 D. H2O2 15. Listed below are examples of empirical formulas, which is the exception? A. Na2SO4 C. C3H8 B. N2O4 D. Al3(SO4)2 Additional Activities Direction: Calculate the empirical and molecular formula of the following, write your complete solution in your answer sheet. 1. A compound is 52.0% Zinc, 9.6% Carbon and 38.4% Oxygen. Calculate the empirical formula of the compound. 2. A compound is 92.2% carbon and 7.76% hydrogen. The formula mass of the compound is 78.1g. Calculate the empirical formula and molecular formula of the compound. 3. A compound is 43.7% Phosphorus and 56.3% oxygen. The formula mass of the compound is 284g. calculate the empirical formula and molecular formula of the compound. 4. Determine the molecular formula of each of the following: a.CH (52.0 amu) b. C2H3F (92.0 amu) c. NaO (78.0 amu) d. C5H5H (79.0 amu) e. C2HNO3 (213.0 amu) 12 13 Activity 2 Activity 3 1. K2C2O4 2. C2H4O4 Activity 1 C4O2H8 1. C3H3O C8H8O2 2. Li2CO3 3. a.CO b.CrK2O4 c.C2H5NO2 d.KClO3 References Books Ilao, L., B. Lontoc, and E.Gayon. 2016. General Chemistry 1. Manila, Philippines: REX. ELECTRONIC SOURCES Website Source: “Chem Libre”, Date of Access May 15, 2020, https: www.chem.libre.texts.org/. Source: “Courses.Lumenlearning”, Date of Access, May 15, 2020, https://www.physicsclassroom.com/. Source: “Cnx.org.”, Date of Access, April 27, 2021, https://:cnx.org/content/col11760/. Source: “Flowers, P., Theopold, and K.,Langley, R.”, Date of Access, Accessed may 15, 2020. http://openstaxcollege.org. Source: “Clipart.”, Date of Access, May 15, 2020. https://www.google.com/search q= finding+empirical+ and+ molecular+formula. Source: “Empirical and Molecular Formulas Worksheet”, Date of Access, May 15,2020, https://faculty.elgin.edu/sbuktenica/Files/EmpMolFormulas. pdf. 14 15
