Percent Composition - General Chemistry 1 PDF

Unit 7: Percent Composition and Chemical Formulas Lesson 7.1 Percent Composition Contents Introduction 1 Learning Objectives 2 Warm Up...

Unit 7: Percent Composition and Chemical Formulas Lesson 7.1 Percent Composition Contents Introduction 1 Learning Objectives 2 Warm Up 2 Learn about It! 4 Percent Composition 5 Calculating Percent Composition from Experimental Data 6 Calculating Percent Composition Using Molar Mass 10 Key Points 14 Key Formulas 15 Check Your Understanding 15 Challenge Yourself 17 Photo Credit 18 Bibliography 18 Key to Try It! 18 Unit 7: Percent Composition and Chemical Formulas Lesson 7.1 Percent Composition Introduction Water is often referred to as the source of life. It is an essential compound, which is one of the reasons why life is possible on Earth. A compound is a substance that is made up of different elements. The atoms of the elements are joined together by chemical bonds. Some compounds, like water and table salt, are simple and might contain two or three different elements joined together easily. Other compounds are very complicated. They might contain several different elements joined together in varying amounts. For example, glucose is a compound made up of carbon, hydrogen, and oxygen. The ratio of atoms that make up a particular compound may be expressed through its chemical formula. 7.1. Percent Composition 1 Unit 7: Percent Composition and Chemical Formulas The composition of a compound may be determined through its chemical formula. In this lesson, you will be able to find out the percent composition or percentage of the elements that comprise a particular compound by referring to its chemical formula. Sample computations on percent composition are included in the discussion for a better understanding of this lesson. Learning Objectives DepEd Competency In this lesson, you should be able to do the Calculate the percent composition of a compound from its chemical following: formula (STEM_GC11PC-If-31). Describe the percent composition. Compute for the percent composition of a given compound or molecule based on its chemical formula. Warm Up Counting Candies 30 minutes There is a specific ratio of atoms that comprise a compound. This ratio is written together with the chemical symbols of the elements in it. From the chemical formula, the percent composition can be calculated. Suppose that a pack of candies is a specific compound, can you write its correct chemical formula? Materials one regular-sized pack of colorful chocolate candies (i.e., M&M’s, Smarties) saucer Procedure 1. Open a regular-sized pack of chocolate candies and place them on a saucer. 2. Take note of the different colors of the chocolate candies. 7.1. Percent Composition 2 Unit 7: Percent Composition and Chemical Formulas 3. Count the number of each color present in the pack. 4. Write the chemical formula of your “compound” by writing the initials of the colors it contains and adding a numerical subscript for each color. Do not include in the chemical formula the color that is not present in your pack of candy. Example: In a pack of candy, you have four blue, three red, and six yellow. The chemical formula of your “compound” is B4R3Y6. 5. Determine the total masses for each color by multiplying the number of each color to its assigned molar mass, which is shown in Table 7.1.1. Example: There are four blue candies in the pack. The molar mass for “blue” is 7 g. Thus, the total mass for “blue” is 28 g. 6. Determine the formula mass of the compound by adding the total masses for each color. 7. Compute the percent composition of each color of candy by using the given formula below. 8. Answer the guide questions below. Data and Results Table 7.1.1. Percent composition of a pack of chocolate candies. Color Total Number Molar Mass Total Mass % Composition red (R) 5g blue (B) 3g yellow (Y) 4g green (G) 2g orange (O) 7g brown (Br) 9g 7.1. Percent Composition 3 Unit 7: Percent Composition and Chemical Formulas Guide Questions 1. What is the chemical formula of your compound? 2. What does each color represent in each pack of chocolate candies? 3. How will you describe the composition of your “compound”? 4. How does the chemical formula help you determine the composition of your compound? 5. Based on the activity, how will you describe the percent composition? Learn about It! How does the chemical formula describe a compound? Chemists commonly use abbreviations called chemical symbols to make elements and their combinations easier to describe in written form. Abbreviations of molecules are known as chemical formulas. In a chemical formula, the type and number of each atom in the smallest unit of the substance are represented by element symbols and numerical subscripts. Remember that when atoms of elements are engaged in a chemical reaction to form a compound, these atoms are combined in a definite proportion. Thus, the combination of these atoms happened in a definite proportion by mass. The chemical formula for water 7.1. Percent Composition 4 Unit 7: Percent Composition and Chemical Formulas How is the percent composition of a compound determined? Percent Composition Each element is a part of a compound’s mass. Chemists can determine the composition of a compound based on the percentage of the elements that comprise it. This composition may be expressed through percent composition or the ratio of the mass of an element and the mass of the compound, multiplied by 100. Simply put, the percent composition is the percent by mass of each element in a compound. Nutritional information provided on the product label indicates the composition of the product based on the mass percent of each nutrient it contains. The calculation of the percent composition is necessary for the chemical analysis of a specific compound. Before calculating the percent composition, the mass of the elements, as well as the compound, must be determined first. The mass of the compound may be given based on the experimental data or can be computed through its molar mass. 7.1. Percent Composition 5 Unit 7: Percent Composition and Chemical Formulas Calculating Percent Composition from Experimental Data Compounds that are prepared and synthesized in a laboratory have definite mass proportions. Thus, when the mass of an element in a sample of a compound is given, its mass percent composition may be computed. You can do this using the equation given below. Equation 7.1.1 Let’s Practice! Example 1 What is the mass percent of N if 7.64 g of N are present in 12.0 g of N2O, “laughing gas,” which is used as an anesthetic for surgery and in dentistry? Solution Step 1: Identify the given. The mass of N is 7.64 g while the mass of the compound, N2O os 12.0 g. Step 2: Identify what is asked in the problem. The problem asks to compute the percent composition of N in N2O. Step 3: Write the working equation. Step 4: Substitute the values and solve for the percent composition of N. Therefore, the percent composition of N in N2O is 63.7%. 7.1. Percent Composition 6 Unit 7: Percent Composition and Chemical Formulas 1 Try It! A 27.0 g sample of a compound contains 7.20 g carbon, 1.35 g hydrogen, and 12.1 oxygen. What is the percent composition of hydrogen in this compound? Example 2 A newly synthesized compound contains elements Zn and O. When a 20.00 g sample of the compound is decomposed, 17.03 g of zinc remains. Determine the percent composition of this compound. Solution Step 1: Identify the given. The mass of the compound is 20.00 g, while the mass of Zn is 17.03 g. Step 2: Identify what is asked in the problem. The problem asks to compute the percent composition of the compound, which consists of Zn and O. Step 3: Write the working equations. To determine the mass of O and to compute its percent composition, subtract the mass of Zn from the mass of the compound. Step 4: Substitute the values and solve for the percent composition of the compound. 7.1. Percent Composition 7 Unit 7: Percent Composition and Chemical Formulas Therefore, the percent composition of the elements in the compound is 85.15% Zn and 14.85% O. 2 Try It! Calculate the percentage composition of a 15.0 g of NaCl with 7.19 g of Na and 7.81 g of Cl. Example 3 In a compound containing Si and O, 3.25 g of the compound contains 1.25 g of Si, and the remainder is O. In another sample of a compound of Si and O, 2.45 g of the compound contains 1.62 g of Si, and the rest is O. Calculate the percentage composition of each compound. Are the two samples the same compound? Justify your answer. Solution Step 1: Identify the given. The mass of the first compound is 3.25 g, and it contains 1.25 g of Si. The mass of the second compound is 2.45 g, and it contains 1.62 g of Si. Step 2: Identify what is asked in the problem. The problem asks to compute the percent composition of each sample and determine whether the samples are of the same compound. Step 3: Write the working equations. To determine the mass of O and compute its percent composition, subtract the mass of Si from the mass of the compound. 7.1. Percent Composition 8 Unit 7: Percent Composition and Chemical Formulas Step 4: Substitute the values and solve for the percent composition of the compound. Sample 1: Sample 2: Therefore, the percent composition of the first sample of the compound is 38.5% Si and 61.5% O. In comparison, the second sample has 66.1% Si and 33.9% O. Given the difference in the percent composition of the two samples, it can be inferred that the samples are from two different compounds. 3 Try It! A sample of a liquid was decomposed into its elements and gave 5.627 g of carbon, 1.034 g of hydrogen, and 3.459 g of oxygen. What is the percentage composition of this compound? 7.1. Percent Composition 9 Unit 7: Percent Composition and Chemical Formulas Tips If the mass of one of the elements that comprise the compound is unknown, you can solve it by subtracting the mass of the element/s from the mass of the compound. Also, consider the number of significant figures of the given mass measurements in expressing the final answer for the percent composition. Calculating Percent Composition Using Molar Mass The molar mass of an element is the quantity in grams that equals the atomic mass of the element. Take note that 6.022 x 1023 atoms of an element are counted when its number of grams equal to its molar mass is weighed out. For example, the atomic mass of carbon in the periodic table of elements is 12.01. Thus, one mole of carbon atoms has a mass of 12.01 g. To obtain 1 mol of carbon atoms, you shall need to weigh out 12.01 g of carbon. Thus, the molar mass of carbon is found by looking at its atomic mass in the periodic table. To determine the molar mass of a compound, multiply the molar mass of each element to its subscript in the formula then add the result The molar masses of silver (Ag), carbon (C), and sulfur (S) 7.1. Percent Composition 10 Unit 7: Percent Composition and Chemical Formulas The molar mass of the compound may be used to calculate its percent composition. Equation 7.1.2 Let’s Practice! Example 4 What is the percentage composition of magnesium chloride, MgCl2? Solution Step 1: Identify the elements in the given compound. The elements in the given compound are Mg and Cl. Step 2: Determine the total mass of each element in the molar mass of a formula. Therefore, the mass of 1 mol of MgCl2 is 95.21 g. Step 3: Write the working equation. Step 4: Substitute the values and solve for percent composition of the compound. Therefore, the percent composition of MgCl2 is 25.53% Mg and 74.47% Cl. 7.1. Percent Composition 11 Unit 7: Percent Composition and Chemical Formulas 4 Try It! What is the percentage composition of CaO? Example 2 In some organic synthesis reactions, dichlorine heptoxide (Cl2O7), which is a highly reactive compound, is used to hasten the reaction. What is the percent composition of this compound? Solution Step 1: Identify the elements in the given compound. The elements in the given compound are Cl and O. Step 2: Determine the total mass of each element in the molar mass of a formula. Therefore, the mass of 1 mol of Cl2O7 is 182.9 g. Step 3: Write the working equation. Step 4: Substitute the values and solve for percent composition of the compound. Therefore, the percent composition of Cl2O7 is 38.76% Cl and 61.24% O. 7.1. Percent Composition 12 Unit 7: Percent Composition and Chemical Formulas 5 Try It! Compute the percent composition of aspartame, an artificial sweetener, with a formula of C14H18N2O5. Example 3 What is the total mass of C used in a 125 g sample of methane, CH4? Solution Step 1: Identify the elements in the given compound. The elements in the given compound are C and H. Step 2: Determine the total mass of each element in the molar mass of a formula. Therefore, the mass of 1 mol of CH4 is 16.05 g. Step 3: Write the working equations. Step 4: Substitute the values and solve for percent composition of each element and the mass of C used in the compound. 7.1. Percent Composition 13 Unit 7: Percent Composition and Chemical Formulas Therefore, 93.6 g of C is used in a 125 g sample of CH4. 6 Try It! Compute the mass of each element present in a 255-g C2H5OH? Remember The mass in grams of one mole of an element is equal numerically to its atomic mass. The molar mass of a compound is equal to the sum of the individual molar masses of the elements in the formula. Why is it important to calculate the percent composition of a compound? Key Points ___________________________________________________________________________________________ In a chemical formula, the type and number of each atom in the smallest unit of the substance are represented by element symbols and numerical subscripts. Each element contributes a fraction of a compound’s mass. This composition may be expressed through percent composition or the ratio of the mass of an element and the mass of the compound, multiplied by 100. Before calculating the percent composition, the mass of the elements, as well as the compound, must be determined first. The mass of the compound may be given based on the experimental data or can be computed through its molar mass. The molar mass of an element is the quantity in grams that equals the atomic mass of the element. To determine the molar mass of a compound, multiply the molar mass of each element to its subscript in the formula then add the results. _________________________________________________________________________________________ 7.1. Percent Composition 14 Unit 7: Percent Composition and Chemical Formulas Key Formulas ___________________________________________________________________________________________ Concept Formula Description Use this formula to compute for the percent composition when the given Percent masses of the Composition elements and/or compound are from experimental data. Use this formula to compute for the percent Percent composition Composition when the given is the molar mass of the compound. Check Your Understanding A. Identify the term that is described in each statement. ____________________ 1. These are abbreviations that are used to represent chemical elements in written form. 7.1. Percent Composition 15 Unit 7: Percent Composition and Chemical Formulas ____________________ 2. It is a combination of chemical symbols and numerical subscripts that tell the composition of a compound. ____________________ 3. This is the ratio of the mass of an element and the mass of the compound, multiplied by 100 ____________________ 4. It is the quantity in grams that equals the atomic mass of the element. ____________________ 5. This is another term used to describe the percent composition. B. Tell whether the statement is true or false. __________ 1. The combination of atoms in a compound has no definite proportion by mass. __________ 2. Chemists can determine the composition of a compound based on the percentage of the elements that comprise it. __________ 3. In obtaining the percent composition, the mass of the elements, as well as the compound, must be determined first. __________ 4. The molar mass of the compound may be used to calculate its percent composition. __________ 5. To determine the molar mass of a compound, multiply the molar mass of each element to its subscript in the formula then divide the results. C. Indicate the molar mass of each compound. ______________________________________ 1. NaHCO3 ______________________________________ 2. Fe2O3 7.1. Percent Composition 16 Unit 7: Percent Composition and Chemical Formulas ______________________________________ 3. C7H5N3O6 ______________________________________ 4. Ca3(PO4)2 ______________________________________ 5. CO(NH2)2 D. Using the molar mass obtained in part C, compute the percent composition of each element in each compound. ___________________ 1. NaHCO3 ___________________ 2. Fe2O3 ___________________ 3. C7H5N3O6 ___________________ 4. Ca3(PO4)2 ___________________ 5. CO(NH2)2 Challenge Yourself A. Answer the given questions. 1. How will you describe the percent composition? 2. How can you determine the mass of the elements in a given sample of a compound using the concept of percent composition? B. Answer the given questions. 3. What is the molar mass of the compound sodium arsenate, Na2AsO4? 4. Carbon will burn in a sufficient amount of oxygen to produce carbon dioxide. In an experiment, 6.40 grams of C reacts with oxygen, and 20.80 grams of carbon dioxide are produced. What mass of oxygen reacted with the 8.40 grams of C? 5. Calculate the percentage composition of the elements in carbon dioxide. 7.1. Percent Composition 17 Unit 7: Percent Composition and Chemical Formulas Photo Credit Spam Can Nutritional Label, by Leh3460 is licensed under CC BY 3.0 via Wikimedia Commons. Bibliography Buckley, Don. Interactive Science: Chemistry. United States of America.: Pearson Education, 2011. Print. Handwerker, Mark J. Science Essentials. San Francisco, CA.: Jossey-Bass, 2005. Print. Hawe, Alan., Davies, Dan., McMahon, Kendra., Towler, Lee., Collier, Christopher., and Scott, Tonie. Science 5 - 11: A Guide for Teachers. 2nd Edition. New York, NY: David Fulton Publishers, 2009. Print. Petrucci, Ralph H. General Chemistry: Principles and Modern Applications. Toronto, Ont.: Pearson Canada, 2011. Print. Wolf, Jonathan S. Easy Physics: Step-by-step. United States of America.: The McGraw Hill Companies, 2013. Print. Key to Try It! 1. 5% 2. 44.9 % Na and 52.1 % Cl 3. 55.60 % C; 10.22 % H; 34.18 % O 4. 71.47 % Ca and 28.53 % O 5. 51.14 % C; 6.16 % H; 9.52 % N; 27.18 % O 6. 65.25 g C; 16.25 g H; 43.50 g O 7.1. Percent Composition 18