PERCENT COMPOSITION EXPERIMENT 6
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1 PERCENT COMPOSITION EXPERIMENT 6 OBJECTIVE The objective of this eperiment is to determine the percent composition of potassium chloride and oygen in a sample of potassium chlorate and to compare the eperimental results with the theoretical values. EQUIPMENT AND CHEMICALS Potassium chlorate (anhydrous KClO 3 ) Ring stand with clay triangle Bunsen burner Triple beam balance DISCUSSION The percent composition of a compound is the relative amount of mass contributed by each element in the compound based on. An eample, which is similar to the percent composition of a compound, is the percent composition of the students in the class. Let us say that there are 20 students in the class. In this class, there are four types of students. Teans 10 students Okies 6 students Yankee 3 students Others 1 student What we want to know is "What is the percent composition of each type of student in the class?" To find the percent composition, we divide the number of each student type by the total of all the students and multiply by. part contributed by the component % compositio n whole From the data given, we can calculate the percent composition of the class. number of Teans 10 % Teans total class % 47
2 number of Okies 6 % Okies total class % number of Yankees 3 % Yamkees total class % number of Others 1 % Others total class 20 5 % The sum of percentages should equal %. Teans 50% Okies 30% Yankees 15% Others 5% Total % The percent composition of a compound is determined the same way. Instead of suing students, we use the various elements. Each element has its own particular mass (atomic mass). This atomic mass is multiplied by the number of atoms of the particular elements in the compound. Eample 1: The atomic oygen 16 amu. If there are three atoms of oygen, the total all the oygen atoms 3 X 16 amu 48 amu. Each element in the compound is determined in the above manner. The individual each element is then added together to get the total mass (formula mass) of the compound. The percent composition of the compound can then be determined by dividing the part (the formula mass contributed by each element) by the whole (the formula the compound) and multiplying by. Eample 2: Calculate the atomic Na 2 SO 4 Step 1 - There are two atoms of sodium, one atom of sulfur, and four atoms of oygen. Step 2 - Write the atomic each element: Atomic weight of Na 23.0 amu Atomic weight of S 32.1 amu Atomic weight of O 16.0 amu Step 3 - Calculate the formula weight contributed by each element: Na 2 atoms amu S 1 atom amu O 4 atoms amu 48
3 Step 4 - The total weight of Na 2 SO 4 is the sum of all the weights of all the elements. Na 46.0 amu S 32.1 amu O 64.1 amu Na 2 SO amu Eample 3: Calculate the % composition of Na 2 SO 4 The formula weight of Na 2 SO amu 46 % Na % % S % O % 45.0 % The total of all the percentages should equal %. Eample 4: % Na 32.4% % S 22.6% % O 45.0% Total.0% When the percent composition is calculated using the formula of a compound and the atomic weights, it is called the theoretical percent composition. In this eperiment we will determine the theoretical percent composition of potassium chloride (KCl) and oygen (O) in a sample of potassium chlorate (KClO 3 ). We will, also, eperimentally determine these percentages. When potassium chlorate is heated, it evolves oygen as a gas. heat 2 KClO3 2 KCl + 3 O2 The initial weight of the KClO 3 is determined, the sample is heated to evolve the oygen, and the remaining KCl is weighed. Eample 5: What is the eperimental percent composition of HgO when heated in a crucible? 2 HgO 2 Hg + O Δ 2 49
4 B. Eperimental Weighing crucible crucible + HgO crucible and residue 15 g 115 g g B. Calculation of mass 1. HgO HgO ( crucible + HgO) (115 g 15 g) g ( crucible) 2. residue ( Hg) residue ( Hg) ( crucible after heating) (106.5 g) (15 g) 91.5 g ( crucible) 3. loss ( oygen) loss ( oygen) ( HgO) ( g) (91.5 g) ( 8.5 g residue) C. Calculation of percent composition residue 91.5 g % Hg total mass g 91.5 % loss 8.5 g % O total mass g 8.5 % We can compare the eperimental result with the calculated theoretical result to determine how accurate our results were. percent error (eperimental value theoretical value) theoretical value We still must calculate the theoretical composition of HgO. The formula HgO amu. The atomic Hg amu and that of oygen 16 amu. The percent composition of Hg 92.6% (200.6 /216.6) and percent oygen 7.4% (16 /216.6). 50
5 Eample 6: Calculate the percent error in the determination of Hg and oygen. percent error (eperimental value theoretical value) theoretical value percent error Hg (91.5 % 92.6 %) 92.6 % 1.2 % percent error Hg (8.5 % 7.45 %) 7.45 % 14.9 % PROCEDURE PART A DETERMINATION OF MASS Potassium chlorate (KClO 3 ) will be heated to evolve oygen. The difference in weight will be due to the evolution of oygen. Using the eperimentally determined weights, the percent composition of KClO 3 can be determined. DO TWO SAMPLES AT THE SAME TIME. 1. Heat a clean 25 ml crucible and lid for approimately five minutes to remove any combustible material. 2. Weigh the cool crucible and lid and record the the crucible. NOTE: all masses should be read to the nearest 0.1 grams. 3. Add approimately three grams of reagent grade KClO 3 to the crucible. Record the the crucible, lid, and KClO Heat the crucible gently for about five minutes. 5. Increase the heat so that the bottom of the crucible becomes a dull red. Heat for 15 minutes. 6. Allow the crucible to cool to room temperature. This is the temperature at which you can comfortably pick up the crucible. Do not place the hot crucible on the tabletop. Use a piece of wire screen to rest the crucible on. 7. Weigh the crucible and lid and record the mass. NOTE: Any future references to the crucible include the lid. 51
6 8. Reheat the crucible to a dull red glow for five minutes more. 9. Cool and weigh the crucible. Record this mass under the 2nd heating. 10. If the two masses are not within 0.1 grams of each other, reheat for an additional five minutes and re-weigh. KClO 3 ( crucible + KClO 3 ) ( crucible) KCl ( crucible after heating) ( crucible) loss (oygen) ( KClO 3 ( crucible after heating) PART B: DETERMINATION OF PERCENT POTASSIUM CHLORIDE AND OXYGEN FROM EXPERIMENTAL RESULTS 1. Calculate the % KCl in the sample of KClO Calculate the % oygen in the KClO 3. PART C: DETERMINATION OF THE THEORETICAL PERCENT COMPOSITION OF % POTASSIUM CHLORIDE AND OXYGEN IN POTASSIUM CHLORATE 1. Calculate the formula KClO 3, KCl, and the oygen atoms. 2. Calculate the theoretical composition of KCl in KClO Calculate the theoretical composition of oygen in KClO 3. PART D: DETERMINATION OF ERROR BETWEEN THE THEORETICAL VALUE AND THE EXPERIMENTAL VALUE 1. Determine the percent error in the KCl determination. 2. Determine the percent error in the oygen determination. 52
7 NAME DATE SECTION PERCENT COMPOSITION OF POTASSIUM CHLORATE REPORT SHEET EXPERIMENT 6 DETERMINATION OF MASSES Sample 1 Sample 2 1. Mass of crucible 2. Mass of crucible and KClO 3 3. Mass of crucible and KCl (1st heating) 4. Mass of crucible and KCl 2nd heating) 5. Mass of crucible and KCl (3rd heating, if needed) Calculate the weight of KClO 3 (total weight) Grams KClO 3 Calculate the weight of KCl remaining in the crucible (residue) Grams KCl Calculate the weight of oygen given off (loss) Grams Oygen 53
8 DETERMINATION OF PERCENT KCL AND OXYGEN FROM EXPERIMENTAL RESULTS 1. Calculate the % KCl in KClO 3 Sample 1 Sample 2 % KCl 2. Calculate the % oygen in KClO 3 3. Does the sum of the % KCl and % oygen equal %? % Oygen DETERMINATION OF THE THEORETICAL PERCENT COMPOSITION OF % POTASSIUM CHLORIDE AND % OXYGEN IN POTASSIUM CHLORATE 1. The formula potassium chloride amu 2. The atomic oygen the number of oygens in KClO 3 amu 3. Calculate the formula KClO 3 Formula mass KClO 3 4. Calculate the theoretical % composition of KCl in KClO 3. % KCl 5. Calculate the theoretical % composition of oygen in KClO Does the sum of % KCl and % Oygen equal %? % Oygen 54
9 DETERMINATION OF EXPERIMENTAL ERROR Sample 1 Sample 2 1. Calculate the % error KCl determination. 2. Calculate the % error in the oygen determination. 3. What does the % error indicate about your eperimental technique? 55
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OpenStax-CNX module: m38210 1 States of Matter and the Kinetic Molecular Theory - Gr10 [CAPS] Free High School Science Texts Project This work is produced by OpenStax-CNX and licensed under the Creative
Name Date Class CHEMICAL QUANTITIES. SECTION 10.1 THE MOLE: A MEASUREMENT OF MATTER (pages 287 296)
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MOLES AND MOLE CALCULATIONS
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