RUN TIME: 50 MIN POGIL 10 Page 1 of 5

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Brianna Gallagher
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1 RUN TIME: 50 MIN POGIL 10 Page 1 of 5 POGIL EXERCISE 10 What Is a Mole and Why Do Chemists Need It? Each member should assume his or her role at this time. The new manager takes charge of the POGIL folder and hands out the GRF and RRF to the appropriate members. The new recorder should record the names of the group members on the new GRF. Table 1. Group Member Role Assignments GROUP TYPE -> GROUPS OF THREE GROUPS OF FOUR MEMBER NO. -> Manager + + Reporter + + Recorder + + Reflector + + Technician Encourager + + SFUC * + * + The manager should give the group a two minutes to read Observation I before beginning the process questions. OBSERVATION I: One of the hardest concepts for beginning chemistry students to understand is the concept of the mole. Perhaps it is because the mole is equally defined by at least five definitions depending on the circumstances two of which we will examine today: The mole (n) is defined by a number and by a mass. The number by which the mole is defined is Avogadro s number (AN). The value we use in this course unless specified otherwise is 6.02 x formula units (FU). You should memorize this value. Remember, when you use this value you will always be limited to three significant figures. Thus, the mole is defined by the equivalence equation below: EQ1: n = 6.02 x FU Equation 1 can be used to derive two useful conversion factors: CF1: 6.02 x FU CF2: n. n 6.02 x FU + 5 MIN

2 RUN TIME: 50 MIN POGIL 10 Page 2 of 5 Suppose you wanted to know how many moles there are in 6.02 x molecules. 1. Which conversion factor would you use to calculate the answer to your question? Circle the correct answer: CF1 CF2 2. What is the answer? Suppose you wanted to know how many formula units there are in moles. 3. Which conversion factor would you use to calculate the answer to your question? Circle the correct answer: CF1 CF2 4. What is the answer to your question? Recorders should submit the group s responses for validation before processing Items 5-9. NOTE: Be sure that you show your calculations. 5. How many moles are in 3.24 x atoms uranium? 6. How many FU are in 2.56 moles sodium chloride? OBSERVATION II: The mass of a mole is the formula mass (FM) of the pure substance. The formula mass is equal to the sum of the atomic masses of the elements in the formula of the pure substance. Equation 2 to is the equivalence equation for this concept. EQ2: n = formula mass (FM) of a pure substance (in grams); n = FM You should know that another widely used term for formula mass is molar mass. The formula (molar) mass of molecular hydrogen (H 2 ) is calculated as described in Equation 3: EQ3: FM H 2 = = g Equation 4 calculates the formula mass of NaCl EQ4: FM NaCl = = g atomic mass Na atomic mass Cl formula mass NaCl All formula masses are not quite so simple to calculate. Take ammonium sulfate for example. Table 2 below demonstrates how to calculate the formula mass of this compound. +15 MIN

3 RUN TIME: 50 MIN POGIL 10 Page 3 of 5 Table2. Calculation of the Formula Mass of (NH 4 ) 2 SO 4 ATOM # Atom X Atomic Mass Total Mass From the Atom Formula Mass* X X (Column 2 X Colum3) N H S O *Adjusted for proper significant figures 7. Use the information and concepts in Observation II and Table 2 to complete Table 3. Table 3. Formula Masses (Molar) Masses of Selected Pure Substances Name of Substance Calcium Phosphate Substance Formula BeCl 2 Formula Mass Of Substance Total Number of FU in One Mole Aluminum Sulfate CuO The recorder should check with instructor to validate group responses. OBSERVATION III: Now we can use Equation 2 to create two useful conversion factors (CF3 and CF4) used to interconvert moles and mass of a pure substance. CF3: n. CF4: FM. FM n Suppose you wished to know the mass of 2.00 n aluminum sulfate: 10. Which conversion factor would you use to calculate the answer to your question? Circle the correct answer: CF3 CF4 11. What is the answer to your question? + 30 MIN

4 RUN TIME: 50 MIN POGIL 10 Page 4 of 5 Suppose you wanted to know how many moles were in g of H 2 : 8. Which conversion factor would you use to calculate the answer to your question? Circle the correct answer: CF3 CF4 9. What is the answer to your question? Recorders should submit the group s responses for validation before moving to Observation IV OBSERVATION IV: So, why do chemists need the mole? In previous class sessions we have studied the Law of Constant Proportions which states that each compound has a fixed ratio of its constituent elements. If you think about it and I want you to think about it it must mean that the constituent elements react with each other in a specific way. This means that the reaction is dependent on the number of atoms of the reactants available. Since atoms and FU are so small they can t be counted by hand, the chemist must weigh them to count them. The mole is an automatic conversion factor between the number of units and the mass of the units. Figure 1 illustrates the interconversion between numbers of FU and the mass of these FU. Figure 1. Diagram of the Interconversion of Numbers of FU and Mass of FU (AN = Avogadro s number; FM = formula mass) Using information contained in Observations I-III and Figure 1, answer the following items: 10. What box in Figure 1 represents moles? 11. Which box in Figure 1 represents mass? 12. Which box in Figure 1 represents numbers? 13. What conclusion can you draw from Figure 1 about the relationship between the specific number of FU and their mass? + 35 MIN

5 RUN TIME: 50 MIN POGIL 10 Page 5 of 5 Ask the instructor to validate your answers to Items After validation, label the boxes in Figure 2 and then use Figure 1 as a guide to solve the following problems. Round atomic masses to the nearest whole number before calculating molar masses and consider these masses as exact numbers. 14. What is the mass of 2.41 x FU of cupric oxide? 15. How many FU are there in g of calcium phosphate? 16. How many moles are there in 68.4 g aluminum sulfate? 17. What is the mass in grams of 1 FU of NaCl? The recorder should check with instructor to validate group responses. EXERCISE END. Managers should collect the GRF and RRF and place them in the left pocket of the group s POGIL folder MIN

POGIL EXERCISE 16 Concentration Terms and There Use

RUN TIME = 80 MIN POGIL 16 Page 1 of 10 POGIL EXERCISE 16 Concentration Terms and There Use Each member should locate his/her role in Table 1 and assume his or her role at this time. The new manager takes