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in a Mixture FINAL Final Exercise – 105 points Gravimetric Determination of NaHCO 3 FINAL 2

Eligible Students are ONLY those who have excused absences from either a TEST (105 point) or PRELIMINARY (55 point) Exercise SHOULD HAVE SIGNED UP ON SHEETS POSTED IN EACH LABORATORY ROOM Everyone does the same make-up exercise. You can download the exercise at: 3 CHE 133 MAKE-UP LABORATORY EXERCISE Mon 11/19 - 2:00 PM or Tue 11/20 – 2:30 PM

4 SunMonTueWedThuFriSat 1819Lab:Makeup20Lab:Makeup21 No Classes Lec: 054a Lab054b27Lab054b28Lab054b29Lab054b30LecReview1 23LecReview Lab:Quiz Chk Out The Calendar

Purpose: percent of NaHCO 3 Determine the percent of NaHCO 3 in a mixture by Gravimetry Concepts: Thermal DecompositionConstant Weight Techniques: Weighing Thermal Decomposition Apparatus: Analytical BalanceHot Plate CrucibleCrucible Tongs 5

This Part of the Exercise is, again Conceptually Simple. 1.Weigh Sample, 2. Decompose the sample by heating lost 3. Weigh product to get weight of CO 2 & H 2 O lost weight loss = w CO2 + w H2O = n CO2 * Molar Mass of CO 2 Molar Mass of H 2 O + n H2O * NaHCO 3 (s)  Na 2 CO 3 (s)+ H 2 O (g)+ CO 2 (g) w Sample. weight = mol X Mol Mass + NaCl (s) 2 NaHCO 3 (s) 6 weight loss

4. Get moles of CO 2, H 2 O and NaHCO 3 lost NaHCO 3 (s)  ½ Na 2 CO 3 (s) + ½ H 2 O (g) + ½ CO 2 (g) n CO2 = n H2O = ½ n NaHCO3 = ½ n NaHCO3 * ( ) or, n NaHCO3 = 5. w NaHCO3 = 6. Compute Percent Composition of Sample Pct NaHCO3 = 100 * w NaHCO3 / w Sample Molar Mass of NaHCO 3 (= n CO2 * n H2O * ) so, weight loss 2 * ( weight loss ) / n NaHCO3 * g / mol 2½½½ 7

w NaHCO3 = Pct NaHCO3 = 100 * w NaHCO3 / w Sample n NaHCO3 * g / mol 2 * ( weight loss ) X = * ( weight loss ) = * ( weight loss ) / w Sample How is Pct NaHCO 3 related to the weight loss? 8 w NaHCO3 = ( weight loss ) / n NaHCO3 = w NaHCO3 = 2 * ( weight loss ) Showed that:

9 w NaHCO3 = ( weight loss ) / How much weight would g of pure NaHCO 3 lose? w NaHCO3 = 1 g 1 g = ( weight loss ) / weight loss = g Pct NaHCO3 = 100 * w NaHCO3 / w Sample * ( weight loss ) / w Sample = * / = %

Each mole of NaHCO 3 produces one mole of CO 2 and one mole of H 2 O when decomposed by heating. 10 A.True B.False

Each mole of NaHCO 3 produces one mole of CO 2 and one mole of H 2 O when decomposed by heating. B False NaHCO 3 (s)  ½ Na 2 CO 3 (s) + ½ H 2 O (g) + ½ CO 2 (g) 11

Unlike gasometric part, this part is NOT complicated by any special adjustments or considerations. Accuracy and precision of result are solely a function of care with which weighings and decomposition are conducted! 12

13 The melting point of NaCl is 800 o C. The peak temperature on a hot plate is well below 800 o C. (Molten NaCl cools to produce a solid that adheres to porcelain. ) Na 2 CO 3 (product) melts at 850 o C AND begins to decompose to CO 2 + Na 2 O at that temperature. NaHCO 3 decomposes rapidly at T > 200 o C forming Na 2 CO 3 How do the Various Substances behave at High Temperature? But make sure it is at least 200 o C Would result in additional weight loss 850 o C is bright red heat

If some sample is lost from the crucible during the heating, the resulting percentage of NaHCO 3 that we calculate will be ______ the actual value. 14 A.smaller than B.unchanged from C.larger than

15 OMG ! ! ! ! ! Let’s try again. This time, with enthusiasm (and discussion)

If some sample is lost from the crucible during the heating, the resulting percentage of NaHCO 3 that we calculate will be ______ the actual value. 16 A.smaller than B.unchanged from C.larger than

If some sample is lost from the crucible during the heating, the resulting percentage of NaHCO 3 that we calculate will be ______ the actual value. C Larger If sample is lost, the apparent weight loss will be too large and, therefore, the % NaHCO 3 will be too large. 17 Pct NaHCO3 = * ( weight loss ) / w Sample

18 Last Year Right to Right 48 Right to Wrong 31 Wrong to Right 46 Wrong to Wrong 114

19 What precision & accuracy should you expect? The only sources of error are: loss of sample during run don’t spill any sample incomplete heating reheat until two successive weighings differ by Δ mg overheating not likely on hotplate weighing errors record numbers carefully To find weight loss, you need the weight of the dry, empty crucible A word from Obviousman To be specified

try to use same balance throughout a run DO NOT TINKER WITH BALANCE SETTINGS! This exercise depends heavily on the proper use of the analytical balance! ?? Standard Penny ?? Weigh a coin before anything else Check the weight of the “standard” coin before each subsequent weighing. 20

21 How do we know when the reaction is done? When all the H 2 O and CO 2 have been lost, the sample will cease to lose weight. I.e., two successive weighings before and after heating should be the same How should we define “the same” (Δ)? Our operational definition is that they differ by less than ± 5 mg. I.e., |w 2 – w 1 | < g w w w w w w w w mg- 5.2 mg- 2.8 mg+ 3.4 mg e.g.  X X NOT 0.5 mg!

22 What are the last 3 digits of your student ID? Bonus Question (5 extra points)

Reproducibility w sample analytical balance ( / ) < 0.1% w residue analytical balance ( / ) < 0.1% 5mg 5mg Our criterion for final weight is one that differs from the previous weight by less than 5 mg, so our weight loss should differ from the “true” weight loss by less than 5 mg. For a 1 g sample, the minimum weight loss is ~369.1/2 mg, and 5mg 5 mg constitutes less than a 100 X 5 X 2 / = 2.7 % error in the weight loss 23 50% NaHCO 3 Uncertainty in Weighing: But

Do the analysis twice, and 24 Handle the crucible ONLY with crucible tongs!

Wt of crucible + sample g Wt of crucible g Wt of sample Wt of crucible + residue –aft heat g Wt of crucible + residue –aft heat g Wt of crucible + residue –aft heat g Wt of crucible + residue –aft heat g 19.7 mg 9.3 mg 3.4 mg Wt of residue [ – ] Wt loss Wt loss [ – ] Wt NaHCO 3 [ / ] % NaHCO 3 [100 X / ] DATA SHEET g g g % g mg Wt loss per gram of NaHCO 3 25

If the NaHCO 3 is not completely decomposed when we record our final weight, the % NaHCO 3 we compute will be ________ the actual value. 26 A.Smaller than B.Equal to C.Larger than

If the NaHCO 3 is not completely decomposed when we record our final weight, the % NaHCO 3 we compute will be ________ the actual value. A Smaller than If NaHCO 3 is not completely decomposed, the apparent weight loss will be too small and, therefore, the % NaHCO 3 will be (too) small. 27 Pct NaHCO3 = * ( weight loss ) / w Sample

Suppose our second result is 52.41% Average = = % Avg Dev = = 0.56 % X 0.56 Pct Dev = = 1.1 % Review - Avg, Avg Deviation, Pct Deviation 28

At the end of the exercise, the crucible should contain only NaCl. 29 A.True B.False

At the end of the exercise, the crucible should contain only NaCl. B False NaHCO 3 (s)  ½ Na 2 CO 3 (s) + ½ H 2 O (g) + ½ CO 2 (g) The crucible contains NaCl and Na 2 CO 3 30

Next Week – No Lecture Nov 19 & 20 - Make-up Labs Fri, Nov 30, Mon, Dec 3 – REVIEW Attendance Optional 31 SuMoTuWeThFrSa

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WEAR YOUR When you prepare, and eat your holiday turkey,pita,tuna,tomatoes,pasta, whatever tofu,turnips, peas,potatoes, 33

34 It has been fun learning with you

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