prelabpostlab answers

prelabpostlab answers - EXPERIMENT#1 GRAVIMETRIC ANALYSIS...

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Unformatted text preview: EXPERIMENT #1: GRAVIMETRIC ANALYSIS ( 20 Points) PRELAB ANSWER KEY: 1. (1 point) 3 OxH ⇄ 3 Ox – + 3 H + Al 3+ + 3 Ox – ⇄ Al(Ox) 3 (s) 2. (1 point) Answers should be reported to four significant figures . As a result, 0.01 M should be reported as 0.01000 M (four sig. figs). 3. (1 point) The “limiting reagent” determines the quantity of product formed . In Experiment #1, the limiting reagent is Al (i.e., Al 3+ ) as this is the target ion for precipitation. 4. (2 points) 0.00168 excess moles Ox – are added . 5. (1 point) OxH(aq) ↔ H + (aq) + Ox – (aq) As the p H is lowered (i.e., pH < 4), the oxinate ion is increasingly tied up in the protonated form, making it unavailable for precipitation. Meanwhile, at high p H (i.e., pH > 8), Al 3+ ions react with OH – ions to form the [Al(OH) 4 ] – complex ion. 6. (1 point) 89.2, 87.6, 91.3, 86.4, 93.0 (Note: this is most easily done in Excel) Average = 89.5 Standard deviation ( σ ) = 2.7 2 σ = 5.4 (w/ sig figs !) 7. (2 points) From Q-test: a 4th trial should be done . 8. (1 point) 1.208 M 9. (0 point) See MSDS Information POSTLAB ANSWER KEY: 1. (1 point) 36.7 g of O 2. (2 points) 84.70 wt % SrCO 3 3. (2 points) YBa 2 Cu 3 O 6.5 4. (3 points) 57.33 wt % KClO 3 (and 42.67 wt % NaCl) 5. (2 points) C 6 H 12 O 6 EXPERIMENT #2: VOLUMETRIC ANALYSIS OF AN ORGANIC ACID PRELAB ANSWER KEY: 1. (1 point) The criteria for a "primary standard" are: Readily available in pure form Easy to dry and not so hygroscopic (i.e., water attracting) that it takes up water during weighing process Large molar mass to minimize the consequences of errors in weighing 2. (1 point) Stoichiometric (equivalence) Point : point in a titration when just enough titrant has been added to react completely with the substance being titrated (i.e., analyte). End Point : point in a titration when the indicator changes color. 3. (3 points) Dissolve 0.800 g of NaOH in (<200 mL) water , and then dilute it to exactly 200.00 mL (in a 200.00 mL volumetric flask). 4. (0 point) It is not critical to measure the volume of water in Step I.4 accurately because it is added to the NaOH solution to prepare a solution that will be standardized in Part II. 5. (3 points) The phenolphthalein pink color at the end point disappears after a few seconds as the newly generated basic form of the indicator (In- , pink) reacts with H 3 O + from newly dissolved CO 2 from the air to form the acid form of the indicator (HIn, colorless) (i.e., it shifts the 3rd equilibrium below back to the left according to Le Chatelier’s principle): CO 2 (aq) + H 2 O (l) ↔ H 2 CO 3 H 2 CO 3 + H 2 O (l) ↔ HCO 3 – (aq) + H 3 O + (aq) HIn (aq) + H 2 O (l) ↔ In – (aq) + H 3 O + (aq) Colorless Pink 6. (2 points) K a = 5.6(2) ⋅ 10-10 (Note: answer should have 2 sig. figs. due to the rule* for sig. figs. on logs and exponentials. However, I will also accept 3 sig. figs. in this case) 7. (0 points) See MSDS Information * Important Reminder on sig. fig. rules for Logarithms and Exponentials:* Important Reminder on sig....
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This note was uploaded on 12/03/2008 for the course CHEM 630888 taught by Professor Berniolles during the Fall '08 term at UCSD.

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prelabpostlab answers - EXPERIMENT#1 GRAVIMETRIC ANALYSIS...

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