Chem 222. Aug 25, 2003 #2

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Ralf Barrett
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1 Chem 222 Aug 25, 2003 #2

2 Announcement Safety No Sandals or High Heels in the Lab Your foot may be hurt by broken beakers or chemicals No short pants in the Lab Your leg may attract some nasty chemicals. Lab Please label your beaker (or chemicals) when you keep yours in an oven or any shared space. Report The due date of the Lab report for Exp 2 is Sep 8 or 9. Submit the report at beginning of your lab. Quiz Quiz will be held on next Thursday. The chapters covered will be 1-3.

3 Quick Review Diluting solution Moles = Molarity (mol/l) Volume (L) From P 18 Ex. 1 Q) HCl stored in most laboratory has molarity of 12.1 M. How do you obtain 100 ml of 1 M HCl solution. Assume you use y ml of 12.1 M HCl solution (12.1 M) ( y ml) = (1 M) ([Q1] ) y = (1 M) ([Q1] )/(12.1 M) =

4 Converting Units of Concentration How to convert? Wt % Molarity Obtain how many moles of substance exist in 1L of solution from wt %. 1) The weight of 1L solution is given by 1000 d [g/ml] = 1000d [g/l], where d [g/ml] is the density of the solution. 2) Then, the weight of the solute in 1L of solution is (wt %/100) 1000d[g/L]) 3) Molarity =Moles/L = {(wt %/100) 1000d[g/L]}/ MW[g/mol]

5 Converting Wt % to Molarity & Dilution Ex. 2 in P18 Q) A concentrated NH 3 solution contains 28.0 wt % NH 3. The density of the solution is g/ml. What volume of this reagent should be diluted to 1 L to make 1 M NH 3 solution. The weight of 1L of solution is ( )[g/L] The weight of NH 3 in the 1L of solution = [Q1] ( )[g/L] = g Hence, the molarity of the concentrated NH 3 = [g/l] /17.03 [g/mol] =14.8 M To dilute y ml of the solution into 1L of 1M 14.8 M y ml = 1 M 1000 ml

6 Unit Conversion in Concentration Wt % Molarity Obtain how much mol of substance exists in 1L of solution M =mol/l = (Wt %/100) (d[g/ml] 1000) /MW[g/mol] d is the density of the solution Molality Wt % (correction) Assume that 1L of solution contains A g of solute and B kg of solvent. Molality (m) A/(MW B) Wt % 100 A/(1000B + A) = 100 (A/B)/( A/B) = 100 (m MW) /{ (MW m)}

7 Problem 1-27, 28 Assume x g of B(OH) 3 is mixed with water to prepare 2 L of M solution M = (x g /[Q1] g/mol )/ (2.00 L) Assume y g of B(OH) 3 is mixed with 2 kg of water m = (y g /61.83 [g/mol] )/ ([Q2] [kg])

8 1-4 Solution and Stoichiometry Analysis of Fe 2+ and Fe 3+ (For Exp. 3) 2) Reduce Fe 2+ to Fe 3+ with H 2 O 2 2Fe 2+ + H 2 O 2 + 2H + 2Fe H 2 O (1-5) 3) Preticipate Fe 3+ as hydrous iron oxide (gel) & bake it and weigh Fe OH - + (x -1) H 2 O FeOOH xh 2 O (gel) ºC Fe 2 O 3 (Solids) (1-6) If you have y g of Fe 2 O 3,(M.W ) Moles of Fe 2 O 3 = y/ Moles of Fe 2+ or Fe 3+ in the solution = 2 y/159.69

9 ppm & ppb ppm (parts per million) ppb (parts per billion) For solids Mass of substance ppm/ppb = Mass of whole sample 10 6 / The concentration of C 20 H 42 (FM ) in winter rainwater in Figure 1-3 is 0.2 ppb. Assume that the density of rainwater is close to 1.00 g/ml; find the molar concentration of C 20 H 42. A. Assume 1 L of rainwater contains y g of C 20 H = (y g/1000 g) [Q1] y = [g]

10 Ppm and ppb for gas For gas, For solids ppm/ppb= Volume of substance 10 6 /10 9 Volume of whole sample Q1-26 A concentration of 5.24 ppm He means 5.24 µl of He per 1 L of air. Moles = PV/RT (See Q1-5 = [Q1] /( ) This number corresponds to the molarity of He in air.

11 2-1 Safety, Ethical Handling of Chemical Waste Labeling on chemicals Q. Label all vessels to indicate what they contain, when (date & year) you prepared the sample, and who prepared (your name). Why? Nobody knows what the sample is without proper labeling. The sample must be analyzed before being disposed. If the sample is too old, it is better not to use it in case the sample is degraded. Your colleague (or we) may dispose your important sample without your name on it.

12 Do not throw away chemicals into sink! It is dangerous to throw any hazardous chemicals to the sink. To identify hazardous chemicals ask your supervisor or check MSDS (Material Safety Data Sheet). MSDS for most chemicals is available at

13 2-2 Lab Notebook The notebook needs to be a bound notebook (no loose leaves). You have to use ink to record your results. Why? Your lab notebooks 1. State what was done 2. State what was observed 3. Be understandable to someone else

14 2-3 Analytical Balance Finger prints may change your mass. Avoid moisture when you cool your sample or vessel. Buoyancy: Correction needed to compensate for the effect of air

15 How buoyancy can be explained To add weights, you have to exclude air from the top of the balance. M M - Vwda - Vda M V w d a = M Vd a Using V=M/d & V W = M /d w M (1 d a /d w ) = M(1-d a /d) Q.What kind of material gives you large measurement error?

16 Low density materials need larger corrections If you weigh water, the error due to buoyancy is less than 0.1 %

17 Please read how analytical balance works!

18 2-4, 2-5, 2-6, 2-7, 2-8 Please read about the usage of Burets Volumetric Flask Pipettes Micropippettes Syringe Desiccator, dessicant

19 Important When you transfer any chemical from a chemical bottle, use a clean and dry pipette. Why? If you use a pipette containing any chemical including water, you contaminate the chemical in the bottle. The contaminated chemicals need to be thrown away. Micro pipette delivers solution from a disposable pipette tip that minimizes contamination. Do not use your fingers to attach the tip in order to avoid contamination.

20 How do you read meniscus? The volume is [Q1] ml

21 Filtration Filtrate

22 2-10 Microsoft Excel If you don t know Excel very much, ACCC offers excellent seminars on Microsoft Excel. These seminars are free for UIC students, but you need registration. About the details, see title.html Creating Spreadsheets and Graphs with Microsoft Excel: Part 1 [Hands-on] Tue 9/7 1-3pm, 408CCC Wed 9/ pm, 179BGRC Mon 9/20 1-3pm, 408CCC

23 Ch. 3 Experimental Error 3-1 Significant Figures The number of significant figures is the minimum number of digits needed to write a given value of scientific notation without loss of accuracy. What does it mean? significant figures The value is within a range from to You can also write (9.25 ±0.01) significant figures significant figures significant figures significant figures significant figures

24 How many digits should we include? If the error is ±200, you can denote (9.25 ±0.02) 10 4 It is often allowed to add one extra digit as Ο (9.250 ±0.020) 10 4 Ο (9.252 ±0.024) 10 4 But it is not meaningful to add more than one digit as ( ±0.0200) 10 4 ( ± ) 10 4 ( ±0.0243) 10 4 ( ± ) (a) Round to 4 significant figures. (b) Round to 4 significant figures. (c) Round to 3 significant figures.

25 How do you read meniscus? The volume is ± [Q1] ml

26 Home work Problems Ch3: 3-A, 3-1, 3-2, 3-3, 3-5 Read Ch 3

27 If measurement error is ±1, is the difference between 9.2 and 9.3 significant?

Chem 222 #3 Ch3 Aug 31, 2004

Chem 222 #3 Ch3 Aug 31, 2004 Announcement Please work in the lab session you registered for. If you are found to work in any other lab without my permission, no points will be given for the lab. Please