Chem 105 Mon. 9 Sept 2009 Prof. John Keller

Transcription

1 Chem 105 Mon. 9 Sept 2009 Prof. John Keller -Don t forget to go to your lab this week! -As of Wed 8:30 AM, 54/105 enrolled in OWL, and 39 clicker IDs registered TODAY: 1. OWL & clickers 2. Elements, Compounds, Mixtures 3. Measurements in chemistry: Temp scales; metric system 4. Significant Figures and math operations 9/9/2009 1

2 Postponed to Monday night points of Chapt 1 OWL homework are due Thurs night at 11:00 PM. So far, about 50 students have registered in OWL and have begun to do their homework. Nice going! OWL total Wed 8 AM 9/9/2009 2

3 Please register your clicker serial number on the OWL website by Sunday Sept. 13, 6 PM. On Monday the 14 th, we will begin recording clicker responses. These will be counted in the class participation portion of the Chem 105 grade. Hallway clicker test. 9/9/2009 3

4 2. Elements, Compounds, and Mixtures 9/9/2009 4

5 All known substances Pure One element (just one kind of atom) Gold bar N 2 gas Hg puddle Mixture = more than one element or compound A Compound Homogeneous Heterogeneous = atoms in constant ratio (one ionic compound or single type of molecule) Salt crystal (NaCl) Water (H 2 O molecules) = composition does not vary with location Air Gasoline Whiskey Natural gas Seawater (filtered) = composition varies with location Seawater (unfiltered) Soil Steel Living cell Fog 9/9/2009 5

6 Let s warm up our clickers! 9/9/2009 6

7 pure water Whiskey 38 The above materials are respectively.. 1. (l) Homogeneous mix; (r) heterogeneous mix 2. (l) Pure compound; (r) homogeneous mix (l) Element; (r) homogeneous mix 0 4. (l) Pure compound; (r) heterogeneous mix (l) Homogeneous mix;... (l) Pure compound;... (l) Element; (.. (l) Pure compound; (.. 9/9/2009 7

8 Only a handful of elements occur in nature in pure form. 9/9/2009 8

9 But there are >25 million known compounds! One element A compound Homogeneous Heterogeneous = 2 or more elements in constant ratio (forming ionic and/or covalent bonds) Gold bar N 2 gas Hg puddle Salt (NaCl) Water (H 2 O molecules) These are important for chemistry We cannot study individual molecules so we must experiment with large collections of molecules (pure compound). And we then INFER the properties of the individual molecules using mental or mathematical models. Air Gasoline Natural gas Seawater (filtered) Gatorade Seawater (unfiltered) Soil Steel Living cell 9/9/2009 9

10 Purification (separation) of chemical compounds from mixtures why is it important? Study the properties of pure compounds - Measure amounts of different components in a mixture, such as toxic impurities. - so we can use different components, for example in petroleum distillation. Evaporating seawater 9/9/

11 Gas chromatography is used to measure amounts of volatile compounds in complex mixtures 9/9/

12 H H H C C C C C C H H Gasoline is a homogeneous mixture. Gas Chromatography of Gasoline H Benzene 9/9/

13 3. Measurements in chemistry (and other sciences!) 9/9/

14 9/9/

15 Demo. Air in the balloon shrank as it was cooled to about -190 C 9/9/

16 Units of length, volume, others (metric system) Length: Basic unit is meter (m). Then apply prefixes to denote different lengths. Please remember and use these prefixes. Kilo Deci Centi Milli Micro Nano Pico Femto /10 1/100 1/10 3 1/10 6 1/10 9 1/ /10 15 km m dm cm mm µm ( micron ) nm pm fm The Ångstrom is a non-metric unit used in molecule diagrams. (1 Ångstrom (Å) = 1/10 nm = 100 pm 9/9/

17 H Atom ~ 1 Å = 100 pm 5.0 Å Nucleus ~ 0.01 pm Benzene molecule 9/9/

18 Average size protein ~ 80 Å (= 8.0 nm) 9/9/

19 length_scales.html Shows animated review of lengths from m --> m (0.1 femtometer) 9/9/

20 1/1000 = 10-3 A millionth A billionth Volume: basic quantity is Liter (L). ml, µl, nl, pl 1 microliter (µl): smallest pipet-able (Pipetman) ~50 picoliter (pl): volume of inkjet printer droplet 9/9/

21 Stop motion photography of Inkjet droplets forming ~50 picoliter (pl) Univ of Florida 9/9/

22 Pipetman 1000 ( microliters) Pipetman 20 (1-20 microliters) 3.0 ± 0.2 µl This is a typical volume measurement used in biochemistry experiments with DNA or protein liquid samples. 9/9/

23 Significant figures The precision of any measurement is limited to about ±1 or ±2 in the last readable digit. Example: If we measure 875 µl with a Pipetman, the actual volume may be anywhere between 874 and 876 µl. Therefore it is incorrect to report this volume as µl or µl. These numbers imply a precision (±0.1 or ±0.01 µl) that does not exist for the Pipetman. Define: significant figures = the reliable digits of a measurement or calculated quantity. 9/9/

24 Accuracy = How close the average of several measurements is to the true value. Precision = How scattered the individual measurements are around the average. We can test both the accuracy and precision of a Pipetmen by weighing the water that is released from several volume measurements. Let s assume the density of water is g/ml. And assume that the dial on the Pipetman is set at 875 µl. Then, if the Pipetman measurements are accurate, the average sample would have a mass of g. Say we did TEN measurements of 875 µl with three different Pipetmen. We might see results like this: g This one is accurate. The average value = g. And the precision is about ±1 µl g This one has about the same precision, but is less accurate. The average value = g. Why? Perhaps the internal mechanism is working correctly, but is offset slightly. This is called a bias g This one is less precise than the first example, say ±2 µl, but is accurate. The average value = g. Why? Perhaps the mechanism is set correctly, but random air leaks are occurring 9/9/2009 around the tip. 24

25 Rules of significant figures 1. All non-zero digits are significant 2. If there is no decimal, only internal zeros are significant 3. If there is a decimal - leading zeros are not significant - trailing zeros are significant 4. Using scientific notation, all zeros are significant # sig fig x x x /9/

26 875 µl has three sig figs 802 µl has three sig figs 810. µl has three sig figs We can also use scientific notation, which is less ambiguous, to report measurements x 10 2 µl has three sig figs. Converting these to different units does not change # sig figs., ml still has three sig figs ml still has three sig figs ml still has three sig figs leading zeros are NOT significant internal zeros, and trailing zeros to right of the decimal, are significant 9/9/

27 See Counting.pdf on course website 9/9/

28 Mathematical operations with measurements: How many significant figures are in the answer? In multiplication, division (and raising to power): The answer has number of sig figs of the multiplier (or divisor) with least number of sig figs. RULE: CARRY OUT THE OPERATION first with excess sig figs, then round off. 120, which has two sig figs, would also be correct x 5055 = = x 10 2 = 1.2 x 10 2 TWO sig figs The underline shows which positions contain sig figs (TWO). Round up because there is a 6 in the next position. 9/9/

29 The correct answer for the following operation is x =? /9/

30 Addition or subtraction: the number of decimal places in the result is equal to the number of decimal places in the addend that has the fewest decimal places. ***Hint write out in vertical format to see decimal places.**** Notice we leave the addends AS IS, and carry through with the complete addition operation for all decimal places. ONLY THEN do we round off to the correct decimal place Answer with correct number of sig figs. In general, the answer for an addition may have more, same as, or fewer sig figs than any of the addends! For example in this case the addends have 6, 5, and 3 sig figs, but the answer has 4! This is correct. 9/9/

31 The End 9/9/