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Ways to Separate Mixtures Filtering— (It works because of particle size.) Distillation • Distillation is used to separate chemical species by taking advantage of the differential strength of intermolecular interactions between and among the components and the effects these interactions have on the vapor pressures of the components in the mixture. • Simply put, if a solution of two or more liquids is being distilled, then it is the differences in their boiling points that makes the separation possible. – The higher the intermolecular forces, the more the molecules tend to “stick” together, so this will lower the vapor pressure, and this means the boiling point will be higher. • Distillation There are 2 steps to the process: Boil Step 1: _________ the solution. Condense Step 2: ________________ the vapor as is escapes and collect it. • Chromatography (paper and column) separates chemical species by taking advantage of the differential strength of intermolecular interactions between and among the components. Paper Chromatography http://chemsite.lsrhs.net/FlashMedia/html/paperChrom.html Column Chromatography http://chemsite.lsrhs.net/Fla shMedia/html/columnChro m.html Column Chromatography Gas Chromatography • Here is a chromatogram that took 30 minutes to produce in a gas chromatograph. It shows peaks for each of the different chemicals. The peaks were measured by a simple detector that can't tell the difference between molecules, but it can give you an approximate measure of the quantity of molecules in the gas as it exits the GC. It doesn't directly tell you which molecule accounts for each peak, but under the same conditions, a particular chemical will always show a peak at the same time. Precision vs. Accuracy • Precision measures how close a set of measurements are to each other. • Precision can also mean how “specific” a measurement is. − π is commonly written with only three digits of precisionī 3.14 − Your age might be measured to the nearest year, or month, or day. Each would have a different level of precision. • Below are two measurements of the mass of the same object. The same quantity is being described at two different levels of precision or certainty. • What is the volume? a) 17 mL b) 17.5 mL c) 17.49 mL d) 17.50 mL e) 17.500 mL • What is the length of the bar? 0 1 2 3 cm 3.22 cm or 3.23 cm or 3.24 cm (2 decimal places) 4 Significant Figures • Sig. Figs are used to indicate the precision of a measurement. • The number of significant figures in a measurement is the number of digits known with certainty plus one uncertain digit. − Example: 2.2405 g means we are sure the mass is _______ 2.240 g but we are uncertain about the nearest 0.0001 g.) • If a measurement is determined simply by counting the number of items… (not estimating!!)… then we say that the precision is ∞. − Some conversions have an infinite amount of sig. figs because they are known to be EXACT. o Examples: 60 seconds/min 2.54 cm = EXACLY 1 inch 1 meter = 1000 mm (Each value in the conversion factor would have an infintie # of sig. figs!!) The Box-and-Dot Method: A Simple Strategy for Counting Significant Figures Suppose we want to determine the number of significant digits in the following measurement. In this method, a “dot” is the decimal point. 0.0020530 meters Step 1 -Draw a box around all nonzero digits, beginning with the leftmost nonzero digit and ending with the rightmost nonzero digit in the number, including any “sandwiched” zeros. 0.0020530 meters Step 2 - If a dot is present, draw a box around any trailing zeros. 0.0020530 meters Step 3 - All the boxed digits are significant. (5 sig. figs.) More Counting Sig. Figs. Practice Problems State the number of significant figures in each measurement. 1) 2804 m 4 s.f. 2) 2.84 km 3 s.f. 3) 5.029 cm 4 s.f. 4) 0.003068 miles 4 s.f. 5) 4.6 x 105 gallons 2 s.f. 6) 4.06 x 10-5 inches 3 s.f. 7) 750 liters 2 s.f. 8) 0.000750 feet 3 s.f. 9) 75,000 seconds 2 s.f. 10) 75.00 meters 4 s.f. 11) 75,000.0 grams 6 s.f. 12) 10 cm 1 s.f. Rounding Rules • Generally speaking, on the AP Exam, you should round off your final numerical answers to 3 sig. figs, but there are rules to follow. • Final calculations are only as significant as the least significant measurement. Significant Figures in Calculations • For Multiplication and Division: - Round to the least number of significant figures. 32 cm2 Example: 6.221 cm x 5.2 cm = _______ • For Addition and Subtraction: - Round to the least number of decimal places. Example: 20.4 g – 1.322 g = _______ 19.1 g Accuracy • Accuracy is the closeness that a single measurement is (or the average of several measurements are) to the accepted value. • Percent Error is a way to measure accuracy. % Error = (Accepted Value – Measured Value) (Accepted Value) x 100 Accurate & Precise Precision without Accuracy Accuracy without Precision No Precision & No Accuracy Density • Density = mass/volume mass Density x volume • Density is what determines if something will sink or float. More dense = sinks Less dense = floats Same density = “hovers” • Helium balloons can float in air because helium is less dense than air, not because it is “lighter” than air. • Density can be used as a “conversion factor” as well. - Examples: 1 g of H2O = 1 mL of H2O @ 4°C 4.0 g of Helium = 22.4 L of Helium @ STP