Lab Practical
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Created by:
lucyrichardson7 on April 21, 2012
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71 terms
Terms | Definitions |
|---|---|
 how to use Ocean Optics spectrophotometer | (should already be calibrated) be careful when handling cuvette so you don't scratch it; put side with V on it facing the light source; read absorbance (in read) from computer screen |
| how to use digital pH meter | (should already be calibrated) keep electrodes wet; stir solution with electrodes to get an accurate pH reading |
| freezing point depression (#11): equation for freezing point depression | Δt(f)=K(f)m, Δt(f) is the change in freezing point, K(f) is the molal freezing point constant, and m is the molality of the solution |
| freezing point depression (#11): adding a solute to a solvent affects freezing point how? | lowers freezing point |
| molality = | moles of solute / kg of solvent |
| freezing point depression (#11): basic procedure | cool substance with an ice bath; temperatures are measured; temperature vs. time is graphed; done for a pure substance and for a known mass of solute added to a given mass of solvent; find Δt(f)=t(f)(pure substance) - t(f)(substance with solute) |
| freezing point depression (#11): how to find Δt(f)'s from graphs of temperature vs. time | if there is a supercooling dip: from the right side of the supercooling dip, extend the cooling line up left; where that extended line intersects with the initial temperature drop, is where you should measure temperature on the y-axis; do for graphs of pure substance and substance with solute added; find Δt(f) |
| freezing point depression (#11): if the unknown solute did not completely dissolve, how would this affect the calculated molar mass of the solute? | mm would be greater (b/c of smaller freezing point depression) |
| freezing point depression (#11): if the bath was too cold, how would this affect the calculated molar mass of the solute? | mm would be smaller (b/c of greater freezing point depression) |
| freezing point depression (#11): why is molality used instead of molarity? | molarity is temperature/volume-dependent |
| freezing point depression (#11): 2 techniques that would improve accuracy? | don't let bath get too cold; completely dissolve the solute |
| quantitative analysis of an alloy (#12) general procedure: | determine % composition of alloy, w/rt [Cu], [Ag]: [Cu]: use spectrophotometer to make a calibration graph (of absorbency vs. g Cu/50 mL of soln) if necessary and then use that graph to find the g of Cu in an unknown solution, based on absorbance; [Ag]: titrate aliquot with KSCN (one-to-one) |
| Beer's Law (2 forms): | A=log(Io/I)=Cε; Io=amount of light absorbed by a solution with known concentration, I=amount of light absorbed by a solution with unknown concentration; ε=molar absorptivity constant |
| quantitative analysis of an alloy (#12): how would scratches/fingerprints on the cuvette affect the [ ] of one of the species, and which one? | would increase [Cu] because would absorb more light |
| quantitative analysis of an alloy (#12): how would over titration with KSCN affect one of the species, and which one? | would increase [Ag] |
| quantitative analysis of an alloy (#12): how would over dilution of the alloy affect the results? | would decrease [Cu], would decrease [Ag] |
| quantitative analysis of an alloy (#12): units of emissivity (for ε)? | L/(cm*mol) |
| rate of chemical reactions (#13) basic procedure: | add known concentrations of 2 solutions and an indicator; measure the time it takes for the reaction to complete (with indicator); use times for different solutions to calculate order of reaction, overall and w/rt the different solutions |
| rate of chemical reactions (#13): how to calculate order of reaction w/rt one species | use 2 trials where [ ] of other species is constant; rate1/rate2 = ([x]1/[x]2)^x; solve for x, x=order |
| rate of chemical reactions (#13): how to determine overall order | add orders of each species |
| rate of chemical reactions (#13): how to write rate law expressions | rate = K[x]^a[y]^b |
| rate of chemical reactions (#13): how to calculate K | K=rate/([x]^a*[y]^b) |
| rate of chemical reactions (#13): how to calculate rate for each trial: | rate=Δ[I2]/Δt; where [I2] can be determined from [ ] of something else |
| rate of chemical reactions (#13): units of K | depends on order of reaction |
| rate of chemical reactions (#13): only physical parameter that can change the rate & k? | temperature |
| rate of chemical reactions (#13): how does increased temperature affect rate? | slower |
| rate of chemical reactions (#13): as time increases, how do rate and K change? | rate decreases, no change to K |
| rate of chemical reactions (#13): how to calculate initial concentrations? | use dilution formula M1V1=M2V2 |
| rate of chemical reactions (#13): how to calculate order of a reaction graphically? | plot log(rate) vs. log[x]; slope of line = order, W/RT X |
| Le Chatelier's Principle (#14): basic procedure? | given a reaction, see what happens when you add more of a product/reactant or take away some of a product/reactant; should be able to see shift by a change in solution color or the formation/removal of a precipitate (ppct) |
| Le Chatelier's Principle | when a system at equilibrium is subjected to an external stress, the system will shift in a direction (left or right) to counteract the stress and achieve a new equilibrium state |
| Le Chatelier's Principle (#14): changes in volume & pressure | PV=nRT; more P=less V, so equilibrium shifts to side with fewer moles of gas because there's less room; and vice-versa; look at stoichiometric coefficients & states |
| Le Chatelier's Principle (#14): changes in temperature | must know if run is exo- or endothermic; write ΔH on correct side and treat it like a product or reactant; if ΔH is positive, rxn is endothermic, so write ΔH on left side of rxn, and if temperature increases, it's like adding more of a reactant, and so the equilibrium shifts right to get rid of it |
| Le Chatelier's Principle (#14): changes in concentration | adding more of a product or reactant; rxn will try to get rid of that extra by shifting to the other side; i.e. if you add more of a reactant, rxn will shift to the right to make more products to maintain the equilibrium expression/equation |
| Le Chatelier's Principle (#14): addition of a catalyst | catalyst DOES NOT AFFECT position of equilibrium, just how fast the rxn gets there |
| Le Chatelier's Principle (#14): how adding solids affects rxns | doesn't really affect them because the concentration is fixed; also, solids are left out of K expressions |
| spectrophotometric determination of K(in) (#15): basic procedure | mix solutions with different specified concentrations and use Ocean Optics spectrophotometer to determine absorbency; use absorbency values to calculate K |
| spectrophotometric determination of K(in) (#15): how to calculate K | use standard K equation, with [ ]'s and coefficients as exponents; find [ ]'s of each species from absorbencies |
| spectrophotometric determination of K(in) (#15): how to adjust absorbencies for impurities | used in calculating ε; at lower wavelength, find A(imp) for basic form; at higher wavelength, find A(imp) for acidic form; A(imp)=absorbance of impurities |
| spectrophotometric determination of K(in) (#15): pH where indicator will change colors? | pH=p(Kin)=-log(Kin) |
| spectrophotometric determination of K(in) (#15): how to tell which reactions a certain indicator would be good for | indicator's K(in) should be in the range of +/- 1.5 pH units of the pH of neutralization (which varies depending on the kind of acid-base titration, like strong acid-strong base or strong acid-weak base) |
| spectrophotometric determination of K (#15): generic titration curves | study curves |
| spectrophotometric determination of K (#15): if average experimental K(in) is greater than literature value, how would pK(in) be affected? | would decrease; think of log scale |
| spectrophotometric determination of K (#15): why is K(in) used instead of K(a)? | so K(in) of indicator and K(a) of acid aren't confused |
| spectrophotometric determination of K (#15): indicator acts as a ___ ___ | weak acid |
| spectrophotometric determination of K (#15): when pK(in)=pH, where are the relative [HIn] and [In-]? | [HIn]=[In-] |
| spectrophotometric determination of K (#15): how would using too much indicator affect the rxn? | using le chatelier's principle, using too much of an indicator will shift the equilibrium to the right as the system works to get rid of it, which would increase [H+] in the products and decrease the pH |
| acid-base (#16): basic procedure | put known quantity of diluted acid in flask and add base until indicator changes colors; monitor pH with digital pH meter |
| acid-base (#16): equivalence point occurs when | moles of acid = moles of base |
| acid-base (#16): how to calculate pH with ICE table | use value of K and find the [H+] at equilibrium |
| acid-base (#16): how to calculate K | use pH to find [H+] and calculate K from corresponding expression |
| acid-base (#16): how would over titration of the base affect the base's molarity? | decreased molarity of base (and acid unknowns) |
| acid-base (#16): how would too much phenolphthalein affect the acid molarity and Ka value? | increase molarity and decrease Ka value |
| acid-base (#16): if the beaker was rinsed but not dried, how would this affect the initial pH and the pH at equilibrium? | higher initial and same at equilibrium |
| acid-base (#16): 2 methods of determining Kb | use ICE table to find equilibrium [ ]'s and then use K expression; or find the pH at the half-equivalence point, which gives pKa, then find Kb |
| Ksp for Cu(IO3)2 (#17): basic procedure | find [ ]'s of Cu by spectrophotometer (using calibration curve) and IO3 by titration (with 2 indicators because you titrate with Na2S2O3, diprotic); use to find Ksp w/ K expression |
| Ksp for Cu(IO3)2 (#17): how would scratches on the cuvette affect the Ksp? | would increase absorbency, increase concentration, and increase Ksp |
| Ksp for Cu(IO3)2 (#17): how would overtitration affect Ksp? | would increase the [IO3] and increase the Ksp |
| qualitative analysis (#18): basic procedure | run a series of tests on an alloy by making precipitates and solutions and testing them; the formation of a ppct or a certain color in solution indicates the presence/absence of an ion |
| qualitative analysis (#18): how to balance redox reactions | split overall reaction into 2 half-reactions, one reduction and the other oxidation; balance the number of atoms other than H and O with coefficients; add H2O to balance the # of O's; add H+ to balance the # of H's; add e-'s to balance the charge; if in basic solution, add enough OH- to both sides to cancel out the H+'s |
| qualitative analysis (#18): why is it important to run a control? | to give a comparison for each color change & ppct for each ion |
| electrolysis (#19): basic procedure | connect 2 cells (one with copper, another with carbon) with wires and connect to current; turn current on and wait until level of water in hydrogen collection tube is at the level of water in the steel beaker; mark level in tube, measure volume of gas collected |
| electrolysis (#19): unit conversions | 1 C = I amp-second; 96,500 C = 1 F = 1 mol e- |
| electrolysis (#19): how to calculate mass of element formed/lost with current and time | use unit conversions and stoichiometric ratios of half-reactions, including moles of e- used |
| electrolysis (#19): adjusting for vapor pressure | find vapor pressure of water at certain temperature and subtract from barometric pressure |
| how to calculate % error | (experimental - theoretical )/theoretical x 100; remember order because if your experimental answer is inflated, the % error should be > 0 |
| electrolysis (#19): what kind of half-reaction occurs at the anode? | oxidation reaction; remember because vowels are together - Oxidation, Anode |
| electrolysis (#19): if air bubble in collection tube is initially too high, how will H2 volume be affected? | H2 volume recorded will be too high |
| electrolysis (#19): if level of wet hydrogen is measured before all the bubbles popped, how will H2 volume by affected? | H2 volume recorded will be too small |
| electrolysis (#19): if electrolysis is stopped after the aqueous level in the test tube is below the water level, how will H2 volume be affected? | H2 volume recorded will be too high |
| Kf of Fe(SCN)2 (#20): basic procedure | find value of ε for substance by finding the absorbance of a known concentration; find the absorbance of a series of diluted solutions and graph [complex] or absorbance vs. mole fraction Fe3+; where max/peak is of graph, find mole fraction of Fe3+ and determine stiochiometric formula of complex from that; calculate Kf from ICE table with various concentrations |
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