# Lab Practical

## 71 terms

### 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

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]

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 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

temperature

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)

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

weak acid

[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