CHEM 221: Midterm Concepts and Vocabulary (pt. 1)

quantitative analysis

how much of a chemical substance is present

qualitative analysis

what chemical substance is present

ordinate

y-axis

abscissa

x-axis

titrimetry

method based on a measured volume

gravimetry

methods based on measured weight

electrochemical analysis

rely on measurement of potential, current, resistance and charge

spectral methods

interaction of an analyte with electromagnetic radiation (spectrophotometry, spectroscopy)

chromatography

separation of materials due to its interaction with two different phases

chemometrics

statistical treatment of data, how we deal with and process data

steps for a typical quantitative analysis

1. choosing a method

2. acquiring a sample

3. processing the sample

4. eliminating inferences

5. calibrating and measuring concentration

6. calculating results

7. evaluating and estimating their reliability

complete analysis

the goal is to determine the amount of each component in a sample

partial analysis

the goal is to determine the amount of one or a limited number of components (without regard for total composition)

factors to consider before an experiment

accuracy and sensitivity, cost, number of samples to be assayed, number of components in a sample

mass vs. weight

mass never changes, weight changes based on gravitation force

buoyancy correction

how to read pipettes, burets, and graduated cylinders

make sure the apparatus is at eye level, the lower point is the volume

solute

minor species in solution

solvent

major species in solution (dissolves the solute)

percent concentration (wt%)

grams of solute per 100 grams of solution

molarity

moles of solute per liters of solution

molality

moles of solute per kilograms of solvent

analytical molarity

total # of moles of a solute in 1L

equilibrium molarity

molar concentrations of particular species after dissociation

strong electrolyte

substances completely dissociations into ions in a solution

weak electrolyte

substance only partially dissociates into ions in solution

volume percent (vol%)

(volume of solute/total volume of solution) x 100

ppm

(mass of substance/mass of sample) x 10^6

specific gravity

ratio of a substance mass and the mass of an equal volume of water

accuracy

the closeness of the measurement to the target or reference value

precision

the repeatability or consistency of a measurement

error

measurement for accuracy, E = measured - target

relative error

(measured-target)/target x 100

relationship between repetitions and error

the more measurements you make, the closer you will get to the average which means the error will be decreased

addition and subtraction sig figs

only digits after the decimal point matter

multiplication and division sig figs

all digits matter

systematic/determinate errors

problem with the methods, all errors are of the same size, magnitude and direction

random/indeterminate errors

based on limits and precision of a measurement, can be treated statistically

examples of determinate errors

potential instrumental errors (variation in temp, contamination of the equipment, component failure), method errors (slow or incomplete reaction, unstable species, side reactions), personal errors

when is indeterminate error present?

ALWAYS

relative error %

(loss or excess/true quantity) x 100

number of measurements and confidence limits relationship

increase the number of measurements, decrease the confidence limits

confidence interval

the range of values within which the population mean is expected to lie with certain probability

confidence level

the probability that the true mean lies within a certain interval, usually expressed as a %

what are linear regressions often used for?

calibration purposes

t-test case 1

compare a small sample mean with a true mean

t-test case 2

compare two sets of measurements

t-test case 3

compare individual differences

three common approaches for quantification

calibration curve, standard addition, internal standard

why do you want a higher slope for a calibration curve?

higher slope value means it has a higher sensitivity

calibration sensitivity

m, the slope of the linear regression

limit of detection

(3.33 x standard deviation of the blank)/slope of the calibration curve

limit of quantitation

(10 x standard deviation of the blank)/slope of the calibration curve

why is standard addition a good calibration technique?

it minimizes the impact of matrix effects by adding known concentrations of the analyte directly to the sample ensuring the standard and unknown samples have identical matrices

internal standard process

add a known amount of a standard analyte similar to the analyte of interest to the sample, measure response of analyte and standard

response factor

compares the signal produced by a substance to the amount of substance that produced the signal

nonelectrolyte

compounds that are soluble in water but do not dissociate in ions (no increase in conductivity is observed)

arrhenius acid

produce hydronium ion in aqueous solutions

arrhenius base

produces OH- in aqueous solution

bronsted-lowry acid

proton donor

bronsted-lowry base

proton acceptor

lewis acid

electron acceptor

lewis base

electron donor

autoprotolysis

chemical reaction where a proton is transferred between two identical molecules

strong acid and bases

completely dissociate in aqueous solutions

weak acids and bases

incompletely dissociate in aqueous solutions

lechatlier’s principle

1. have a system that is in equilibrium

2. subject the system to change

3. equilibrium is disturbed

4. system proceeds back to equilibrium such that the change is offset

K (equilibrium constant)

Kw (ionic product of water)

= [H₃O⁺][OH^-]

Ksp (solubility product)

used for ionic materials that are slightly soluble in water (no solids)

constant of formation (Kf)

constant of decomposition

complex formation

two or more species join together to form a single species

equilibrium constant of acids (Ka)

equilibrium constant of bases (Kb)

dissociation for conjugate acid/base pairs

Ka x Kb = Kw

effect of ion charge on equilibria

electrostatic forces cause each ion from a dissociated species/reactant to be surrounded by a “sheath” of oppositely charged ions from the added electrolyte, this reduces ion affinity, increases solubility

electrolyte effect

• affinity of the ion is reduced

• solubility increases as more ions are “pulled” into the solution

• the greater number of electrolyte ions, the greater the effect

• in dilute solutions, “sheaths” are not likely

ionic strength

activity coefficient

“effective” concentration of a species depends on the ionic strength

properties of the activity concentration

measures the “effectiveness” of a species in an equilibrium, independent of the nature of the electrolyte

• assumed 1 for unchanged molecules

• the more charge an ion carries, the faster it will change from 1

dubye-huckel equation

extended debye-huckel equation

debye-huckel equation for the mean activity coefficient

limits for debye-huckel equation

only works for extremely dilute solutions, it ignores ion-dipole interactions

analyte

substance whose concentration is being determined (unknown)

titrant

solution of known concentration that is added to an analyte to determine its concentration