Exams Tmr

NH4 +

ammonium

CH3CO2 -

acetate

CN-

cyanide

SCN-

thiocyanate

ClO-

hypochlorite

ClO2 -

chlorite

ClO3 -

chlorate

ClO4 -

perchlorate

BrO3 -

bromate

IO3 -

iodate

CO3 2-

carbonate

HCO3 -

bicarbonate

C2O4 2-

oxalate

OH-

hydroxide

MnO4 -

permanganate

MnO4 2-

manganate

NO2 -

nitrite

NO3 -

nitrate

CrO4 2-

chromate

Cr2O7 2-

dichromate

O2 2-

peroxide

SO3 2-

sulfite

SO4 2-

sulfate

S2O3 2-

thiosulfate

S4O6 2-

tetrathionate

HSO4 -

hydrogen sulfate (bisulfate)

HSO3 -

hydrogen sulfite (bisulfite)

S 2-

sulfide

PO4 3-

phosphate

HPO4 2-

hydrogen phosphate

H2PO4 -

dihydrogen phosophate

AsO4 3-

arsenate

UO2 2+

uranyl

VO 2+

vanadyl

I3 -

triiodide

Ce 3+

cerous

Ce 4+

ceric

Cu+

cuprous

Cu 2+

cupric

Fe 2+

ferrous

Fe 3+

ferric

Hg2 2+

mercurous

Hg 2+

mercuric

Sn 2+

stannous

Sn 4+

stannic

significant figures

minimum digits needed for a given value without loss of accuracy

always use these

what so SF include

all certain figures and the first uncertain figure

1. initial zeros dont count as SF

2. disregard final zeros, except when follow a decimal point

3. all other digits are significant

SF with addition and subtraction

only concerned with SF from after decimal to the same degree of uncertainty

SF with multiplication and division

limited to the number with the fewest SF

how to deal with SF with scientific notation with addition and subtraction

put everything in the same exponent, then add or subtract

how to deal with SF with scientific notation with multiplication or division

no influence on SF in multiplication and division

characteristic

the integer (values to the left of the decimal point)

mantissa

all digits after the decimal

SF rule for log

# of digits in the mantissa of logx = #SF in x

value inside log SF = # decimal places in answer

SF rule for antilog

# digits in antilog = # SF in mantissa of x

opposite of log SF rules

error in chemical analysis

- every measurement has some uncertainty

- conclusions are never made with complete certainty

- true value for any quantity is always unknown

how must you evaluate error in chemical analysis

evaluate the magnitude of error then establish limits within which the true value of the measurements lies at a given level of probability

systematic errors

determinate errors

errors that are directional which can be determined, corrected, and accounted for

systematic error due to (2)

flaws in equipment or experimental design

how can you account for systematic error (4)

- analyze standard reference material from NIST

- analyze a blank sample

- use 2nd reliable analytical method

- have different people in different lab analyze identical samples using same/different method

types of systematic error

- instrument

- method

- constant

- proportional

instrument errors

imperfections in measuring devices (ex: calibration, delivering quantity different than indicated, instrument limitations)

how are instrument errors corrected

are found and corrected by calibration (calibrate instrument with time)

method errors

most difficult to identify (especially if assay based like acid or binding interatcion)

incompleteness of a reaction

constant errors

independent of sample size; are easier errors to work with

ex: precipitate lot to solubility

proportional error

increase or decrease according to sample size

gross errors

personal error/equipment failure

ex: power and/or water

random errors

aka indeterminant

is always present

cant be eliminated/controlled

ultimate limitation on the determination of a quantity

what is the sign (+ or -) of random errors

could be positive or negative

examples of random error

fluctuations due to electrical noise in an instrument

instability on a meter

how can random error be reduced or corrected

statistics used to treat/evaluate these types of errors

might be reduced by a better experiment

take replicate measurements to fluctuate randomly around the mean of a set

precision

associated with random error

reproducibility of a result

how closely do several repetitions agree

accuracy

associated with systematic error

nearness to true value

absolute error/uncertainty

raw difference between measured value and true value

percent relative error/uncertainty

expresses error as ratio/percentage of true value

uncertainty

typically expressed as the standard deviation of a measurement

SF rule for uncertainty

dec places of y = uncertainty dec places

the first uncertain figure is the last significant figure

propagation of error key takeaways (2)

- how to calculate propagated error is important but not always necessary

- in cases where error is proportionate across variables, error propagation can reasonable be estimated without calculation

key takeaway for applying error propagation

when developing a method, look for the largest sources of error and try to reduce replace and try to reduce, replace or find alternate methods

equivalence point

when the quantity of added titrant is the exact amount for stoichiometric reaction with the analyte

not always but can be 1 to 1

end point

small difference between end point and equivalence

this is what we can measure since there is a sudden change in observable physical property (color, precipitation)

end point example within titration

the first trace of persistent purple color is the end point

this is when there is excess unreacted MnO4-

titration error

is the difference between the end point and the equivalence point

can be measured in a blank (ex: sample without an analyte)

primary standard

is prepared by dissolving a weighed amount of a high purity (> 99.9%) reagent, its concentration can be calculated

primary standard property requirements

must be stable (doesnt decompose at standard conditions and at elevated temperatures so it can be dried to remove ambient absorbed water

what cant be a primary standard

strong acids and bases but they can be standardized by a primary standard

standardization

determine concentration of reagents that are not available as primary standards

do this with a primary standard

standard solution

titrant is this

solution thats concentration has been determined against primary standard

direct titration

addition of titrant until reaction is complete

ex: standardizing HCl

back titration

add an excess of one standard reagent and titrate the excess with a second standard reagent

back titration how to calculate how much was consumed by analyte

react with analyte which has known concentration

then measure how much was left behind

however much was left behind, subtract from how much added to find how much was consumed by analyte

what is a back titration helpful for

helpful when the end point is clearer for the reverse than the forward reaction

what do we rely on statistical analysis for

to make objective judgements about the validity and quality of the experimental data

population

infinite results (assuming we have infinite amount of time to collect it)

sample

small/confined data set, a subset of a data population of data

replicates

same size samples analyzed the exact same way

is needed to help minimize errors

gaussian distribution

a normal or bell shaped distribution curve when an experiment is repeated many, many, many times

only random errors are present in the data (indeterminant creates distribution)

what happens to the curve the more times the experiment is repeated

the more likely the result approaches the ideal smooth curve

what can a small set of results help estimate

estimate the parameters in the big set

ex: statistical behavior

unimodal

mean is where mode is (one peak)

width is determined experimentally