Chem 220 final section 1
dilution calculations
M1V1=M2V2
Adding 45 mL of concentrated HCl to a beaker of water
graduated cylinder
Glassware used to accurately transfer small volumes
graduated pipet
concentration
how much solute is contained in a given volume/mass
volume %
volume solute/volume of total solution x 100
rel. standard dev
% RSD= s/x x 100
back titration
- step 1- a known excess of a standardized reagent is added to an analyte step 2: the amount of leftover reagent is determined by a titration calculation: the analyte concentration is determined by subtracting the number of excess mol found in step 2 form the total mol added in step 1
what makes a reaction a good basis of a titration
- a large K (constant is very product favored) -fast rxn
types of titration reactions
-acid base titration -redox titration -complexometric titration (ex. EDTA, ligand) -precipitation titration (forming AgCl)
random error
-always present -equal frequency of being too high or low -to reduce: repeat experiemnt -uncertainty represented as the st. dev. of confidence interval, x+- ex
multiplication/division propagation of error
-change absolute uncertainty to % rel. uncertainty -ex/x x 100= % rel. uncer. -% et= sq rt (%e1^2.......) -change back to absolute uncer. -%et= 100( ex/ final total)
end point
-change you see in the lab
two types of t tests
-comaparing experimental data to a known value -comparing two averages each with their own st. dev. -must do an F test to see if they have similar levels of random error to see if you can pool the s -if the Fcalc<Ftable, then you can pool them
grubs test
-how to determine if a value is an outlier -Gcalc<Gtable- value is not an outlier
end point of a titration
-indicator color change -sudden change in an instrumental measurement (usually pH) -change you observe in the lab
measuring mohr pipet
-less accurate -straight
analytical balance
-most accurate, goes to 4 decimal places -nearly every measurement made in lab can be trced back to accurate weighing
sig figs for log(x)
-number of digits in the answer are the same as the sig figs of x in log(x) -log (9.9 x10^3)= 4.00
confidence interval
-range of value in which the "true" value probably lies
sig figs for propagation of error
-report only 1 sig fig for error -final answer is rounded to same # of decimals as the error
equivalence point
-the theoretical point when exactly all the analyte has been consumed by titrant
equivalence point
-theoretical point when exactly all the analyte has been consumed by titrant
volumetric flask
-to contain -at 20 degrees, temp matters
buret
-to deliver -read going down -thickness of a line is about .02 mL
transfer/volumetric pipet
-use rubber bulb -has the bubble halfway down -drain the liquid onto the wall of the vessel -don't blow out last drop -more accurate than mohr pipet and graduated cylinder
t tests
-used to see if values agree with each other
preparing solution
-weigh correct mass of pure reasgent -dissolve in vol flask -dilute with more solvent to desied vol -invert flask
how is st dev used with average
average (+- st. dev)
Glassware best used when greater access to the contents is needed
beaker
Delivering different volumes of 0.200 M HCl solution for titration of a base.
buret
Glassware used to deliver a volume not known in advance
buret
adsorption
chemicals stick to the surface of glass -acid wash can help stop this
Glassware used to prevent splashing or evaporation
erlynmeyer
addition/ subtraction propagation of error
ex= sq rt (e1^2+ e2^2....)
titration
known quantity of reagent (titrant) is added incrementally with a buret to an unknown quantity of an analyte
weight %
mass solute/mass solution
weight %
mass solute/mass total solution x 100
ppm
mg solute/L
Transferring 50 μL of an enzyme solution to a test tube
micropipet
ppb
mirogram solute/L
% rel. tolerance
ml tolerance/ mL capacity x 100
Dispensing 6.6 mL portions of a solution to a series of test tubes
mohr pipet
molarity
mol solute/L soln
molality
mol solute/kg solvent (not solution) -not temp dependent, molarity is
molecular mass
number of grams containing avos number of moleucles
atomic mass
number of grms containing avogadros number of atoms
sig figs for 10^x
number of sig figs in 10^x should be equal to the number of digits after the decimal point in x 10^1.24= 1.7 x 10^1
the real rule of sig figs
only report one sig fig for error ( ex. 0.02) -the final result is rounded to the same number of decimals as the error
tolerance
permissable deviation from the specified value -class A has lower tolerance
K
product/reactant -want a large K
accurate
results are close to the "true" value
precision
results are reproducable
systematic error
results are reproducable (precise), but not accurate -sources: imporper tech, faculty equip, poorly prepared standards -consistently too high or too low
Glassware used to make accurate solutions.
volumetric flask
Glassware used to accurately transfer a small, single volume
volumetric pipet
Transferring 5.00 mL of solution from one container to another
volumetric pipet
when is standardization necessary
when a titrant is not available at high enough purity to act as a primary standard
When should a plastic volumetric flask be used instead of a more accurate glass flask?
when perfoming trace analysis and the analyte may adsorb to a glass surface
parallax error
when your eye isnt at the same height as the liquid