STOICHIOMETRY Chapter 9 Mass-Mass Stoichiometry Mole-Mole Mass-Mole Volume Mixed mole.
Analytical Chemistry. Mole Amount of a substance A way to compare amounts of substances that react...
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Transcript of Analytical Chemistry. Mole Amount of a substance A way to compare amounts of substances that react...
Analytical Chemistry
Mole
Amount of a substance
A way to compare amounts of substances that
react
The number of particles in 12.000g of carbon-12
Equivalent to Avagadro’s Number: 6.02 x 1023
Molar MassCan be derived from the periodic table4 significant figures
m = n M n = m / M M = m / n
Units of g mol-1
Significant FiguresFinal answer to contain the same number of
significant figures as least precise piece of data
Do not round until final answer
Count from first non zero digit
Supportive Questions
Calculate the number of moles in 2.2g of CO2
m = n M n = m / M
n = 2.2 / (12.01 + 16.00 x 2)
n = 0.050 mol (2 sf)
Supportive QuestionsCalculate the mass of 0.200 mol of Na2CO3.m = n Mm = 0.200 x (22.99 x 2 + 12.01 + 16.00 x 3)21.198 g 21.2 g (3sf)
Molar ConcentrationConcentration involving moles is also referred to
as molarity
Molar concentration (C) measures in mol L-1
C = n / v
You will occasionally see molarity written using M, eg 0.2M is equivalent to 0.2 mol L-1
Supportive Questions 2Calculate the molarity when 0.2 mol of HCl is
dissolved in 500 ml of water.
C = n / v
C = 0.2 / 0.5
C = 0.4 mol L-1
Mole RatiosA balanced equation provides the mole ratios of
the substances that react or are produced during a reaction
We can use these ratios to calculate unknown quantities
Supportive Questions 3Complete combustion of propanolC3H7OH(g) + 4½ O2(g) 3 CO2(g) + 4 H2O(g)
Propanol : Oxygen = 1:4½ or 2:9Propanol : Carbon dioxide = 1:3
Supportive Questions 3m = n M n = m / Mn = 220 / (12.01 x 3 + 8 x 1.008 + 16)n = 3.66 moln(O2) / n(C3H7OH) = 4.5 / 1n = 4.5 x 3.66n = 16.47 molm = 16.47 x (16 x 2)m = 527.17 g m = 530 g (2sf)
Redox ReactionsOxidation is loss of electronsReduction is gain of electrons
Oxidation is an increase in oxidation numberReduction is a decrease in oxidation number
Balancing Redox Reactions Balance everything other than O and HBalance O by adding waterBalance H by adding H+
Balance charge by adding e-
Write full equations by multiplying half equations to give equivalent electrons
Cancel common molecules
Oxidation NumbersOverall number of molecule is equivalent to its
charge
Hydrogen is +1 (except in metal hydrides)
Oxygen is -2 (except in peroxides or OF2)
Halogens are -1
Supportive Questions 5MnO4
-
Mn + 4 x -2 = -1Mn – 8 = -1Mn = +7Mn2+
Mn = +2+7 -> +2, oxidation number decreased =
reduction
ConcentrationCan always be determined by
The concentration = amount of solute volume of solvent
Analytical TechniquesConcentration Conversions
Concentrationmol L-1 (moles per litre) = n (moles) / V (litres)g L-1 (grams per litre) = mass (grams) / V (litres)% w/v (percent weight per volume) =
mass (grams) / 100 mLppm (mg L-1) (parts per million) =
mass (milligrams, mg) / V (litres) ppb (g L-1) (parts per billion) =
mass (micrograms, g) / V (litres)
Concentration
mol L-1
%w/v
g L-1
ppm
ppb
x M
/ M
x 10
/ 10
x 1000 / 1000
x 1000
/ 1000
x 106
/ 106
The concentration conversion table: know it, love it
Hint: moving left you divide, moving right you multiply
Example0.20M Ca2+ = 0.2 mol L-1 x M= 0.2 x 40.01 gL-1= 8.002 gL-1 /10= 8.002 / 10= 0.80 % w/v (2sf)
Example0.22 % w/w concentration of NaFCalculate mass in grams in a 160 g tube0.22 % w/w x 10 = 2.2 g kg-1
2.2 x 0.160 = 0.352 g
2.2 g kg-1 x 1000 = 220 ppm
DilutionsWhen diluting a solution, the volume is changed
but there is no change in the amount of substancen = C1 V1
n = C2 V2
These two equations are therefore equalso C1 V1 = C2 V2
The dilution factor is the ratio of these two volumes, ie df = V2 / V1
ExampleCalculate the volume of concentrated sulfuric acid
of concentration 20 mol L-1 which is needed to make up 5 L of 2 mol L-1 sulfuric acid
So C1 = 20
V1 is unknown
C2 = 2
V2 = 5
ExampleC1 V1 = C2 V2
20 x V = 2 X 520V = 10V = ½Dilution factor is V2 / V1
Dilution factor = 5 / ½ Dilution factor = 10
Example
Df = V2 / V1
therefore V1 = V2 / Df
V1 = 200 / 20
V1 = 10 ml
Analytical ChemistryVolumetric Analysis
RinsingVolumetric flask
Rinse with distilled waterNeed to control moles of substance
Pipette and BuretteFirst rinse with distilled waterFinal rinse with solution they will containNeed to control the concentration of substance
Conical FlaskRinse with distilled waterNeed to control moles of each substance
Techniques: PipettePipette should be held verticallyFill so bottom of meniscus is at etched line – check
at eye levelWhen draining, hold against the side of flask and
allow to drain – don’t shake to remove last drop
Techniques: BuretteBurettes do not need to be refilled between
titrationsMake sure to remove air bubbles, droplets on the
side and the funnelUse left hand turn – right hand swirl techniqueEndpoint should be approached drop wiseUse wash bottle to ensure any liquid that leaves
the burette reaches the reaction liquid
Volumetric FlaskWeigh mass of solute on watch glassTransfer to flask using dry funnelWash watch glass and funnel a number of time
into flaskHalf fill flask with water and swirl to dissolveAdd further water until level with meniscusDO NOT overshoot the meniscus – you will have to
start again!
Analytical ChemistryErrors
ErrorsThere are 2 types of errors in an experiment
Random errorsSystematic errors
Neither are experimental errors or mistakes!!
When discussing errors, only talk about unavoidable errors, not mistakes that you have made
Systematic errorHave a distinct and definite magnitudeAffect the accuracy of the experimentCaused by imperfections in equipmentCause consistent deviation from true valueCannot be reduced through averages etcRepeating with different equipment allows for
identification of systematic errors
Random errorsNot fixed and will vary in extent and
magnitudeRandom errors affect precisionResults randomly scattered around true valueReasonable estimate can be found through
repetition, averages and line of best fit. ie can be minimised but never eliminated
Greater number of samples taken will minimise the effect of random errors
Accuracy
Correctness of a single measurement
Assessed by comparing value with the true or accepted value
Precision
Precision is reproducibility
Compare how close values are to each other
Accuracy and Precision
Good precision & good accuracy
Good accuracy, poor precision
Good precision, poor accuracy
Poor accuracy & poor precision
Single and Group measurements
Precision of a single measurement relates to the quality of the instrument used
Precision of a group of measurements is how close they are to each other
Accuracy of a single measurement is how close it is to the true value
Accuracy of a group of measurements is how close the average is to the true value
Significant FiguresThe final result of a calculation can be no
more precise than the least precise value or resolution of the instrument
The resolution of an instrument is the number of significant figures or decimal places that a measurement can reliably be made
Analytical TechniquesChromatography
Chromotography
ChromatographyAll forms of chromatography involve a mobile
phase passing over a stationary phaseChromatography separates different components
by way of their different attraction to the mobile and stationary phase
The strength of this attraction is due to their relative polarities
Polar substances will adsorb onto the more polar phase
Polarity Revision“Like dissolves like” – a good reminder, but never
an answer to a question!Polar bonds are caused by differing
electronegativities causing uneven sharing of electrons
Polar molecules are asymmetrical molecules that contain polar bonds
Polar bonds attract each other via electrostatic interactions
Phase InteractionsIf the mobile phase is polar:
The more polar chemical will adsorb more strongly to the mobile phase, and so will travel the furthest (in paper/TLC) and exit earlier (column/HPLC/GC)
Larger Rf and shorter retention time
If the stationary phase is polarThe more polar chemical will adsorb more strongly to
the stationary phase, and so will travel the least distance (paper/TLC) and exit later (column/HPLC/GC)
Smaller Rf and longer retention time
Retardation FactorRetardation Factor (Rf) is a measure of how far the
solute travels relative to the solvent frontThis enables comparisons between experimentsRf = distance moved by solute
distance moved by solvent
Rf is dimensionless and always less than 1 (usually as a decimal)