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)