Outcome 1Analysis by Mass (Gravimetric Analysis), Volumetric Analysis (Acid-base titrations, Redox titrations and Back Titrations)

Qualitative Analysis is used to identify the chemicals present in a substanceQuantitative Analysis provides information about the amount of chemicals present in a substance

Analysis by MassGravimetric Analysis is the process of determining the composition of a sample by forming a precipitate and measuring its mass

Empirical Formula is the simplest whole-number ratio of the atoms present in the compoundMolecular Formula gives the actual number of atoms of each element present in a molecule of a compound

A precipitate should have the following properties: Have a known compositions (molar mass/formula) Be insoluble Be stable when heated during drying Be the produce of one specified reaction, i.e. other ions in a sample should

not interfere and from different precipitateMost common precipitatesAgCl (Cl-) – 143.4BaSO4 (SO4

2-) – 233.4CaCO3 (CO3

2-) – 100.1AgBr (Br-), AgI (I-), Fe2O3 (Fe3+), Mg2P2O7 (Phosphorous and Mg2+), BaSO4 (Ba2+)

Solubilities of common ionic compoundsHigh SolubilityCompounds containing the following ions are generally soluble in water:

Na+, K+, NH4+, NO3

-, CH3CHOO-

Cl-, Br-, I- (unless combined with Ag+ or Pb2+)

SO42- (except PbSO4 and BaSO4;

Ag2SO4 and CaSO4 are slightly soluble

Low solubilityCompounds containing the following ions are generally insoluble, unless combined with Na+, K+ or NH4

+

CO32-, PO4

3-, S2-, OH- (unless combined with Ba2+ or Sr2+, Ca(OH)2 is slightly soluble

Volumetric AnalysisVolumetric Analysis is the process of determining the concentration of one solution by reacting it with another solution of known concentration (a standard solution)

Precision indicates the degree of reproducibility of a measurement. It depends on how well you make a measurement. Results that are precise are close in value.Accuracy describes how close a measure value is to the true value. It depends upon the quality and calibration of the measuring device. Accurate results are close (or the same) as the true value

A standard solution is a solution of Precisely known composition Exact concentration

A standard solution can be prepared from a primary standard by dissolving an accurately known mass to make up an accurately known volume of solution.

Another way to prepare a standard solution is by standardizing a solution. i.e. by titration of the solution with a solution of known concentrationThe solution standardized becomes known as a secondary solution.

The common strong acids and bases cannot be made directly into standard solutions. The concentrations of commercial supplies of hydrochloric, nitric and sulfuric acids are not accurately specified, and strong bases such as sodium hydroxide and potassium hydroxide are usually impure as they absorb water and react with carbon dioxide in the air

A primary standard is a substance that is so pure that the amount of substance, in mole, can be calculated accurately from their masses.It has the following properties

It can be readily obtained in a pure uncontaminated form Have a known formula Be easy to store without deteriorating or reacting with the atmosphere Have a high molar mass to minimize the effect of errors in weighing Be inexpensive

E.g. anhydrous sodium carbonate (Na2CO3 – a basic primary standard) or potassium hydrogen phthalate (KHC8H4O4 – an acidic primary standard)NaOH is an unsuitable primary standard because it will react with atmosphere (forms carbon dioxide)

Primary Standardization is the most basic way to prepare a standard solution, which is dissolving an exact amount of a solid primary standard in a volumetric flask.

The equivalence point of a reaction occurs when the reagents have been mixed in the mole ratio shown by the reaction equation

The stage at which an indicator changes colour is called the end point

Titration RinsingBurette – solutionPipette – solution

Conical flask – deionized waterUnits of concentrationParts per million (ppm) parts per million (mg L-1)Parts per billion (ppb) parts per billion% w/w g solute/100g solution% w/v g solute/100ml solution% v/v ml solute/100ml solution

ErrorsExperimental Errors can come from

The endpoint is identified incorrectly Incorrect concentration of the standard solution Dilution of solutions with water left from rinsing Using contaminated glassware Incorrect use of glassware (e.g. overfilling a pipette, misreading the

burette)Systematic Errors are always in the same direction and usually associate with measuring equipmentRandom Errors –

pHThe concentration of H3O+ ions in a solution is referred to as the solution’s acidity. For convenience , acidity is measure using a logarithmic scale, called the pH scale.

pH = -log[H3O+]

IndicatorsIndicators are weak acids with their acid form being one colour and their

conjugate base being another

Back TitrationSome acids and bases are so weak that they do not produce a sharp colour

change at the end point of a titration. A technique known a s back titration is used to overcome this problem

A back titration involves the addition of a measured excess of a solution and then a direct titration to calculate the amount of the excess substance

RedoxRedox reactions involved the transfer of electrons from one chemical to

anotherOxidation involves the loss of electrons and an increase in oxidation

number and reduction involves the gain of electrons and a decrease in oxidation number.

A substance that accepts electrons and undergoes reduction is called an oxidant. A substance that donates electrons and is oxidized is called a reductantAddition of oxygen = removal of hydrogen

Oxidation numbersH = +1 except in metal hydrides where H=-10 = -2 except in peroxides where O=-1 e.g. F2O (because fluorine is more electronegative), H2O2 (as H can’t be +1)

Common redox reactionsCr2O7

2- +14H+ + 6e- 2Cr3+ + 7H2OMnO4

- +8H+ +5e- Mn2+ +4H20CH3CH2OH +H2O CH3COOH + 4H+ + 4e-

GlossaryAscorbic acid (C6H8O6) or Vitamin C is a water-soluble antioxidant that plays a vital role in protecting the body from infection and disease. Oxidises into dehydroascorbic acid (C6H6O6)Anhydrated/anhydrous means ‘without water’ (internal)Dehydrated externalIodine I2

Iodide I-