Experimental Techniques in Chemistry notes

MDCAT Chemistry

This chapter explains common laboratory methods used to separate, purify, identify and analyse substances. It covers filtration, crystallisation, sublimation, distillation, solvent extraction, chromatography, titration, safe laboratory practices and selected measurements.

Laboratory Measurements and Analytical Chemistry

Analytical chemistry is the branch of chemistry concerned with the complete qualitative and quantitative analysis of substances. Qualitative analysis identifies the substances or ions present, while quantitative analysis determines their amounts.

Accurate measurements are essential in laboratory work. Mass, volume, temperature, pressure and time are measured using suitable instruments and units.

An electronic balance is used to measure the mass of a substance accurately.
Qualitative analysis determines what is present in a sample.
Quantitative analysis determines how much of a substance is present.
Volume is commonly measured with a measuring cylinder, pipette, burette or volumetric flask.
Temperature is measured with a thermometer.
Read the lower meniscus of a liquid at eye level when using ordinary laboratory glassware.
The volume of a liquid should be recorded with suitable precision for the instrument used.

Filtration

Filtration separates an insoluble solid from a liquid by passing the mixture through a porous filter. The liquid passes through the pores, while the insoluble particles are retained on the filter paper.

The liquid collected below the filter paper is called the filtrate. The solid left on the filter paper is called the residue. Filtration cannot separate a substance that is completely dissolved in a liquid.

Filtration separates insoluble particles from liquids.
Filtrate is the clear liquid that passes through the filter paper.
Residue is the solid retained on the filter paper.
In a filtration setup, a funnel supports the filter paper and directs the filtrate into a receiver.
The rate of filtration can be increased by applying gentle suction.
Gentle suction is developed effectively when the filter paper fits tightly against the funnel and forms a good seal.
Filtration is different from crystallisation. Filtration removes an insoluble solid, while crystallisation obtains a dissolved solid as crystals.

Crystallisation

Crystallisation is used to obtain a pure soluble solid from its solution. The solution is concentrated by careful evaporation until it becomes nearly saturated, and then it is allowed to cool. Pure crystals form because the solubility of many solids decreases on cooling.

The crystals are separated from the remaining liquid by filtration. The liquid left after crystals form is called the mother liquor. In some laboratory descriptions, it may be referred to as the remaining filtrate, while the crystals are collected as the solid residue.

Crystallisation is used to purify a soluble solid from a solution.
A hot saturated solution contains the maximum amount of dissolved solute at that temperature.
On cooling, excess solute separates as crystals.
The solution remaining after crystal formation is called the mother liquor or remaining filtrate.
The crystals are separated from the mother liquor by filtration.
Wash crystals with a small amount of cold solvent to remove adhering impurities.
Dry the crystals between filter papers or in a suitable drying apparatus.
Evaporation to dryness is avoided when the solid may decompose or when soluble impurities are present.

Sublimation

Sublimation is the direct conversion of a solid into vapour on heating, followed by conversion of the vapour directly back into a solid on cooling. It is useful when one component of a solid mixture sublimes and the other component does not.

Benzoic acid can be purified by sublimation. On heating, benzoic acid changes into vapour. The vapour cools on a cold surface and deposits as pure solid benzoic acid.

Sublimation involves solid to vapour and vapour to solid changes.
The solid that sublimes is separated from non-sublimable impurities.
Benzoic acid is purified by sublimation.
Ammonium chloride and iodine are other substances that can sublime under suitable conditions.
Sublimation is not suitable if all components of a mixture sublime at similar temperatures.
The deposit formed on the cool surface is collected after the apparatus has cooled.

Simple and Fractional Distillation

Distillation separates a liquid from dissolved substances or separates liquids with sufficiently different boiling points. The mixture is heated in a flask. The more volatile component vaporises, passes through the condenser, cools and changes back into a liquid called the distillate.

Simple distillation is suitable for obtaining a solvent from a solution or for separating liquids with widely different boiling points. Fractional distillation is used to separate miscible liquids with close boiling points, including gases from a mixture.

Vaporisation takes place in the heated distillation flask.
Condensation takes place in the Liebig condenser.
The liquid collected after condensation is the distillate.
Dissolved ammonia gas that is carried over with the vapour becomes part of the distillate after distillation.
Simple distillation can separate water from dissolved salts.
Fractional distillation is used for separation of gases from a mixture.
A fractionating column provides repeated vaporisation and condensation.
The thermometer bulb should be placed near the side arm so that it measures the temperature of vapour entering the condenser.

Solvent Extraction and Chromatography

Solvent extraction separates a substance by using its different solubilities in two immiscible solvents. The substance distributes itself between the two layers. A separating funnel is used when the layers are liquids that do not mix, such as an aqueous layer and an organic layer.

Chromatography separates components because they have different attractions for a stationary phase and a mobile phase. In paper chromatography, the paper is the stationary phase and the solvent is the mobile phase.

Solvent extraction is based on the different solubilities of a substance in immiscible solvents.
A separating funnel is used to separate the two liquid layers.
The denser liquid forms the lower layer, but the layer should be identified rather than assumed.
In paper chromatography, a small spot of sample is placed above the solvent level.
The solvent rises through the paper by capillary action.
Components that are more soluble in the mobile solvent move farther up the paper.
The retention factor is calculated as: Rf = distance travelled by solute divided by distance travelled by solvent front.
The comparative rates at which solutes move in paper chromatography are expressed by their retention factors.
Chromatography may be classified according to the stationary phase, mobile phase or separation process. Solvent extraction is a related separation method based on distribution between phases.

Titration and Indicators

Titration is a quantitative method used to determine the concentration of an unknown solution. A solution of known concentration is placed in a burette and added gradually to a measured volume of the unknown solution in a flask.

An indicator shows the end point by changing colour. Phenolphthalein is colourless in acidic solution and pink in alkaline solution. When sodium hydroxide is added to an acidic solution, the end point is reached when a faint pink colour appears and remains after swirling.

The burette is rinsed with the titrant before filling it.
The pipette is rinsed with the solution that will be transferred by it.
Rinsing a burette with distilled water instead of the titrant dilutes the titrant and can cause an error in the calculated analyte concentration.
Phenolphthalein gives a pinkish colour in alkaline solution.
After adding 15 mL of NaOH and phenolphthalein to a flask, a pinkish colour appears throughout the solution if the solution is alkaline.
The end point is reached when the colour disappears and does not return during swirling, or, for phenolphthalein in an acid flask, when a very pale pink colour persists, depending on the direction of titration.
The first temporary colour that disappears is not the final end point.
Equal volumes of HCl and NaOH do not necessarily represent the end point because their concentrations may be different.
The burette reading is recorded at the end point, and concordant titres are used for calculation.

Safety, Reaction Rate and Other Laboratory Calculations

Laboratory techniques also include safe heating, dilution of acids, calorimetry and gas-volume measurements. Correct apparatus handling reduces errors and protects the experimenter.

In a rate experiment, the volume of carbon dioxide produced from marble chips and hydrochloric acid is measured against time. Factors such as acid concentration, temperature, surface area of marble chips and the amount of reactants affect the rate, but the volume of the reaction flask does not affect the rate of carbon dioxide production.

Concentrated acid must be added slowly to water, never water rapidly to concentrated acid, because the process releases heat.
Wear suitable eye protection and follow laboratory safety procedures when diluting acids.
A yellow or orange burner flame that blackens the flask indicates incomplete combustion and insufficient air.
Allow more air into the collar of the burner to obtain a hotter blue flame.
For heat absorbed by water, use q = m c ΔT.
For 100 g of water, c = 4.18 J/g°C and a temperature increase of 20.0°C, q = 100 × 4.18 × 20.0 = 8360 J.
The volume of a reaction flask does not affect the rate of CO2 production when other conditions remain unchanged.
At STP, 0.040 g Mg produces approximately 37 mL H2 when it reacts completely with an acid: Mg + 2HCl → MgCl2 + H2.
In a simple calorimetry experiment, heat lost by the fuel is assumed to be transferred to the water, although some heat is lost to the surroundings.
Atmospheric pressure corrections are applied when a liquid column changes the pressure measured by a manometer. A 40.8 mm difference in a suitable liquid column may correspond to a correction of about 3.0 mm Hg, depending on the liquid and apparatus arrangement.

Key terms

Analytical chemistry: The branch of chemistry concerned with qualitative and quantitative analysis of substances.
Filtration: A separation method in which an insoluble solid is retained by a porous filter while liquid passes through.
Filtrate: The liquid that passes through filter paper during filtration.
Residue: The solid left on the filter paper after filtration.
Crystallisation: A method of obtaining a pure soluble solid as crystals from its solution.
Mother liquor: The solution left after crystals have formed and been separated.
Sublimation: The direct change of a solid into vapour and the reverse change from vapour to solid.
Distillation: A separation method based on vaporisation followed by condensation.
Distillate: The liquid collected after vapour has been condensed during distillation.
Fractional distillation: Distillation using a fractionating column to separate miscible liquids or gases with close boiling points.
Solvent extraction: Separation based on the different solubilities of a substance in two immiscible solvents.
Chromatography: A separation method based on different distributions of substances between stationary and mobile phases.
Retention factor, Rf: The distance travelled by a solute divided by the distance travelled by the solvent front.
Titration: A quantitative analysis method in which a solution of known concentration reacts with a measured amount of another solution.
End point: The stage in a titration at which the indicator shows that the reaction is complete for the required accuracy.
Phenolphthalein: An indicator that is colourless in acidic solution and pink in alkaline solution.
Calorimetry: The measurement of heat transferred during a physical or chemical process.
Specific heat capacity: The heat required to raise the temperature of one gram of a substance by one degree Celsius.

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Chemistry shortcuts

Finding the limiting reactant and percentage composition

Convert every given mass or volume into moles first. The reactant that produces the least amount of the required product is the limiting reactant.

Write the balanced equation and calculate moles using n = mass/Mr.
Use the mole ratio to calculate the product. For percentage composition, use percentage = mass of element in one mole of compound divided by molar mass, multiplied by 100.
Example: Percentage of nitrogen in KNO3 = 14/101 × 100 = 13.86%.
Answer: 13.86% nitrogen.

Use gas volume at molar volume only when the gas conditions are stated or are standard conditions.

Using gas volume, pressure and temperature relations

At the same temperature and pressure, gas volume is directly proportional to the number of molecules. For changing conditions, use P1V1/T1 = P2V2/T2.

At constant temperature and pressure, divide or multiply the volume in the same ratio as the number of molecules.
Example: 10 mL H2 contains 2 × 10^3 molecules. Oxygen in 200 mL contains 20 × 2 × 10^3 = 4 × 10^4 molecules.
Answer: 4 × 10^4 molecules.
For a rigid container, increasing temperature increases molecular speed and mean free path if the gas remains in the same phase.

The direct volume to molecule ratio does not apply when temperature or pressure changes.

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