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Experimental Techniques and Chemical Analysis - Separation and purification

Grade 12IGCSEChemistry

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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A pure substance consists of only one substance and has no effector impurities. Purity is determined by checking for sharp, fixed melting points (TmT_m) and boiling points (TbT_b). Impurities lower the TmT_m and increase the TbT_b.

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Filtration is used to separate an insoluble solid from a liquid. The solid remaining on the filter paper is the residue, and the liquid passing through is the filtrate.

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Crystallization is used to obtain a soluble solid (solute) from a solution. The solution is heated to the point of crystallization (saturated solution) and then allowed to cool, forming crystals of the pure solute.

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Simple distillation separates a solvent from a solution (e.g., obtaining pure H2OH_2O from NaClNaCl solution). It relies on the difference between the boiling points of the solute and the solvent.

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Fractional distillation is used to separate miscible liquids with different boiling points, such as EthanolEthanol (Tb=78∘CT_b = 78^{\circ}C) and WaterWater (Tb=100∘CT_b = 100^{\circ}C). The fractionating column allows for repeated evaporation and condensation cycles.

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Paper chromatography separates substances (like dyes or amino acids) based on their relative solubility in a mobile phase and their affinity for the stationary phase. Colorless substances are made visible using locating agents.

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The RfR_f (retention factor) value is a characteristic ratio used to identify substances in chromatography. It depends on the solvent and the temperature.

📐Formulae

Rf=Distance moved by the substance from the baselineDistance moved by the solvent front from the baselineR_f = \frac{\text{Distance moved by the substance from the baseline}}{\text{Distance moved by the solvent front from the baseline}}

Percentage Purity=Mass of pure substanceTotal mass of sample×100%\text{Percentage Purity} = \frac{\text{Mass of pure substance}}{\text{Total mass of sample}} \times 100\%

💡Examples

Problem 1:

In a paper chromatography experiment to identify a food coloring, the solvent front moved 12.5 cm12.5\text{ cm} from the start line. A red dye spot moved 7.5 cm7.5\text{ cm} from the same start line. Calculate the RfR_f value of the red dye.

Solution:

Rf=7.5 cm12.5 cm=0.6R_f = \frac{7.5\text{ cm}}{12.5\text{ cm}} = 0.6

Explanation:

The RfR_f value is calculated by dividing the distance traveled by the component by the distance traveled by the solvent. Since it is a ratio of two lengths, it has no units and is always ≤1\le 1.

Problem 2:

A student has a mixture of EthanolEthanol (Tb=78∘CT_b = 78^{\circ}C) and PropanonePropanone (Tb=56∘CT_b = 56^{\circ}C). Which liquid will be collected first during fractional distillation, and why?

Solution:

PropanonePropanone will be collected first.

Explanation:

In fractional distillation, the liquid with the lower boiling point (56∘C56^{\circ}C for PropanonePropanone vs 78∘C78^{\circ}C for EthanolEthanol) evaporates more readily. Its vapors reach the top of the fractionating column first, enter the condenser, and are collected as the first fraction.

Problem 3:

A 5.00 g5.00\text{ g} sample of impure KNO3KNO_3 was found to contain 0.25 g0.25\text{ g} of sand. Calculate the percentage purity of the KNO3KNO_3.

Solution:

Mass of pure KNO3=5.00 g−0.25 g=4.75 g\text{Mass of pure } KNO_3 = 5.00\text{ g} - 0.25\text{ g} = 4.75\text{ g} Percentage Purity=4.755.00×100%=95%\text{Percentage Purity} = \frac{4.75}{5.00} \times 100\% = 95\%

Explanation:

To find the purity, we first subtract the mass of the impurity from the total mass to find the mass of the pure compound, then divide by the total sample mass.

Separation and purification - Revision Notes & Key Formulas | IGCSE Grade 12 Chemistry