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Experimental Techniques and Chemical Analysis - Separation and purification

Grade 12A LevelChemistry

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.