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Experimental Techniques and Chemical Analysis - Measurement and apparatus

Grade 12IGCSEChemistry

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

🔑Concepts

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Measurement of Mass: Measured using a digital balance, typically recorded to two decimal places (e.g., 0.01 g0.01 \text{ g}) for high precision in quantitative analysis.

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Measurement of Volume: Liquids are measured using a measuring cylinder (approximate), a pipette (fixed accurate volumes like 25.0 cm325.0 \text{ cm}^3), or a burette (variable accurate volumes up to 50.00 cm350.00 \text{ cm}^3).

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Measurement of Gas Volume: Collected using a gas syringe or by displacement of water in a graduated tube. The choice depends on the solubility and density of the gas.

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Paper Chromatography: A technique used to separate and identify mixtures. The RfR_f (retention factor) value is characteristic of a specific substance in a given solvent.

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Locating Agents: Used in chromatography to visualize colorless substances (e.g., amino acids) by reacting with them to form colored spots.

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Purity Determination: Pure substances have sharp melting and boiling points. Impurities broaden the melting range and typically lower the melting point while raising the boiling point.

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Gas Collection Methods: Upward delivery is used for gases less dense than air (e.g., H2H_2, NH3NH_3); downward delivery is used for gases denser than air (e.g., CO2CO_2, Cl2Cl_2, HClHCl).

📐Formulae

Rf=Distance traveled by the substanceDistance traveled by the solvent frontR_f = \frac{\text{Distance traveled by the substance}}{\text{Distance traveled by the solvent front}}

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\%

Density (ρ)=Mass (m)Volume (V)\text{Density } (\rho) = \frac{\text{Mass } (m)}{\text{Volume } (V)}

Percentage Yield=Actual YieldTheoretical Yield×100%\text{Percentage Yield} = \frac{\text{Actual Yield}}{\text{Theoretical Yield}} \times 100\%

💡Examples

Problem 1:

In a chromatography experiment, the solvent front moved 12.0 cm12.0 \text{ cm} from the baseline. A dye spot moved 9.6 cm9.6 \text{ cm} from the baseline. Calculate the RfR_f value of the dye.

Solution:

Rf=9.6 cm12.0 cm=0.80R_f = \frac{9.6 \text{ cm}}{12.0 \text{ cm}} = 0.80

Explanation:

The RfR_f value is calculated by dividing the distance traveled by the solute by the distance traveled by the solvent. It is a dimensionless ratio always ≤1\le 1.

Problem 2:

A sample of NaClNaCl is suspected to be impure. Its melting point was measured as 785 ∘C−798 ∘C785 \text{ }^\circ\text{C} - 798 \text{ }^\circ\text{C}, whereas the literature value for pure NaClNaCl is 801 ∘C801 \text{ }^\circ\text{C}. Determine if the sample is pure and explain the effect of the impurity.

Solution:

The sample is impure because the melting point is lower than 801 ∘C801 \text{ }^\circ\text{C} and occurs over a range (785−798 ∘C785 - 798 \text{ }^\circ\text{C}) rather than at a sharp, single temperature.

Explanation:

Impurities disrupt the regular lattice structure of a crystal, meaning less energy is required to break the bonds, thus lowering the melting point and causing it to melt over a temperature range.

Measurement and apparatus - Revision Notes & Key Formulas | IGCSE Grade 12 Chemistry