Experimental techniques and chemical analysis: Practice Questions

A student needs to accurately measure exactly \(25.0\text{ cm}^3\) of sodium hydroxide solution for an acid-base titration.

Which piece of apparatus is most suitable for this purpose?

A colorless gas is produced when an unknown solid reacts with dilute hydrochloric acid. When this gas is bubbled through limewater, the limewater turns cloudy. Which anion is present in the solid?

A solid salt sample is heated with aqueous sodium hydroxide and aluminium foil. A gas is evolved that turns damp red litmus paper blue. Which anion does this indicate is present in the salt?

The diagram of a paper chromatography experiment shows the following measurements:
- Distance from the baseline to the solvent front = \(10.0\text{ cm}\)
- Distance from the baseline to the center of a blue spot = \(4.2\text{ cm}\)

What is the \(R_f\) value of the blue spot?

A colorless solution is tested with a few drops of aqueous ammonia, and a white precipitate forms. When excess aqueous ammonia is added, the precipitate dissolves to form a colorless solution.

Which cation is present in the solution?

Describe how a student can determine if a sample of water is pure using only its boiling point, and state the expected observation if the water contains dissolved impurities.

Outline the method used to obtain pure water from aqueous copper(II) sulfate, and explain why simple distillation is chosen as the separation technique instead of filtration.

Define the term saturated solution and explain how a student can use melting point information to assess whether a solid substance is pure or a mixture.

A student uses paper chromatography to analyze a mixture of three different food dyes, A, B, and C. The chromatogram is produced using a suitable solvent.

(a) Explain why the starting line must be drawn in pencil rather than ink.
(b) The solvent front travels \( 12.5 \text{ cm} \) from the starting line. Dye B travels \( 8.0 \text{ cm} \). Calculate the \( R_f \) value for dye B, providing the formula used.
(c) After the experiment, dye A is found to have traveled further than dye C. State what this indicates about the relative solubility of dye A in the solvent compared to dye C.

The purification of water for domestic supply involves several distinct stages to ensure it is safe for human consumption.

(a) Describe the difference between the processes of sedimentation and filtration in water treatment.
(b) Explain why carbon is specifically used during the treatment process.
(c) Chlorine is added to the water in the final stages. State the purpose of chlorination and describe a chemical test to identify chlorine gas in the lab, including the positive result.