Chemical analysis: Practice Questions

Flame tests are used to identify specific metal ions in unknown compounds. According to the syllabus, what is the distinctive flame colour observed when a potassium compound is heated in a Bunsen burner flame?

In a paper chromatography experiment to identify a food dye, a student measures the movement of the components. The solvent front travels a distance of \( 12.0\text{ cm} \) from the baseline. If a specific dye spot has an \( R_f \) value of \( 0.75 \), what is the distance from the baseline to the centre of that dye spot?

An unknown white solid is analysed using two chemical tests.
1. A flame test results in a red flame colour.
2. The solid is dissolved in water, then dilute nitric acid and silver nitrate solution are added. A cream precipitate forms.
What is the identity of the unknown solid?

A student uses a table to record the results of chemical tests on four different salt solutions: W, X, Y, and Z.

Solution W: Yellow flame test; No precipitate with sodium hydroxide.
Solution X: Lilar flame test; No precipitate with sodium hydroxide.
Solution Y: Green flame test; White precipitate with sodium hydroxide.
Solution Z: No flame test result recorded; Green precipitate with sodium hydroxide.

Which solution contains iron(II) ions?

Identify the color of the flame produced during a flame test when a compound containing potassium ions is heated in a Bunsen burner flame.

A student adds barium chloride solution to an unknown sample in the presence of dilute hydrochloric acid and observes a white precipitate. Identify the anion present in the sample and state the chemical name of the precipitate formed.

Describe how a student can use sodium hydroxide solution to distinguish between a solution containing aluminium ions and a solution containing magnesium ions.

A technician performs flame tests on two unknown chloride salts, X and Y, to identify the metal ions they contain.

(a) Salt X produces a lilac flame. Identify the metal ion present in Salt X.
(b) Salt Y produces a crimson flame. Identify the metal ion present in Salt Y.
(c) Describe the procedure for cleaning the wire used in the flame test and explain why this step is necessary before testing a new sample.

A student uses paper chromatography to analyze a mixture of three food dyes. The student marks a baseline and places a spot of the mixture on it. After the solvent moves up the paper, the following measurements are recorded:
- Distance from baseline to solvent front: \(15.0\) cm
- Distance from baseline to Dye 1: \(3.0\) cm
- Distance from baseline to Dye 2: \(7.5\) cm
- Distance from baseline to Dye 3: \(12.0\) cm

(a) Calculate the \(R_f\) value for Dye 2. Show your working.
(b) Define the stationary phase and the mobile phase in this experiment.
(c) Explain why Dye 3 traveled further than Dye 1 in terms of its solubility and attraction to the paper.