Separation and purification：练习题

Which piece of apparatus is most suitable for measuring exactly \( 25.0\text{ cm}^{3} \) of a solution during a titration?

A student is given a sample of an unknown liquid. Which observation provides the best evidence that the liquid is a pure substance?

A solid mixture contains ammonium chloride (which sublimes on heating), sand (insoluble in water), and sodium chloride (soluble in water).
Which sequence of steps should be used to isolate samples of pure, dry sand and pure, dry sodium chloride from this mixture?

A student uses paper chromatography to identify a component in a food coloring. The distance travelled by the substance is \( 4.5\text{ cm} \) and the distance travelled by the solvent is \( 18.0\text{ cm} \).
Calculate the \( R_f \) value of the substance.

A student uses paper chromatography to analyze a mixture of colorless substances. Which statement about the procedure and analysis is correct?

Name the laboratory technique used to separate a mixture of two or more miscible liquids with different boiling points.

Describe the experimental steps required to obtain pure, dry crystals of a soluble salt from its aqueous solution.

A chemist needs to prepare pure, dry crystals of magnesium sulfate starting from solid magnesium carbonate (an insoluble base) and dilute sulfuric acid.

(a) Explain why the magnesium carbonate is added in excess to the acid.
(b) How is the excess magnesium carbonate removed from the solution?
(c) Describe the process of crystallization, explaining why the solution should not be evaporated to dryness.
(d) How are the crystals finally dried without decomposing them?

Fractional distillation is used on an industrial scale to separate crude oil (petroleum).

(a) Explain the physical principle that allows the components of crude oil to be separated.
(b) Describe the trend in viscosity and volatility for fractions collected at the bottom of the fractionating column compared to those at the top.
(c) Name the fraction used as fuel for aircraft and the fraction used for making roads.