Question

ГИА-9-2011, С3 Для установления качественного состава неизвестного вещества школьникам была выдана соль металла, который входит в состав метеоритов и обладает ярко выраженными магнитными свойствами. Коричневые кристаллы этой соли ученики растворили в воде, затем полученный раствор жёлто-коричневого цвета разлили в две пробирки. В одну из них добавили раствор гидроксида калия, при этом образовался осадок бурого цвета. В другую прилили раствор нитрата серебра, в результате чего выпал белый "творожистый" осадок. Запишите химическую формулу и название неизвестной соли. Составьте два молекулярных уравнения реакций, которые были проведены в процессе ее распознавания.

Answer 1

Identification of Unknown Salt

To determine the qualitative composition of an unknown substance, the students were given a metal salt that is found in meteorites and exhibits strong magnetic properties. The brown crystals of this salt were dissolved in water, and the resulting yellow-brown solution was divided into two test tubes. In one test tube, potassium hydroxide solution was added, resulting in the formation of a brown precipitate. In the other test tube, silver nitrate solution was added, resulting in the formation of a white "curdy" precipitate.

Chemical Formula and Name of the Unknown Salt: The chemical formula and name of the unknown salt can be determined based on the reactions observed.

Chemical Formula: To determine the chemical formula, we need to analyze the reactions that occurred when the unknown salt was mixed with potassium hydroxide and silver nitrate solutions.

Reaction with Potassium Hydroxide (KOH): When the unknown salt was mixed with potassium hydroxide solution, a brown precipitate was formed. This indicates the presence of a metal cation that forms an insoluble hydroxide compound.

Molecular Equation: Unknown Salt + Potassium Hydroxide → Brown Precipitate

Chemical Formula of the Unknown Salt: The brown precipitate formed suggests the presence of a metal cation that forms an insoluble hydroxide compound. One possible metal cation is Fe^3+ (iron(III)).

Reaction with Silver Nitrate (AgNO3): When the unknown salt was mixed with silver nitrate solution, a white "curdy" precipitate was formed. This indicates the presence of a chloride anion, which reacts with silver ions to form a white precipitate of silver chloride.

Molecular Equation: Unknown Salt + Silver Nitrate → White Precipitate

Chemical Formula of the Unknown Salt: The white precipitate formed suggests the presence of a chloride anion. One possible chloride anion is Cl^- (chloride).

Based on the reactions observed, the chemical formula of the unknown salt is likely FeCl3 (iron(III) chloride).

Molecular Equations of the Reactions: 1. Reaction with Potassium Hydroxide: FeCl3 + 3KOH → Fe(OH)3 + 3KCl

2. Reaction with Silver Nitrate: FeCl3 + 3AgNO3 → Fe(NO3)3 + 3AgCl

Please note that these equations are balanced based on the stoichiometry of the reactants and products.