Question Video: Explaining the Steps in the Preparation of a Soluble Salt from Soluble Reactants Chemistry

Sodium chloride can be prepared by reacting sodium hydroxide with hydrochloric acid. In an experiment, a fixed volume of sodium hydroxide and a few drops of phenolphthalein were added into an Erlenmeyer flask. Hydrochloric acid was then added, from a buret, until the reaction was complete. The reaction was then repeated, without the indicator but using the exact volume of hydrochloric acid determined from the previous experiment. The resulting solution was then heated to produce white crystals of sodium chloride. Why were a few drops of phenolphthalein added to the solution at the start? [A] To increase the solubility of sodium chloride [B] To increase the basicity of sodium hydroxide [C] To remove any impurities in the solution [D] To act as an indicator and inform us when the reaction is complete [E] To act as a catalyst in the reaction, Why would this method not work for producing crystals of copper sulfate from copper(II) oxide and sulfuric acid at room temperature? Why could the sodium chloride not be obtained by filtering the solution?

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Video Transcript

Sodium chloride can be prepared by reacting sodium hydroxide with hydrochloric acid. In an experiment, a fixed volume of sodium hydroxide and a few drops of phenolphthalein were added into an Erlenmeyer flask. Hydrochloric acid was then added, from a buret, until the reaction was complete. The reaction was then repeated, without the indicator but using the exact volume of hydrochloric acid determined from the previous experiment. The resulting solution was then heated to produce white crystals of sodium chloride. Why were a few drops of phenolphthalein added to the solution at the start? (A) To increase the solubility of sodium chloride, (B) to increase the basicity of sodium hydroxide, (C) to remove any impurities in the solution, (D) to act as an indicator and inform us when the reaction is complete, or (E) to act as a catalyst in the reaction.

Let’s first review the methods used in this experiment before answering the first part of our question. First, a fixed volume of sodium hydroxide, which is a basic solution and can be represented using its chemical formula NaOH with the state symbol aq representing that it is aqueous, was added to an Erlenmeyer flask, which can also be called a conical flask. In this same Erlenmeyer flask, a few drops of phenolphthalein were also added. Then, hydrochloric acid was reacted with sodium hydroxide using a buret.

Hydrochloric acid can be represented using the chemical formula HCl with the state symbol aq. Once this first reaction was completed and the exact volume of hydrochloric acid needed was determined, the process was repeated without the indicator. From this reaction, an aqueous solution is produced of sodium chloride, which can be represented using the chemical formula NaCl. This solution was then heated to drive off the second product of the reaction, which was liquid water or H2O with the state symbol l. With the water driven off, the remaining white crystals of sodium chloride could be collected.

The question asks us to consider why the phenolphthalein was added at the start of this experiment. Phenolphthalein is a compound whose color is determined by the pH of the solution to which it is added. In solutions with a pH higher than approximately eight, phenolphthalein appears pink. But in solutions with pHs lower than approximately eight, phenolphthalein is colorless. In this reaction between sodium hydroxide, which we can see from the diagram is a colorless solution, and it is a basic solution, which means it has a pH of above seven. Sodium hydroxide is reacted with hydrochloric acid, which we can see from the diagram is also a colorless solution and being that it is an acid has a pH of below seven.

In this experiment, it was critical to determine the exact volume of hydrochloric acid needed to completely react with the fixed volume of sodium hydroxide so that the resulting solution contained only the desired salt and water. At the start, phenolphthalein appeared pink as it was in a basic solution with a pH of above approximately eight but turned colorless when the reaction was complete, producing a resulting solution with a pH of below approximately eight. In this experiment, phenolphthalein served as a pH indicator, which is a substance that changes color within certain pH ranges and in this case informed us when the reaction was complete. Therefore, the reason why a few drops of phenolphthalein were added to the solution at the start is answer choice (D) to act as an indicator and inform us when the reaction is complete.

Why would this method not work for producing crystals of copper sulfate from copper(II) oxide and sulfuric acid at room temperature? (A) Copper sulfate precipitates out of solution and so filtration would be required. (B) Copper oxide is insoluble and would not form an aqueous solution. (C) Copper oxide is acidic and would not react with sulfuric acid. (D) Copper sulfate only forms when copper oxide reacts with sulfur dioxide. Or (E) copper oxide does not react with sulfuric acid.

Keeping the method of our experiment in mind, let’s clear some space on screen so we can answer this part of the question. In the method used to produce sodium chloride crystals, a base which is aqueous was reacted with an acid that is also aqueous. So, in this method, both the base and acid reacted were aqueous solutions, which produced an aqueous salt and liquid water. This question features copper sulfate as the desired salt product, which would be soluble in water. We can represent this using the chemical formula CuSO4 aqueous in the products of this reaction. The acidic reactant in this question is sulfuric acid whose chemical formula is H2SO4 aqueous.

Copper(II) oxide is the other reactant of this reaction whose chemical formula is CuO. According to water solubility rules, copper(II) oxide is an insoluble compound, which we can represent using the state symbol s for solid. While this reaction could feasibly occur to produce the salt copper sulfate and water as products, the method used required both reactants to be aqueous solutions and would not be appropriate for a reaction involving a solid insoluble reactant. Other methods could be used such as adding excess solid reactant to the acid, then stirring and gently warming the mixture, where once the reaction is complete, excess solid reactant can be removed through filtration. The solution can be heated to drive off the water and the salt crystals can be collected.

Therefore, the answer choice which explains why this method used to collect sodium chloride crystals would not work for producing crystals of copper sulfate from copper(II) oxide and sulfuric acid at room temperature is answer choice (B). Copper oxide is insoluble and would not form an aqueous solution.

Why could the sodium chloride not be obtained by filtering the solution? (A) Any insoluble impurities would not be separated through filtration. (B) Only the sodium ions would be filtered and not the chloride ions. (C) Any excess hydrochloric acid would burn the filter paper. Or (D) sodium chloride is soluble and would therefore not be separated through filtration.

In our reaction, sodium chloride was produced as an aqueous solution, which means in water, the ions which make up this salt disassociate and move freely from each other. However, they are extremely small that if in an attempt to separate the sodium and chloride ions from the water using filtration, the ions would be small enough to pass through the filter paper and remain in the water. Filtration, which is a technique that separates solid and liquid substances based on particle size, can be used to collect insoluble substances when poured through a filter paper in a funnel positioned above a container to collect the soluble or liquid substances, which can be referred to as the filtrate, while the substance remaining separated by the filter paper is referred to as the residue.

This specific type of filtration is called gravity filtration. Because our salt sodium chloride is water-soluble, it would not be separated using this method. Therefore, the answer choice that best describes why sodium chloride could not be obtained by filtering the solution is answer choice (D). Sodium chloride is soluble and would therefore not be separated through filtration.

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