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Chapter 4: Problem 28

Separate samples of a solution of an unknown ionic compound are treated with dilute \(\mathrm{AgNO}_{3}, \mathrm{~Pb}\left(\mathrm{NO}_{3}\right)_{2}\), and \(\mathrm{BaCl}_{2}\). Precipitates form in all three cases. Which of the following could be the anion of the unknown salt: \(\mathrm{Br}^{-}, \mathrm{CO}_{3}{ }^{2-}, \mathrm{NO}_{3}{ }^{-}\)?

Short Answer

Expert verified
The anion of the unknown salt must form precipitates with AgNO3, Pb(NO3)2, and BaCl2. Based on our analysis, Br- and CO3^2- can form precipitates with these compounds, while NO3- cannot. Thus, the anion of the unknown salt could be either Br- or CO3^2-.

Step by step solution

01

Analyze the reactivity of Br-

In the presence of AgNO3, Br- would react as follows: AgNO3(aq) + XBr(aq) 鈫� AgBr(s) + XNO3(aq) AgBr is almost insoluble and would precipitate. Br- would also react with Pb(NO3)2 and BaCl2 in similar ways: Pb(NO3)2(aq) + 2XBr(aq) 鈫� PbBr2(s) + 2XNO3(aq) BaCl2(aq) + 2XBr(aq) 鈫� BaBr2(s) + 2XCl-(aq) Both PbBr2 and BaBr2 are slightly soluble in water, allowing the formation of precipitates. So, based on these reactions, Br- could possibly be the anion of the unknown salt.
02

Analyze the reactivity of CO3^2-

In the presence of AgNO3, CO3^2- would react as follows: 2AgNO3(aq) + XCO3^2-(aq) 鈫� Ag2CO3(s) + 2XNO3(aq) Ag2CO3 is almost insoluble and would precipitate. CO3^2- would also react with Pb(NO3)2 and BaCl2 in similar ways: Pb(NO3)2(aq) + XCO3^2-(aq) 鈫� PbCO3(s) + 2XNO3(aq) BaCl2(aq) + XCO3^2-(aq) 鈫� BaCO3(s) + 2XCl-(aq) Both PbCO3 and BaCO3 are also almost insoluble in water, allowing the formation of precipitates. So, based on these reactions, CO3^2- could also possibly be the anion of the unknown salt.
03

Analyze the reactivity of NO3-

In the presence of AgNO3, NO3- would react as follows: AgNO3(aq) + XNO3(aq) 鈫� AgNO3(aq) + XNO3(aq) Since AgNO3 is soluble in water, there wouldn't be any precipitate formed with NO3-. The same will happen with reactions of NO3- with Pb(NO3)2 and BaCl2. So, NO3- cannot be the anion of the unknown salt as it doesn't form precipitates.
04

Conclusion

The anion of the unknown salt must form precipitates with AgNO3, Pb(NO3)2, and BaCl2. Based on our analysis, Br- and CO3^2- can form precipitates with these compounds, while NO3- cannot. Thus, the anion of the unknown salt could be either Br- or CO3^2-.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Precipitation Reactions
Precipitation reactions occur when two soluble salts react in aqueous solution to form an insoluble solid, called a precipitate. When you mix solutions containing ionic compounds, they dissociate into individual ions in water. If a combination of these ions forms an insoluble compound, a precipitate forms, visually signaling the reaction.

In the given exercise, unknown ions react with
  • 础驳狈翱鈧� to form AgBr or Ag鈧侰O鈧�
  • 笔产(狈翱鈧�)鈧� to form PbBr鈧� or PbCO鈧�
  • 叠补颁濒鈧� to form BaBr鈧� or BaCO鈧�
These reactions result in a precipitate because the products are not soluble in water. Identifying which precipitates form helps determine the anion in the unknown ionic compound.
Anion Identification
Identifying an anion involves testing reactions that can distinctly reveal the presence of specific ions. In this exercise, the goal is to determine which anion forms precipitates with silver, lead, and barium compounds.

By forming a precipitate with
  • 础驳狈翱鈧�
  • 笔产(狈翱鈧�)鈧�
  • 叠补颁濒鈧�
Bromide (Br鈦�) and carbonate (CO鈧兟测伝) ions indicate their presence. Since these ions create insoluble compounds, observing precipitates cues us to their identity. In contrast, nitrate (NO鈧冣伝) does not form a precipitate, helping rule out its presence in the tested compound.

Hence, testing which reactions result in precipitation provides the necessary clues for identifying the unknown anion.
Chemical Solubility
Understanding chemical solubility is key to predicting which compounds will form precipitates. Solubility rules tell us which ionic compounds dissolve in water and which do not. Solubility is influenced by factors like lattice energy of the solid and the hydration energy of the ions.

For example, most nitrate ( NO鈧冣伝) compounds are soluble, so they don't typically form precipitates. Bromides ( Br鈦�), however, are only slightly soluble with lead and silver, and most carbonates (like CO鈧兟测伝) are insoluble, forming a solid in solution when mixed with ions like Ag鈦�, Pb虏鈦�, or Ba虏鈦�.

This knowledge allows chemists to predict precipitation reactions and identify unknown ions by observing results in a controlled setting.

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Most popular questions from this chapter

Some sulfuric acid is spilled on a lab bench. You can neutralize the acid by sprinkling sodium bicarbonate on it and then mopping up the resultant solution. The sodium bicarbonate reacts with sulfuric acid according to: $$ \begin{aligned} 2 \mathrm{NaHCO}_{3}(s)+\mathrm{H}_{2} \mathrm{SO}_{4}(a q) \longrightarrow & \mathrm{Na}_{2} \mathrm{SO}_{4}(a q)+\\\ & 2 \mathrm{H}_{2} \mathrm{O}(l)+2 \mathrm{CO}_{2}(g) \end{aligned} $$ Sodium bicarbonate is added until the fizzing due to the formation of \(\mathrm{CO}_{2}(g)\) stops. If \(27 \mathrm{~mL}\) of \(6.0 \mathrm{M} \mathrm{H}_{2} \mathrm{SO}_{4}\) was spilled, what is the minimum mass of \(\mathrm{NaHCO}_{3}\) that must be added to the spill to neutralize the acid?

Determine the oxidation number of sulfur in each of the following substances: (a) barium sulfate, \(\mathrm{BaSO}_{4}\), (b) sulfurous acid, \(\mathrm{H}_{2} \mathrm{SO}_{3}\), (c) strontium sulfide, \(\mathrm{SrS}\), (d) hydrogen sulfide, \(\mathrm{H}_{2} \mathrm{~S}\). (e) Locate sulfur in the periodic table in Exercise 4.47; what region is it in? (f) Which region(s) of the period table contains elements that can adopt both positive and negative oxidation numbers?

Specify what ions are present upon dissolving each of the following substances in water: (a) \(\mathrm{MgI}_{2}\), (b) \(\mathrm{K}_{2} \mathrm{CO}_{3}\), (c) \(\mathrm{HClO}_{4}\), (d) \(\mathrm{NaCH}_{3} \mathrm{COO}\).

Pure acetic acid, known as glacial acetic acid, is a liquid with a density of \(1.049 \mathrm{~g} / \mathrm{mL}\) at \(25^{\circ} \mathrm{C}\). Calculate the molarity of a solution of acetic acid made by dissolving \(20.00 \mathrm{~mL}\) of glacial acetic acid at \(25^{\circ} \mathrm{C}\) in enough water to make \(250.0 \mathrm{~mL}\) of solution.

A 0.5895-g sample of impure magnesium hydroxide is dissolved in \(100.0 \mathrm{~mL}\) of \(0.2050 \mathrm{M} \mathrm{HCl}\) solution. The excess acid then needs \(19.85 \mathrm{~mL}\) of \(0.1020 \mathrm{M} \mathrm{NaOH}\) for neutralization. Calculate the percentage by mass of magnesium hydroxide in the sample, assuming that it is the only substance reacting with the \(\mathrm{HCl}\) solution.
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