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Chapter 8: Problem 8

Aluminum is determined gravimetrically by precipitating \(\mathrm{Al}(\mathrm{OH})_{3}\) and isolating \(\mathrm{Al}_{2} \mathrm{O}_{3} .\) A sample that contains approximately \(0.1 \mathrm{~g}\) of \(\mathrm{Al}\) is dissolved in \(200 \mathrm{~mL}\) of \(\mathrm{H}_{2} \mathrm{O},\) and \(5 \mathrm{~g}\) of \(\mathrm{NH}_{4} \mathrm{Cl}\) and a few drops of methyl red indicator are added (methyl red is red at pH levels below 4 and yellow at \(\mathrm{pH}\) levels above 6 ). The solution is heated to boiling and \(1: 1 \mathrm{NH}_{3}\) is added dropwise until the indicator turns yellow, precipitating \(\mathrm{Al}(\mathrm{OH})_{3} .\) The precipitate is held at the solution's boiling point for several minutes before filtering and rinsing with a hot solution of \(2 \%\) \(\mathrm{w} / \mathrm{v} \mathrm{NH}_{4} \mathrm{NO}_{3} .\) The precipitate is then ignited at \(1000-1100^{\circ} \mathrm{C},\) form- ing \(\mathrm{Al}_{2} \mathrm{O}_{3}\) (a) Cite at least two ways in which this procedure encourages the formation of larger particles of precipitate. (b) The ignition step is carried out carefully to ensure the quantitative conversion of \(\mathrm{Al}(\mathrm{OH})_{3}\) to \(\mathrm{Al}_{2} \mathrm{O}_{3} .\) What is the effect of an incomplete conversion on the \(\% \mathrm{w} / \mathrm{w}\) Al? (c) What is the purpose of adding \(\mathrm{NH}_{4} \mathrm{Cl}\) and methyl red indicator? (d) An alternative procedure foraluminum involves isolating and weighing the precipitate as the 8 -hydroxyquinolate, \(\mathrm{Al}\left(\mathrm{C}_{9} \mathrm{H}_{6} \mathrm{NO}\right)_{3} .\) Why might this be a more advantageous form of Al for a gravimetric analysis? Are there any disadvantages?

Short Answer

Expert verified
The procedure promotes larger particles through controlled precipitation and heating. Incomplete conversion lowers %w/w Al. NH4Cl and methyl red control pH. The hydroxyquinolate form is accurate but may pose solubility challenges.

Step by step solution

01

Encouraging Larger Particles

The procedure encourages larger particle formation by holding the precipitate at the solution's boiling point for several minutes. This allows for the growth of larger crystals due to Ostwald ripening. Additionally, the gradual addition of \( \mathrm{NH}_3 \) dropwise ensures controlled precipitation, which reduces the formation of fine particles and promotes larger crystalline particles.
02

Effect of Incomplete Conversion

If \( \mathrm{Al(OH)}_3 \) is not completely converted to \( \mathrm{Al}_2 \mathrm{O}_3 \), the measured mass of \( \mathrm{Al}_2 \mathrm{O}_3 \) will be lower than expected. This results in a calculated %w/w Al that is lower than the actual value, as \( \mathrm{Al(OH)}_3 \) weighs less than \( \mathrm{Al}_2 \mathrm{O}_3 \) when completely converted.
03

Purpose of NH4Cl and Methyl Red

\( \mathrm{NH}_4 \mathrm{Cl} \) is added to help control the pH of the solution by acting as a buffer, ensuring the precipitation of \( \mathrm{Al(OH)}_3 \) occurs at the correct pH. The methyl red indicator is used to visually signal when the pH is above 6, indicating the completion of the precipitation process as the color changes from red to yellow.
04

Alternative Procedure Considerations

Using \( \mathrm{Al(C_9H_6NO)_3} \) may be advantageous because it is less prone to loss upon ignition, ensuring more accurate gravimetric analysis results. It is also less likely to adsorb impurities. However, disadvantages include potential complexity in synthesis and possible solubility issues when trying to isolate this compound.

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Key Concepts

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

Aluminum Precipitation
Aluminum precipitation is a process used in gravimetric analysis to measure the amount of aluminum in a sample. It involves the reaction of aluminum ions with substances that cause them to form a solid precipitate, usually aluminum hydroxide \( \mathrm{Al(OH)}_3 \). This precipitate can then be isolated and weighed, providing an accurate measurement of the aluminum content. To achieve larger particle formation, the procedure involves keeping the precipitate at the boiling point for several minutes. This technique, known as Ostwald ripening, helps larger crystals grow by dissolving and redepositing smaller ones. Additionally, the slow addition of ammonia \( \mathrm{NH}_3 \) ensures a controlled environment for the formation of the precipitate, reducing fine particles and allowing larger crystalline structures to develop.
Chemical Indicators
Chemical indicators play a critical role in analytical procedures by providing visual cues about the chemical environment, such as pH levels. In the described procedure, methyl red is used as a pH indicator. Methyl red changes color based on the pH of the solution. It appears red in acidic solutions (pH below 4) and turns yellow when the pH is above 6. This color change is crucial because it signals that the solution has reached the optimal pH level for \( \mathrm{Al(OH)}_3 \) precipitation. By observing the color shift from red to yellow, chemists can accurately determine when enough ammonia has been added to the solution. This ensures the completeness of the aluminum precipitation, leading to more precise results in gravimetric analysis.
Stoichiometry
Stoichiometry is a fundamental concept in chemistry that deals with the quantitative relationships between reactants and products in a chemical reaction. In gravimetric analysis of aluminum, stoichiometry is important when converting aluminum hydroxide \( \mathrm{Al(OH)}_3 \) into aluminum oxide \( \mathrm{Al}_2 \mathrm{O}_3 \). If the conversion is incomplete, the mass of aluminum oxide formed will be less than expected. Consequently, this results in an erroneously low calculation of the percentage of aluminum \( \%\ \mathrm{w}/\mathrm{w}\ \mathrm{Al} \). Understanding the stoichiometry allows chemists to accurately predict the amounts needed and accounts for any discrepancies in mass calculations, ensuring accurate analytical outcomes.
Analytical Chemistry Techniques
Analytical chemistry techniques, such as gravimetric analysis, are vital for accurately determining the composition of substances. These techniques involve isolating and measuring the mass of a compound formed from the unknown substance. The described exercise employs this method to determine aluminum content through precipitation and conversion to aluminum oxide. The alternative procedure suggests isolating aluminum as Al(8-hydroxyquinolate) \( \mathrm{Al(C_9H_6NO)_3} \), which may offer distinct advantages. For instance, this compound is less prone to be altered during ignition, which can lead to more accurate measurements. However, isolating it may involve more complex synthesis steps and potential solubility issues. By choosing the most appropriate analytical technique, chemists can optimize the accuracy and reliability of the analysis results.

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Most popular questions from this chapter

Calcium is determined gravimetrically by precipitating \(\mathrm{CaC}_{2} \mathrm{O}_{4} \cdot \mathrm{H}_{2} \mathrm{O}\) and isolating \(\mathrm{CaCO}_{3}\). After dissolving a sample in \(10 \mathrm{~mL}\) of water and \(15 \mathrm{~mL}\) of \(6 \mathrm{M} \mathrm{HCl}\), the resulting solution is heated to boiling and a warm solution of excess ammonium oxalate is added. The solution is maintained at \(80^{\circ} \mathrm{C}\) and \(6 \mathrm{M} \mathrm{NH}_{3}\) is added dropwise, with stirring, until the solution is faintly alkaline. The resulting precipitate and solution are removed from the heat and allowed to stand for at least one hour. After testing the solution for completeness of precipitation, the sample is filtered, rinsed with \(0.1 \% \mathrm{w} / \mathrm{v}\) ammonium oxalate, and dried for one hour at \(100-120^{\circ} \mathrm{C}\). The precipitate is transferred to a muffle furnace where it is converted to \(\mathrm{CaCO}_{3}\) by drying at \(500 \pm 25^{\circ} \mathrm{C}\) until constant weight. (a) Why is the precipitate of \(\mathrm{CaC}_{2} \mathrm{O}_{4} \cdot \mathrm{H}_{2} \mathrm{O}\) converted to \(\mathrm{CaCO}_{3} ?\) (b) In the final step, if the sample is heated at too high of a temperature some \(\mathrm{CaCO}_{3}\) is converted to \(\mathrm{CaO}\). What effect would this have on the reported \(\% \mathrm{w} / \mathrm{w}\) Ca? (c) Why is the precipitant, \(\left(\mathrm{NH}_{4}\right)_{2} \mathrm{C}_{2} \mathrm{O}_{4},\) added to a hot, acidic solution instead of a cold, alkaline solution?

The amount of iron and manganese in an alloy is determined by precipitating the metals with 8 -hydroxyquinoline, \(\mathrm{C}_{9} \mathrm{H}_{7} \mathrm{NO}\). After weighing the mixed precipitate, the precipitate is dissolved and the amount of 8-hydroxyquinoline determined by another method. In a typical analysis a 127.3 -mg sample of an alloy containing iron, manganese, and other metals is dissolved in acid and treated with appropriate masking agents to prevent an interference from other metals. The iron and manganese are precipitated and isolated as \(\mathrm{Fe}\left(\mathrm{C}_{9} \mathrm{H}_{6} \mathrm{NO}\right)_{3}\) and \(\mathrm{Mn}\left(\mathrm{C}_{9} \mathrm{H}_{6} \mathrm{NO}\right)_{2},\) yielding a total mass of \(867.8 \mathrm{mg}\). The amount of 8 -hydroxyquinolate in the mixed precipitate is determined to be \(5.276 \mathrm{mmol}\). Calculate the \(\% \mathrm{w} / \mathrm{w} \mathrm{Fe}\) and \(\% \mathrm{w} / \mathrm{w} \mathrm{Mn}\) in the alloy.

In the presence of water vapor the surface of zirconia, \(\mathrm{ZrO}_{2}\), chemically adsorbs \(\mathrm{H}_{2} \mathrm{O},\) forming surface hydroxyls, \(\mathrm{ZrOH}\) (additional water is physically adsorbed as \(\mathrm{H}_{2} \mathrm{O}\) ). When heated above \(200^{\circ} \mathrm{C}\), the surface hydroxyls convert to \(\mathrm{H}_{2} \mathrm{O}(g),\) releasing one molecule of water for every two surface hydroxyls. Below \(200^{\circ} \mathrm{C}\) only physically absorbed water is lost. Nawrocki, et al. used thermogravimetry to determine the density of surface hydroxyls on a sample of zirconia that was heated to \(700^{\circ} \mathrm{C}\) and cooled in a desiccator containing humid \(\mathrm{N}_{2}{ }^{15}\) Heating the sample from \(200^{\circ} \mathrm{C}\) to \(900^{\circ} \mathrm{C}\) released \(0.006 \mathrm{~g}\) of \(\mathrm{H}_{2} \mathrm{O}\) for every gram of dehy- droxylated \(\mathrm{ZrO}_{2}\). Given that the zirconia had a surface area of \(33 \mathrm{~m}^{2} / \mathrm{g}\) and that one molecule of \(\mathrm{H}_{2} \mathrm{O}\) forms two surface hydroxyls, calculate the density of surface hydroxyls in \(\mu \mathrm{mol} / \mathrm{m}^{2}\).

A \(516.7-\mathrm{mg}\) sample that contains a mixture of \(\mathrm{K}_{2} \mathrm{SO}_{4}\) and \(\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4}\) is dissolved in water and treated with \(\mathrm{BaCl}_{2},\) precipitating the \(\mathrm{SO}_{4}^{2-}\) as \(\mathrm{BaSO}_{4}\). The resulting precipitate is isolated by filtration, rinsed free of impurities, and dried to a constant weight, yielding \(863.5 \mathrm{mg}\) of \(\mathrm{BaSO}_{4} .\) What is the \(\% \mathrm{w} / \mathrm{w} \mathrm{K}_{2} \mathrm{SO}_{4}\) in the sample?

The water content of an \(875.4-\mathrm{mg}\) sample of cheese is determined with a moisture analyzer. What is the \(\% \mathrm{w} / \mathrm{w} \mathrm{H}_{2} \mathrm{O}\) in the cheese if the final mass was found to be \(545.8 \mathrm{mg} ?\)
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