Problem 99 - Chemistry and Chemical Reactivity

Question

A common method for analyzing for the nickel content of a sample is to use a precipitation reaction. Adding the organic compound dimethylglyoxime to a solution containing \(\mathrm{Ni}^{2+}\) ions precipitates a red solid. Derive the empirical formula for the red solid based on the following composition: \(\mathrm{Ni}, 20.315 \%\) \(\mathrm{C}, 33.258 \% ; \mathrm{H}, 4.884 \% ; \mathrm{O}, 22.151 \% ;\) and \(\mathrm{N}\) \(19.392 \%\).

Step-by-Step Solution

Answer

The empirical formula is \( \mathrm{NiC_8H_{14}O_4N_4} \).

Step 1: Obtain Molar Mass of Each Element

First, determine the molar mass of each element in the compound from the periodic table: \( \mathrm{Ni} = 58.69 \text{ g/mol} \), \( \mathrm{C} = 12.01 \text{ g/mol} \), \( \mathrm{H} = 1.008 \text{ g/mol} \), \( \mathrm{O} = 16.00 \text{ g/mol} \), \( \mathrm{N} = 14.01 \text{ g/mol} \).

Step 2: Convert Percentages to Grams for a 100 g Sample

Assume there is a 100 g sample of the compound. Convert the percentages of each element in the sample to grams:\( \mathrm{Ni} = 20.315 \text{ g} \), \( \mathrm{C} = 33.258 \text{ g} \), \( \mathrm{H} = 4.884 \text{ g} \), \( \mathrm{O} = 22.151 \text{ g} \), \( \mathrm{N} = 19.392 \text{ g} \).

Step 3: Convert Grams to Moles

Convert the mass of each element to moles using their molar masses:\( \text{Moles of Ni} = \frac{20.315}{58.69} = 0.346\text{ moles} \)\( \text{Moles of C} = \frac{33.258}{12.01} = 2.77\text{ moles} \)\( \text{Moles of H} = \frac{4.884}{1.008} = 4.85\text{ moles} \)\( \text{Moles of O} = \frac{22.151}{16.00} = 1.384\text{ moles} \)\( \text{Moles of N} = \frac{19.392}{14.01} = 1.384\text{ moles} \).

Step 4: Divide by the Smallest Number of Moles

Divide each mole value by the smallest number of moles, which is \(1.384\):\( \text{Moles of Ni} = \frac{0.346}{1.384} \approx 0.25\)\( \text{Moles of C} = \frac{2.77}{1.384} \approx 2\)\( \text{Moles of H} = \frac{4.85}{1.384} \approx 3.5\)\( \text{Moles of O} = \frac{1.384}{1.384} = 1\)\( \text{Moles of N} = \frac{1.384}{1.384} = 1\).

Step 5: Simplify the Ratios to Integer Values

Multiply each ratio by a factor to obtain the smallest whole numbers. Here, we can multiply by 4 to clear decimals:- \( 0.25 \times 4 = 1 \) for Ni - \( 2 \times 4 = 8 \) for C - \( 3.5 \times 4 = 14 \) for H - \( 1 \times 4 = 4 \) for O and N This results in an empirical formula: \( \mathrm{NiC_8H_{14}O_4N_4} \).

Key Concepts

Precipitation Reaction

A precipitation reaction is a process in which an insoluble solid, called a precipitate, forms when two solutions are combined. In the context of nickel content analysis, this type of reaction is critical because it helps isolate nickel ions from other components in a sample. By introducing \(\text{dimethylglyoxime}\), which is an organic compound, into a nickel-containing solution, a distinctive red solid forms effectively and specifically.

This red solid is the nickel dimethylglyoxime complex, which precipitates because the solubility of the compound in water is very low. The formation of this solid is a clear indicator of nickel ions present in the solution. Precipitation occurs because the attraction between particles in the precipitate is stronger than the forces keeping them dispersed in the liquid. Thus, the particles bond together to create this visible solid.

• Precipitates provide a straightforward method to remove ions from a solution.
• They are often used in chemical analysis to identify and quantify the components within a mixture.
• In nickel analysis, the unique red color of the precipitate helps confirm the presence of nickel.

Nickel Content Analysis

Nickel content analysis aims to determine the amount of nickel in a sample, using quantitative or qualitative methods. The precipitation reaction involving dimethylglyoxime is one quantitative method widely utilized due to its specificity and the distinctive red color of the precipitate. This method is particularly advantageous because of its accuracy in measuring nickel concentration, even in complex mixtures.

The procedure involves transforming nickel ions into an insoluble compound that can then be separated out, usually by filtration. By weighing the precipitate, chemists can determine the amount of nickel present in the original solution through stoichiometric calculations. This is possible because the mass of the precipitate is consistently related to the mass of nickel ions through the ratio reflected in its empirical formula.

• Precipitation provides a method to quantitatively analyze metal content in diverse samples.
• The mass of the precipitate indicates the concentration of nickel when derived using established calculations.
• Stoichiometry links the mass of nickel ions to the mass of the precipitate, ensuring precise measurement capabilities.

Dimethylglyoxime

Dimethylglyoxime is an organic compound commonly used as a reagent in chemical analysis, especially for detecting nickel ions. Its molecular structure, \(\text{C}_\text{4}\text{H}_\text{8}\text{N}_\text{2}\text{O}_\text{2}\), allows it to bond with nickel ions effectively and form a strong, insoluble complex. This is part of why it has become indispensable in nickel assays.

When dimethylglyoxime is added to a solution containing nickel ions, it complexes with them, resulting in the characteristic red precipitate of nickel dimethylglyoxime. This compound is notable not only for its vivid coloration but also for its low solubility in water, which makes it easy to separate from the solution. This specificity to nickel ions ensures that dimethylglyoxime can be reliably used for analysis in systems that may have multiple potential metal contaminants.

• Dimethylglyoxime is highly specific to nickel, making it ideal for selective metal ion analysis.
• The red nickel dimethylglyoxime complex is easy to handle due to the low solubility and clear visual indication.
• Overall, it acts as a precise and reliable reagent for confirming nickel presence in solutions.