Problem 91 - 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: \(\mathrm{Ni}, 20.315 \% ; \mathrm{C}, 33.258 \% ; \mathrm{H}\) \(4.884 \% ; \mathbf{O}, 22.151 \% ;\) and \(\mathrm{N}, 19.392 \%\).

Step-by-Step Solution

Answer

The empirical formula for the red solid is NiC₈H₁₄O₄N₄.

Step 1: Convert Percentages to Grams

Assume you have 100 grams of the compound. This allows you to directly convert the given percentages into grams. Hence, you have: 20.315 g of Ni, 33.258 g of C, 4.884 g of H, 22.151 g of O, and 19.392 g of N.

Step 2: Convert Grams to Moles

Utilize the molar masses to convert from grams to moles: - For Ni: \( \frac{20.315}{58.69} = 0.346 \) moles- For C: \( \frac{33.258}{12.01} = 2.769 \) moles- For H: \( \frac{4.884}{1.008} = 4.846 \) moles - For O: \( \frac{22.151}{16.00} = 1.384 \) moles - For N: \( \frac{19.392}{14.01} = 1.384 \) moles

Step 3: Find the Simplest Integer Ratio

To achieve the simplest integer ratio, one must divide each mole value by the smallest calculated value (in this instance, for Ni): - Ni: \( \frac{0.346}{0.346} = 1 \) - C: \( \frac{2.769}{0.346} \approx 8 \) - H: \( \frac{4.846}{0.346} \approx 14 \) - O: \( \frac{1.384}{0.346} \approx 4 \) - N: \( \frac{1.384}{0.346} \approx 4 \)

Step 4: Determine Empirical Formula

Combine the ratios to produce the empirical formula. The formula is composed of: NiC₈H₁₄O₄N₄.

Key Concepts

Precipitation Reaction

A precipitation reaction is a type of chemical reaction where two solutions, typically containing soluble salts, are mixed together to form an insoluble solid. This insoluble solid is known as the precipitate.
When dealing with nickel compounds, a common method involves adding the organic compound dimethylglyoxime. This reacts with the nickel ions (\(\mathrm{Ni}^{2+}\)) in solution to form a distinct red solid precipitate. This solid can then be isolated and analyzed to assess the nickel content within a sample, capitalizing on its characteristic color and insolubility.

• This approach is useful for identifying specific metals in a solution because of the predictable formation of a solid.
• The reaction demands precise measurement to ensure complete precipitation and accurate results.

Understanding how precipitation reactions work is essential to devising efficient methods for metal ion analysis in chemical laboratories.

Mole Conversion

Mole conversion is a critical step in chemical calculations that enables chemists to relate masses of substances to the number of particles they contain.
When calculating the empirical formula of a compound, one must first convert the percentages or masses of elements into moles. This involves utilizing the molar mass of each element.
For example, calculating moles involves dividing the mass of each component by its molar mass:

• Nickel (\(\mathrm{Ni}\)):\( \frac{\text{mass of Ni}}{\text{molar mass of Ni}} = \text{moles of Ni} \)
• Carbon (\(\mathrm{C}\)), Hydrogen (\(\mathrm{H}\)), Oxygen (\(\mathrm{O}\)), and Nitrogen (\(\mathrm{N}\)) follow the same pattern.

Accurately performing these conversions is crucial for determining the correct ratios needed to find the empirical formula, as they reflect the composition of molecules at the atomic level.

Chemical Composition Analysis

Chemical composition analysis involves determining the amounts and ratios of elements within a compound. This type of analysis is essential in chemistry for understanding how compounds form and behave.
Given a percentage composition, the first step is to assume a convenient total mass, often 100 grams, to ease the conversion from percentage to actual mass.

• Convert these masses into moles for each element using their respective molar masses.
• Determine the simplest ratio by dividing each mole value by the smallest mole value obtained.
• This ratio provides insight into the empirical formula of the compound.

Conducting a thorough chemical composition analysis allows chemists to produce accurate and reliable formulas, which are vital for further scientific calculations and practical applications.

Nickel Compound Analysis

Nickel compound analysis is a specialized field focused on understanding nickel's role and behavior in various chemical forms.
When analyzing a nickel compound, understanding its empirical formula is critical. This not only provides insight into its structure but also aids in determining its properties.

• To derive the empirical formula, precipitation reactions can isolate nickel ions, making subsequent mole calculations straightforward.
• By thoroughly analyzing the chemical composition, one can ensure precise empirical formulas, essential for predicting reactivity and interactions in broader chemical contexts.

The analysis of nickel compounds such as those utilizing dimethylglyoxime can uncover essential information about the metal's binding, offering valuable insight into industrial and research applications.