Question

A mixture of \(0.5 \mathrm{~g}\) of ethanol and acetaldehyde when heated with Fehling's solution gave \(1.2 \mathrm{~g}\) of red precipitate of \(\mathrm{Cu}_{2} \mathrm{O}\). What is the percentage of acetaldehyde in the mixture? a. \(54 \%\) b. \(47 \%\) c. \(74 \%\) d. \(36 \%\)

Short answer

The percentage of acetaldehyde in the mixture is 74% (option c).

Step-by-step solution

Understand the Reaction

When acetaldehyde reacts with Fehling's solution, it produces a red precipitate of copper(I) oxide, \ \( \text{Cu}_2\text{O} \). The chemical reaction can be represented as follows: \ \[ \text{CH}_3\text{CHO} + 2\text{Cu}^{2+} + 4\text{OH}^- \rightarrow \text{CH}_3\text{COO}^- + \text{Cu}_2\text{O} + 2\text{H}_2\text{O} \] This reaction shows that one molecule of acetaldehyde is required to produce one molecule of \ \( \text{Cu}_2\text{O} \) precipitate.

Find the Molar Masses

The molar mass of acetaldehyde (\( \text{CH}_3\text{CHO} \)) is calculated as follows:\ \( = 12.01 \times 2 + 1.01 \times 4 + 16.00 = 44.05 \, \text{g/mol} \) The molar mass of copper(I) oxide (\( \text{Cu}_2\text{O} \)) is:\ \( = 63.55 \times 2 + 16.00 = 143.1 \, \text{g/mol} \)

Calculate Moles of Cu2O

Given that \(1.2\, \text{g}\) of \(\text{Cu}_2\text{O} \) was formed, calculate the number of moles:\ \[ \text{Moles of Cu}_2\text{O} = \frac{1.2\, \text{g}}{143.1\, \text{g/mol}} \approx 0.0084\, \text{mol} \]

Calculate Moles of Acetaldehyde

Each mole of \( \text{Cu}_2\text{O} \) comes from one mole of acetaldehyde:Therefore, the moles of acetaldehyde are also \( 0.0084 \) moles.

Calculate Mass of Acetaldehyde

Using the number of moles, calculate the mass of acetaldehyde:\ \[ \text{Mass of } \text{CH}_3\text{CHO} = 0.0084\, \text{mol} \times 44.05\, \text{g/mol} \approx 0.370\, \text{g} \]

Calculate Percentage of Acetaldehyde

Now calculate the percentage of acetaldehyde in the mixture:\ \[ \text{Percentage} = \left( \frac{0.370\, \text{g}}{0.5\, \text{g}} \right) \times 100 \approx 74\% \]

Key concepts

Stoichiometry

Stoichiometry is an incredible concept in chemistry that allows us to quantify the relationships between substances in a chemical reaction. Each balanced equation provides a wealth of information about the proportions of reactants and products in a chemical process. For example, in the reaction of acetaldehyde with Fehling's solution to produce copper(I) oxide, the equation tells us that one mole of acetaldehyde will produce one mole of copper(I) oxide.

• This is crucial because it allows chemists to use these molar ratios to calculate the amounts of various substances needed or produced in a reaction.
• Understanding and using stoichiometry helps in predicting yields and optimizing reactions.
• It also plays a vital role in scaling chemical reactions from small laboratory procedures to industrial production.

Keeping track of these relationships requires practice, but once mastered, it becomes an invaluable tool for solving chemical problems efficiently.

Molar Mass

Molar mass is a fundamental property in chemistry, representing the mass of one mole of a given substance. To find the molar mass, you sum up the atomic masses of all atoms in a molecule. In this exercise, we calculated the molar mass of acetaldehyde, which is made up of two carbon atoms, four hydrogen atoms, and one oxygen atom, totaling 44.05 g/mol.

• Understanding how to calculate molar mass is essential for converting between grams and moles—a necessary skill in all stoichiometric calculations.
• This conversion capacity helps in determining the weight of reactants required or the weight of products formed in a reaction.

Performing these calculations builds a strong foundation in chemical problem-solving and aids in further chemical exploration and research.

Precipitation Reaction

Precipitation reactions occur when two soluble substances react in a solution to form an insoluble solid known as a precipitate. In this exercise, the reaction of acetaldehyde with Fehling's solution involves a precipitation reaction where the copper ions in the solution react to form the red copper(I) oxide precipitate.

• Precipitation reactions are important in identifying ions in a solution by observing the color, form, or amount of precipitate.
• They are also widely used in various fields such as environmental science for removing contaminants from water.

By understanding the principles behind precipitation reactions, students can predict the formation of precipitates, helping them to understand the outcome of complex chemical systems.