Question

Bronze is a solid solution of \(\mathrm{Cu}(s)\) and \(\mathrm{Sn}(s) ;\) solutions of metals like this that are solids are called alloys. There is a range of compositions over which the solution is considered a bronze. Bronzes are stronger and harder than either copper or tin alone. (a) A 100.0 -g sample of a certain bronze is \(90.0 \%\) copper by mass and \(10.0 \%\) tin. Which metal can be called the solvent, and which the solute? (b) Based on part (a), calculate the concentration of the solute metal in the alloy in units of molarity, assuming a density of \(7.9 \mathrm{~g} / \mathrm{cm}^{3} .\) (c) Suggest a reaction that you could do to remove all the tin from this bronze to leave a pure copper sample. Justify your reasoning.

Short answer

Copper is the solvent; tin is the solute. Tin has a molarity of approximately 6.66 M. Reacting with HCl can remove tin.

Step-by-step solution

Determine Solvent and Solute

In an alloy, the solvent is the metal present in the greater amount, whereas the solute is the metal present in the lesser amount. In the given bronze sample, copper constitutes 90.0% and tin constitutes 10.0% of the mass. Therefore, copper is the solvent and tin is the solute.

Calculate Moles of Tin

Calculate the moles of tin (Sn) in the alloy using the mass percentage and molar mass:1. Mass of tin in the alloy = \( 100.0 \text{ g} \times 0.10 = 10.0 \text{ g} \).2. Molar mass of tin = \( 118.71 \text{ g/mol} \).3. Moles of tin = \( \frac{10.0 \text{ g}}{118.71 \text{ g/mol}} \approx 0.0843 \text{ moles} \).

Calculate Volume of Alloy

Using the density of the alloy, calculate its volume:1. Density of the alloy = \( 7.9 \text{ g/cm}^3 \).2. Volume of alloy = \( \frac{100.0 \text{ g}}{7.9 \text{ g/cm}^3} \approx 12.66 \text{ cm}^3 \).

Calculate Molarity of Tin

Molarity is defined as moles of solute per liter of solution. Convert the volume of the alloy from cubic centimeters to liters by noting that 1 L = 1000 cm³.1. Volume in liters = \( \frac{12.66 \text{ cm}^3}{1000} \approx 0.01266 \text{ L} \).2. Molarity of tin = \( \frac{0.0843 \text{ moles}}{0.01266 \text{ L}} \approx 6.66 \text{ M} \).

Suggest a Reaction to Remove Tin

To remove tin from the bronze, one could react the alloy with hydrochloric acid (HCl) to produce tin chloride (SnCl₂), which is soluble. The reaction is:\[ \text{Sn} + 2\text{HCl} \rightarrow \text{SnCl}_2 + \text{H}_2 \]This reaction would dissolve the tin while leaving the copper intact, as copper reacts minimally with HCl under these conditions.

Key concepts

Solvent and Solute

In chemistry, the term 'solvent' refers to the substance in a solution present in the larger amount, while 'solute' is the substance present in the smaller amount. In the context of alloys, which are solid solutions of metals, identifying the solvent and solute follows the same principle. For instance, in a bronze alloy composed of copper and tin, the metal with the greater mass percentage acts as the solvent.
In this example, the bronze consists of 90% copper and 10% tin. Due to its higher mass percentage, copper is the solvent, and tin, making up the lesser share, is the solute.
Understanding this distinction is essential in grasping how alloys function, as it defines the primary substance (solvent) which generally maintains the alloy's overall structure, while the solute contributes to its unique characteristics.

Molarity Calculation

Molarity is a term used to describe the concentration of a solute in a solution. It is defined as the number of moles of solute per liter of solution. Calculating molarity helps in understanding how concentrated a solution is, which is critical for various chemical calculations and reactions.

• First, calculate the amount of the solute tin in grams. Since it makes up 10% of a 100 g bronze sample, there are 10 grams of tin.
• The molar mass of tin is approximately 118.71 g/mol, so the moles of tin in the alloy are: \[\frac{10 \, \text{g}}{118.71 \, \text{g/mol}} \approx 0.0843 \, \text{moles}\]
• Next, calculate the volume of the bronze alloy using its density of 7.9 g/cm³. For a 100 g sample, its volume is: \[\frac{100 \, \text{g}}{7.9 \, \text{g/cm}^3} \approx 12.66 \, \text{cm}^3\]

Convert the volume from cm³ to liters (since 1 L = 1000 cm³):
\[0.01266 \, \text{L}\]
Finally, calculate the molarity of tin: \[\frac{0.0843 \, \text{moles}}{0.01266 \, \text{L}} \approx 6.66 \, \text{M}\]
This means the concentration of tin in the bronze alloy is approximately 6.66 M.

Chemical Reactions to Remove Metal

In metallurgical processes, separating metals from an alloy involves chemical reactions that target specific metals based on their reactivity. To remove a particular metal from an alloy, such as tin from bronze, you can use a reaction that preferentially reacts with tin while leaving copper unaffected.
A practical approach is to react tin with hydrochloric acid (HCl). In this reaction:

• Hydrochloric acid reacts with tin to form soluble tin chloride (SnCl₂).
• The chemical equation is: \[\text{Sn} + 2\text{HCl} \rightarrow \text{SnCl}_2 + \text{H}_2\]
• This process dissolves tin, facilitating its removal from the alloy.

Crucially, copper does not react significantly with HCl, which means that the reaction does not affect the copper in the bronze. This selective reactivity allows for the extraction of tin, leaving behind a purer copper mass.