Problem 148 - Chemistry An Atoms First Approach

Question

Commercial brass, an alloy of Zn and Cu, reacts with hydrochloric acid as follows: $$\mathrm{Zn}(s)+2 \mathrm{HCl}(a q) \longrightarrow \mathrm{ZnCl}_{2}(a q)+\mathrm{H}_{2}(g)$$ (Cu does not react with HCl.) When 0.5065 g of a certain brass alloy is reacted with excess \(\mathrm{HCl}, 0.0985 \mathrm{g}\) \(\mathrm{ZnCl}_{2}\) is eventually isolated. a. What is the composition of the brass by mass? b. How could this result be checked without changing the above procedure?

Step-by-Step Solution

Answer

The composition of the brass alloy is 9.31% Zn and 90.69% Cu by mass. To check the result without changing the procedure, ensure that there was enough HCl reacted and compare the yield of ZnCl2 produced in this reaction with the theoretical yield given the mass of the alloy and its composition.

Step 1: 1. Calculate the moles of ZnCl2 produced

We will use the given mass of ZnCl2 isolated to find the moles of ZnCl2 produced by using its molar mass. The molar mass of ZnCl2 is 65.38 g/mol (Zn) + 2 x 35.45 g/mol (Cl) = 136.28 g/mol. \(moles\, of \,ZnCl_2 = \frac{mass\, of\, ZnCl_2}{molar\, mass\, of\, ZnCl_2} = \frac{0.0985\, g}{136.28\,g/mol} = 7.22 \times 10^{-4}\, mol\)

Step 2: 2. Calculate the moles of Zn in the alloy

From the given balanced chemical equation, 1 mole of Zn reacts with 1 mole of ZnCl2 produced. Therefore, the moles of Zn present in the alloy are equal to the moles of ZnCl2 produced. moles of Zn = moles of ZnCl2 = 7.22 x 10^{-4} mol

Step 3: 3. Calculate the mass of Zn in the alloy

Now, we will find the mass of Zn present in the alloy using the moles of Zn and its molar mass. The molar mass of Zn is 65.38 g/mol. mass of Zn = moles of Zn x molar mass of Zn = (7.22 x 10^{-4} mol) x (65.38 g/mol) = 0.0472 g

Step 4: 4. Calculate the mass percentage of Zn in the brass alloy

Using the mass of Zn in the alloy calculated above, we can now find the mass percentage of Zn in the brass alloy. mass percentage of Zn = \(\frac{mass\, of \,Zn}{mass\, of\, alloy} \times 100\) = \(\frac{0.0472 \,g}{0.5065\, g} \times 100\% = 9.31\%\) Since Zn and Cu are the only elements in the brass alloy, the remaining mass percentage comes from Cu, so mass percentage of Cu = 100 - mass percentage of Zn mass percentage of Cu = 100 - 9.31 = 90.69% So, the composition of the brass alloy is 9.31% Zn and 90.69% Cu by mass. #b. How could this result be checked without changing the above procedure?#

Step 5: 1. Determine the amount of excess reactant

One way to check the result without changing the procedure is making sure the hydrochloric acid (HCl) was actually in excess. Ensure that there was enough HCl reacted so that the Zn in the brass alloy was all consumed.

Step 6: 2. Check the yield of ZnCl2 produced

Another way would be to compare the yield of ZnCl2 produced in this reaction with the theoretical yield given the mass of the alloy and its composition. Calculate the theoretical yield of ZnCl2 based on the given mass of the alloy and its composition and compare it to the actual yield obtained. This would give an indication of the reaction's efficiency.

Key Concepts

Understanding Zinc in Brass

Zinc is a crucial component in the creation of brass, an alloy primarily composed of copper and zinc. Brass is celebrated for its diverse applications, from musical instruments to decorative items, because zinc contributes to its strength and ductility. By reacting with hydrochloric acid, zinc forms zinc chloride and hydrogen gas. This experiment helps determine the amount of zinc present by measuring the zinc chloride produced.

In the exercise, when the brass alloy was treated with hydrochloric acid, zinc reacted fully while copper remained unreacted. This selective reactivity allows for the calculation of zinc's mass using the equation:

• The amount of ZnCl₂ produced
• The stoichiometry from the balanced chemical equation

From this, you can determine the percentage of zinc in the alloy.

Identifying Copper in Brass

Copper is the primary constituent of brass, providing its attractive appearance and corrosion resistance. Unlike zinc, copper does not react with hydrochloric acid, a fact leveraged in analyzing brass composition.

In the exercise, after determining the zinc content, the remaining portion of the alloy is assumed to be copper. This simplification arises because copper is the other significant metal in brass. Thus, the mass percentage of copper can be found by subtracting the zinc percentage from 100%.

This approach is useful because:

• It assumes no other metal impurities are present.
• Relies on the absence of copper reaction with HCl.

Alloy Composition Analysis Technique

Alloy composition analysis involves techniques to determine the proportion of elements within an alloy. For brass, this is usually done by chemical reactions, as we have seen with the zinc and hydrochloric acid method.

By calculating the actual yield of zinc chloride and comparing it with theoretical predictions, the efficiency of the reaction process can be assessed. This includes ensuring hydrochloric acid is in excess to guarantee complete zinc reaction, thus providing accurate results.

Analyzing the alloy composition helps in verifying:

• The purity of the alloy
• The accuracy of elemental analysis methods
• The effectiveness of the reaction process

Understanding these fundamentals is crucial for chemical engineers and materials scientists who work with alloys like brass.