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Chapter 5: Problem 148

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?

Short Answer

Expert verified
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 by step solution

01

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. \(moles\, of \,ZnCl_2 = \frac{mass\, of\, ZnCl_2}{molar\, mass\, of\, ZnCl_2} = \frac{0.0985\, g}{136.28\,g/mol} = 7.22 \times 10^{-4}\, mol\)
02

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
03

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
04

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?#
05

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.
06

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.

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