Question

Commercial brass, an alloy of \(Z n\) and \(\mathrm{Cu}\), 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 \(0.5065 \mathrm{~g}\) 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

The composition of the brass alloy by mass is approximately 9.32% Zn and 90.68% Cu. To check this result without changing the procedure, we could weigh the sample again once the reaction with HCl is complete and the Cu is separated; the residual mass should be close to the mass of Cu calculated in Step 2, which is 0.4593 g.

Step-by-step solution

Determine the mass of zinc in the alloy

Given that the alloy weight of 0.5065 g reacts with excess HCl, we're told that 0.0985 g of ZnCl2 is formed. First, we need to find how much zinc is in the isolated 0.0985 g of ZnCl2. The molar mass of Zn is 65.38 g/mol, and the molar mass for ZnCl2 is 136.30 g/mol (65.38 g/mol + (2 * 35.45 g/mol)). Now, we'll find the amount of Zn in moles within ZnCl2: $$ \text{moles of Zn} = \frac{\text{mass of ZnCl}_2}{\text{molar mass of ZnCl}_2} = \frac{0.0985 \mathrm{~g}}{136.30 \mathrm{~g/mol}} = 0.000722 \mathrm{~mol} $$ Next, we'll find the mass of Zn in the alloy: $$ \text{mass of Zn} = \text{moles of Zn} \times \text{molar mass of Zn} = 0.000722 \mathrm{~mol} \times 65.38 \mathrm{~g/mol} = 0.0472 \mathrm{~g} $$ The mass of Zn in the alloy is 0.0472 g.

Determine the mass of copper in the alloy

Since the alloy consists of only Zn and Cu, we can find the mass of Cu in the sample by subtracting the mass of Zn from the total mass of the alloy. $$ \text{mass of Cu} = \text{mass of alloy} - \text{mass of Zn} = 0.5065 \mathrm{~g} - 0.0472 \mathrm{~g} = 0.4593 \mathrm{~g} $$ The mass of Cu in the alloy is 0.4593 g.

Calculate the mass percentages of Zn and Cu in the alloy

Now, we'll calculate the mass percentages of Zn and Cu in the alloy. For Zn: $$ \text{percentage of Zn} = \frac{\text{mass of Zn}}{\text{mass of alloy}} \times 100\% = \frac{0.0472 \mathrm{~g}}{0.5065 \mathrm{~g}} \times 100\% = 9.32\% $$ For Cu: $$ \text{percentage of Cu} = \frac{\text{mass of Cu}}{\text{mass of alloy}} \times 100\% = \frac{0.4593 \mathrm{~g}}{0.5065 \mathrm{~g}} \times 100\% = 90.68\% $$ The composition of the brass alloy by mass is around 9.32% Zn and 90.68% Cu.

Check the result

To answer part b and check the result without changing the procedure, we could use the following method: Weigh the sample again once the reaction with HCl is complete, ensuring that the Cu and any excess HCl are removed completely. The residual mass should be close to the mass of Cu that was calculated in Step 2, which is 0.4593 g.

Key concepts

Understanding Stoichiometry

Stoichiometry is the backbone of chemical reactions, which involves the calculation of reactants and products in chemical processes. It's a section of chemistry that deals with the relationships between the amounts of substances that are involved in reactions.

• It starts with a balanced chemical equation which provides the ratio of the number of moles of all reactants and products.
• Using this ratio, chemists can predict the amount of product that will form from a given amount of reactant (or vice versa).
• In the case of brass reacting with hydrochloric acid, stoichiometry allows us to determine the mass of zinc that reacts due to the known mass of zinc chloride produced.

In educational exercises, examples like this one help reinforce the principles of stoichiometry and demonstrate its importance in practical applications such as determining the chemical composition of an alloy like brass. Students can improve their understanding of stoichiometry by practicing with a variety of balanced equations and by focusing on the step-by-step progression from reactants to products.

Molar Mass Calculation Explained

In chemistry, the molar mass is a crucial quantity that is used to convert between the mass of a substance and the number of moles. It represents the mass of one mole of a substance (be it an element or compound) and is expressed in grams per mole (g/mol).

• To calculate it, you add up the atomic masses (from the periodic table) of all atoms in the molecule. For zinc chloride, this is the sum of the atomic mass of zinc and twice the atomic mass of chlorine.
• Often molar mass helps in identifying how much of a substance is present in a particular sample.
• Understanding the molar mass concept is essential when performing the stoichiometric calculations, such as determining the mass of zinc in zinc chloride.

The calculation of molar mass is a foundational skill in chemistry which assists in the quantification of chemical reactions and is a fundamental step in exercises that involve stoichiometry.

Mastering Mass Percentage Calculations

Mass percentage is a way of expressing the concentration of an element in a compound or a component in a mixture. It is the mass of a particular substance divided by the total mass of the mixture, multiplied by 100% to give a percentage.

• This concept is routinely used to describe the composition of mixtures and solutions.
• In the case of brass, calculating the mass percentages of zinc and copper gives a clear idea of the alloy's composition.
• For students, understanding how to calculate mass percentage is vital in a wide range of chemistry applications, including determining purity or the concentration of solutions.

Students can practice this skill by solving problems that ask them to find the concentration of various components in different types of mixtures or alloys, as in the provided brass and hydrochloric acid reaction example.