Question

An old coin found in an ancient temple is composed of zinc coated with copper. In an experiment to find the percent zinc in the coin, a student determined the weight of the coin to be 3.0 g. Then the student made several scratches in the copper coating (to expose the underlying zinc) and put the scratched coin in hydrochloric acid, where the following reaction occurred between the zinc and HCl (copper remained undissolved):

Zn(s) + 2 HCl (aq) → H2(g) + ZnCl2 (aq)
The student collected the hydrogen produced over water at 27 °C. The collected gas occupied a volume of 1 L at a total pressure of 1.02 bar. The percent zinc in the coin, the student found would be. (Assume that all the Zn in the coin dissolves.)

( a) 67.7 %

(b) 98.2 %

(c) 96.7 %

(d) 88.3 %

(e) 25.0 %

Answer 1

The percent zinc in the coin can be calculated using the stoichiometry of the reaction between zinc and hydrochloric acid, which gives off hydrogen gas, and applying the ideal gas law to find the number of moles of hydrogen produced.

Step-by-step explanation:

To calculate the percent zinc in the coin, we need to use the reaction between zinc and hydrochloric acid (HCl) which produces hydrogen gas (H2) and zinc chloride (ZnCl2). The reaction equation is: Zn(s) + 2 HCl(aq) → H2(g) + ZnCl2(aq).

From the ideal gas law PV = nRT, where P is pressure, V is volume, n is the number of moles, R is the universal gas constant, and T is temperature, we can calculate the number of moles of hydrogen gas. Using the given information (1 L of H2 at 1.02 bar or 1.02 x 10^5 Pa and 27 °C or 300.15 K), and assuming that the vapor pressure of water at 27 °C is negligible, we have: n = PV / RT. After converting the temperature into Kelvin and pressure into pascals, we can calculate n.

Next, we use the stoichiometry of the reaction where 1 mole of Zn releases 1 mole of H2. This allows us to find the mass of the zinc that reacted from the number of moles of hydrogen produced. Since the atomic mass of zinc is 65.4 g/mol, we can then calculate the mass of zinc.

Finally, we find the percent zinc in the coin by the mass of zinc divided by the total mass of the coin and multiply by 100%: (mass of Zn / mass of coin) × 100%. We use this method to determine which option (a through e) correctly represents the percent zinc in the coin.