Chem Problem: Calculating Zinc Percentage in Alloy from Hydrogen Gas Volume

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Discussion Overview

The discussion revolves around a chemistry problem involving the calculation of the percentage of zinc in an aluminum-zinc alloy based on the volume of hydrogen gas produced during a reaction with hydrochloric acid. The scope includes theoretical reasoning and mathematical calculations related to stoichiometry and gas laws.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Technical explanation

Main Points Raised

  • Post 1 presents the problem statement and outlines the reaction equations for aluminum and zinc with hydrochloric acid, along with the initial calculations for the number of moles of hydrogen gas produced using the ideal gas law.
  • Post 2 suggests a method to express the masses of aluminum and zinc in terms of a variable, leading to a system of equations that can be solved for the mass of zinc.
  • Post 3 reiterates the approach from Post 2, confirming the relationship between the masses and moles of the metals in the alloy but seeks clarification on how to relate the moles of zinc to the moles of hydrogen gas produced.
  • Post 4 inquires about the stoichiometric relationships between the moles of aluminum and zinc and the moles of hydrogen produced, prompting further exploration of how the total hydrogen produced relates to both metals.

Areas of Agreement / Disagreement

Participants generally agree on the need to establish relationships between the moles of metals and the moles of hydrogen produced, but there is no consensus on the next steps or how to directly connect the calculated moles of hydrogen to the moles of zinc.

Contextual Notes

Participants have not yet resolved how to utilize the moles of hydrogen gas to find the moles of zinc, indicating potential gaps in the application of stoichiometric relationships and the ideal gas law in this context.

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Homework Statement




0.2500 g sample of an Aℓ-Zn alloy reacts with HCℓ to form hydrogen gas:
Aℓ(s) + 3H+(aq) → Aℓ3+(aq) + 3/2H2(g)
Zn(s) + 2H+(aq) → Zn2+(aq) + H2(g)
The hydrogen produced has a volume of 0.147 L at 25°C and 755 mm Hg. What is the
percentage of zinc in the alloy?

Homework Equations



PV=NRT



The Attempt at a Solution




Frist I start out finding the number of moles of Hydrogen gas produced by using PV=NRT
N=((.993atm)(.147L))/((.0821)(298K))= .00596 mol of H2
1mol of Al = 26.98g
1mol of Zn = 65.93g
now what should I do?
Help please.
 
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Well, you know that the sum of the masses of aluminum and zinc is 0.250 g. Let x = mass of aluminum in the alloy and y = mass of zinc in the alloy.

x+y=0.250

but

y = 0.250 - x, so the mass of zinc in the alloy is 0.250 g - x

You also know the number of moles of aluminum in the sample of the alloy is x/26.98 and the number of moles of zinc is (0.250 - x)/65.93. Can you take it from there?
 
chemisttree said:
Well, you know that the sum of the masses of aluminum and zinc is 0.250 g. Let x = mass of aluminum in the alloy and y = mass of zinc in the alloy.

x+y=0.250

but

y = 0.250 - x, so the mass of zinc in the alloy is 0.250 g - x

You also know the number of moles of aluminum in the sample of the alloy is x/26.98 and the number of moles of zinc is (0.250 - x)/65.93. Can you take it from there?

I know about that but how do you calculate the mol of Zn by using .00596mol of H2 I found in the gas law.
 
Do you know of a relationship between the number of moles of Al and the number of moles of H2 produced from it's treatment with HCl? Do you know of a relationship between the number of moles of Zn and the number of moles of H2 produced from it's treatment with HCl? If you add the hydrogen produced from Al to the hydrogen produced from Zn how much will you have?
 

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