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### Topic: Help calculating mass of Zinc in an alloy by how much H2 it produces with HCL  (Read 8662 times)

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#### _rob

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##### Help calculating mass of Zinc in an alloy by how much H2 it produces with HCL
« on: November 06, 2010, 05:11:20 PM »
The question asks to calculate the percent (does not specify whether it wants % by mass or % by moles which i find ambiguous) of Zinc in a Zinc-Copper alloy with mass 6.1g, when reacting with excess HCL. The volume of Hydrogen gas produced is 1.26mL at 22C and pressure 728mmHg.

I have a rough idea of the answer, but seems like a very very small percent to be a real alloy, so it's making me question it.

I would firstly go about calculating the moles of H2 produced using Pv=nRT, which i get to be roughly 5x10-5 moles which makes sense.

Secondly I would say that 1 mole of Zinc reacts to make one mole of H2 gas

Zn + 2HCL ----->ZnCl2 + H2

so moles of Zinc reacted = roughly 5 x10-5

I would then use mass = moles x MM to get the mass of Zinc as 3.26x10-3g, which is obvously going to be a very small percentage of Zinc in a 6.1g alloy sample!

Am i doing something stupid or is this just a very low % Zinc alloy?!

Thank you for any help guys...

Rob

#### Borek

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##### Re: Help calculating mass of Zinc in an alloy by how much H2 it produces with HCL
« Reply #1 on: November 06, 2010, 05:53:37 PM »
You can be sure it asks for mass % w/w.

3.26e-3g Zn is correct for a given data. Low, but correct.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

#### _rob

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##### Re: Help calculating mass of Zinc in an alloy by how much H2 it produces with HCL
« Reply #2 on: November 06, 2010, 06:05:30 PM »
Thank you very much