Aluminum reacts with hydrochloric acid, but copper does not react with the acid.

Question

Aluminum reacts with hydrochloric acid, but copper does not react with the acid. When a mixture of aluminum and copper having a mass of 0.3579 grams is combined with excess hydrochloric acid, 315.50 mL of hydrogen gas, measured over water, is generated. The temperature at the time of collection is 27.3 oC (vapor pressure of water at 27.3 oC = 27.214 torr), and the total pressure in the collection container is 764.1 torr. Determine the mass percentage of aluminum in the original mixture.

I've pretty much got my chem exam down for tomorrow, but I really need guidance for this question. Please show work and explain what you did (doesn't have to be so detailed).

Explanation / Answer

Total pressure is 764.1 torr

vapor pressure of water at 27.3 oC = 27.214 torr

Therefore pressure for hydrogen = 764.1 torr - 27.214 torr = 736.886 torr = (736.886 / 760) atm

vilume of hydrogen evolved = 315.50 mL = 315.50 x 10-3 L

T = 27.3 oC = 273 + 27.3 = 300.3 K

Gas onstant R = 0.082 L atm K?1 mol?1

Now PV = nRT where n = number of moles

or n = PV/ RT

therefore number of moles of hydrogen evolved

= (736.886 / 760) atm x 315.50 x 10-3 L / 0.082 L atm K?1 mol?1 x 300.3 K = 12.42 x 10-3 mol

Now the reaction between Al and HCl is

2Al + 6HCl ----> 2AlCl3 + 3H2

So 3 moles of hydrogen is produced from 2 moles of Al

so 12.42 x 10-3 moles of hydrogen is produced from 2 x 12.42 x 10-3 / 3 moles of Al = 8.28 x 10-3 moles of Al

Now 1 mole of Al has mass 26.982 g

so 8.28 x 10-3 moles of Al has mass 26.982 x 8.28 x 10-3 g = 0.2234 g

Therefore mass % of Al in the mixture = (0.2234/ 0.3579) x 100 = 62.24 %

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