A student reacts a 0.050g sample of magnesium in excess hydrochloric acid in ord

Question

A student reacts a 0.050g sample of magnesium in excess hydrochloric acid in order to produce hydrogen gas collected at room temperature (23.2 degree C). 48.6 mL hydrogen gas is collected in an eudiometer tube with water initially at 20.0 degree C. How many moles of magnesium are reacted? Moles of magnesium reacted: From the balanced chemical equation, how many moles of hydrogen gas are created? Moles of hydrogen gas created If the pressure inside the eudiometer tube is equalized to atomspheric pressure (1.00 atm), what is the pressure of ONLY the hydrogen gas in the tube (without water vapor)? Pressure of Hydrogen gas only:

Explanation / Answer

1.

0.050 g/24.3 g/mol = 2.06 x10-3 mol Mg

2.

Mg + 2HCl = MgCl2 + H2

1 mol                            1 mol

2.06 mol H2 are created.

3.

Using the gas law

P = nRT/V

   = 2.06 x10-3 mol x 0.08206 L.atm.K-1.mol-1 x 293K / 0.0486 L

   = 1.017 atm

This result is not good, it must be < 1 atm.

Here the author of the question is wrong. At 1. he rounded the value to 2 sif fig. Now your good results are:

1.   2.0 x10-3 mol Mg

2. 2.0 mol H2are created.

3. P = nRT/V

   = 2.0 x10-3 mol x 0.08206 L.atm.K-1.mol-1 x 293K / 0.0486 L

   = 0.98 atm

Verification: The accepted value for water vapor pressure at 20oC is 0.023 atm.

In your case 1.00 atm – 0.98 atm = 0.02 atm for water vapor pressure in the tube.

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