Question

How many milligrams of magnesium metal must react with excess HCI to produce 50.50 mL of H2 (g), measured at 25 degrees Celsius and 775 torr? Show all calculations in this problem.

Mg (s) + 2 HCl (aq) --> MgCl2 (aq) + H2 (g)

Answer 1

First of all we calculate number of moles of H₂ gas

We know that PV = nRT

Where

T = Temperature = 25+273 = 298 K

P = pressure = 775 torr = 775x0.0013 atm = 1.0075 atm                Since 1 torr = 0.0013 atm

n = No . of moles = ?

R = gas constant = 0.0821 L atm mol ^-1 K^-1

V= Volume of the gas=50.50 mL = 0.0505 L                   Since 1 L = 1000 mL

Plug the values we get

                               

Mg (s) + 2 HCl (aq) MgCl₂ (aq) + H₂ (g)

according to the balanced equation ,

1 mole of Magnesium produces 1 mole of H₂ gas

0.0021 mole of Magnesium produces 0.0021 mole of H₂ gas

So mass of Mg produced, m = No.of moles x molar mass

                                         = 0.0021 mol x 24.3 g mol ^-1

                                         = 0.5103 g

                                         = 510.3 mg                         Since 1 g = 1000 mg