Gas Laws and Stoichiometry

Question

1. If the volume of H2 gas collected was 47.92 ml at the temperature, 20.0 C and pressure given, then calculate the moles of H2 gas produced.
2. Calculate the mass of Mg metal reacted using stoichiometry and the balanced reaction.
3. Determine the mass of Mg reacted using the unit equation approach involving converting the volume of H2 gas to STP conditions, then to moles H2, and then mass of Mg.

Answer 1

The "given" pressure is not given for this problem. However, regardless of the initial pressure, the first step will be the same. I will assume the pressure is at 2 atm for the sake of this problem.

Step #1

Given: V = 47.92 mL = 0.04792 L; T = 20.0 °C = 298.15 K; P = 2 atm; R = 0.08206 (atm•L/(mol•K))

PV = nRT

(2 atm)(0.04792 L) = n(0.08206 (atm•L/(mol•K)))(298.15 K)

n = ((2 atm)(0.04792 L))/((0.08206 (atm•L/(mol•K)))(298.15 K))

n = 3.917242689•10^-3 mol

Steps #2 and #3 require more information than you've given to solve properly.