When active metals such as magnesium are immersed in acid solution, hydrogen gas is evolved. Calculate the volume of H2 (g) at 30.1°C and 0.85 atm that can be formed when 275 mL of 0.725 M HCl solution reacts with excess Mg to give hydrogen gas and aqueous magnesium chloride.

The volume of H₂ (g) obtained is 22.4L

Explanation:

First of all, think the reaction:

2HCl (aq) + Zn (s) → ZnCl₂ (aq) + H₂ (g)

You have to add a 2, in the HCl to get ballanced.

Now we should know how many moles of each reactant, do we have.

Volume. Molarity = moles

Notice that volume is in mL, so I must convert to L.

275 mL = 0.275L

0.275L. 0.725mol/L = 0.2 moles of HCl

Molar mass of Zn: 65.41 g/m

50 g / 65.41 g/m = 0.764 moles

Ratio between reactants is 2:1, so I need the double of moles of HCl to react, and a half moles of Zn to react.

My limiting reactant is the HCl, for 0.764 moles of Zinc, I need 1.528 (0.764.2) of HCl, and I only have 0.2 moles.

Ratio between HCl and H₂ is 1:1, so 0.2 HCl make 0.2 moles of gas

Now apply the Ideal Gas Law, to find out the volume

P. V = n. R. T

2 atm. V = 0.2 mol. 0.08206L atm/K mol. 273K

V = (0.2 mol. 0.08206L atm/K mol. 273K) / 2 atm

V = 2.24 L