The balanced chemical equation below represents the formation of magnesium chloride. Mg(s) + 2 HCl(aq) → MgCl2(aq) + H2(g)

At STP, what is the total number of liters of hydrogen gas produced when 3.00 moles of hydrochloric acid solution-HCl(aq) is completely consumed?
A.22.4 L

B.11.2 L

C.33.6 L

D.44.8

C. 33.6L

Explanation:

Based on the reaction, 2 moles of HCl reacts producing 1 mole of hydrogen.

To solve this question we must find the moles of hydrogen produced using the reaction. Then, with combined gas law (PV = nRT) we can find the volume produced:

Moles H2:

3.00 moles HCl * (1mol H2 / 2mol HCl) = 1.50 moles H2 are produced

Volume:

PV = nRT

V = nRT / P

Where P is pressure = 1atm at STP

V is volume = Our incognite

n are moles of the gas = 1.50 moles

R is gas constant = 0.082atmL/molK

T is absolute temperature = 273.15K

V = nRT / P

V = 1.50mol*0.082atmL/molK*273.15K / 1atm

V = 33.6L

n2(g) + 3h2-> 2nh3(g)   this is the balanced equation

note the mole ratio between n2, h2 and nh3.   it is 1 : 3 : 2   this will be important.

moles n2 present = 28.0 g n2 x 1 mole n2/28 g = 1 mole n2 present

moles h2 present = 25.0 g h2 x 1 mole h2/2 g = 12.5 moles h2 present

based on mole ratio, n2 is limiting in this situation because there is more than enough h2 but not enough n2.

moles nh3 that can be produced = 1 mole n2 x 2 moles nh3/mole n2 = 2 moles nh3 can be produced

grams of nh3 that can be produced = 2 moles nh3 x 17 g/mole = 34 grams of nh3 can be produced

note:   the key to this problem is recognizing that n2 is limiting, and therefore limits how much nh3 can be produced.