Given 2HCl + Cr ---> CrCl2 + H2, determine the molarity of HCl used if 500 mL produces 32.5 L of hydrogen gas. Assume STP.
From the given information and the Ideal Gas Law, we can determine how many moles of hydrogen gas are produced:
n=PV/RT, from STP we know T= 273.15K, P=1.00atm. R=0.0821Latm/molK
n = 1.00atmx32.5L/(0.0821Lx273.15k) = 1.45mol
From the balanced equation we get the relationship that 2mol HCl are required to produce 1mol H2. So
1.45mol H2x(2mol HCl/1 mol H2) = 2.90 mol HCl.
From the definition of molarity, we can determine the concentration of HCl:
M = #mol/L = 2.90mol HCl/ 0.500L = 5.80 mol/L
So, [HCl] = 5.80 M