Given 2HCl + Cr ---> CrCl2 + H2, determine the molarity of HCl used if 500 mL produces 32.5 L of hydrogen gas. Assume STP.



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Posted on (Answer #1)

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

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