Question

Given the following chemical reaction:
Zn+HCl-->ZnCl2+H2(g)
If you measured the total volume of collected gas over water at 20 degrees celcius t be 428ml at a total pressure of 845 torr, how many milliliters of 2.6M HCl would be required to produce the correct amount of Hydrogen?

Answer 1

Use PV = nRT to calculate moles H2 released.
First, look up vapor pressure H2O at 20 C. It's approximately 20 mm.
P = 845-vapor pressure H2O @ 20C and convert that number to atmospheres by dividing by 760.
V = 428 mL, convert to L.
n is what you want to know.
R is 0.08206
T is 20 C converted to Kelvin.

Now that you know moles H2, use the coefficients in the balanced equation to convert moles H2 to moles HCl. Finally, M = mols/L. You know moles HCl and M HCl, solve for L and convert to mL.