Question

In an experiment, 238 mL of 0.45 molar hydrochloric acid completely reacts with excess iron to produce hydrogen gas and iron (III) chloride. What volume will the hydrogen gas occupy if it is collected over water at 35 degrees Celsius and 735 torr? The vapor pressure of water at 35 degrees Celsius is 42 mm Hg.

My work so far:
6HCl(aq) + 2Fe(s) ---> 3H2(g) + 2FeCl3(s)

Total pressure= Pressure of H2 + Pressure of H20
Solve for Pressure of H2:
Pressure of H2=Total pressure - Pressure of H20

Conversions:
Total Pressure= 735 torr X (1 atm/760 torr)=0.967 atm

Pressure of the water= 42mm Hg X (1atm/760 torr)=0.055atm

So...Pressure of H2= 0.967atm-0.055atm=0.912atm

I'm lost. I have no idea where to go next from here. Any help is greatly appreciated!

Answer 1

You've done the hard part.
How much HCl did you start with? M x L = ??
Now convert moles HCl to moles H2 gas using the coefficients in the balanced equation.
moles HCl x (x moles H2/y moles HCl) = zz
Then substitute for n in PV = nRT and solve for volume. Your calculation for P is correct although I think you went the long way around the barn to get there. I would have done this.
P = 735-42 = xx mm
Then xxmm/760 = 0.912 atm.

Answer 2

Thank you so much! I was really stuck and my brain went completely blank.

Answer 3

912 mm Hg to standard atmosphere