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!

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.

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

912 mm Hg to standard atmosphere

To find the volume of hydrogen gas, we can use the ideal gas law, which states that PV = nRT, where P is the pressure, V is the volume, n is the number of moles, R is the ideal gas constant, and T is the temperature in Kelvin.

First, we need to find the number of moles of hydrogen gas. We can use the balanced equation to determine the stoichiometry of the reaction.

From the balanced equation, we see that 6 moles of hydrochloric acid react to produce 3 moles of hydrogen gas.

0.45 moles of hydrochloric acid is in 238 mL of the solution. To find the number of moles, we can use the formula:

moles = concentration x volume
moles = 0.45 M x 0.238 L
moles = 0.1071 moles

Since the stoichiometry of the reaction is 6HCl : 3H2, we multiply the number of moles of hydrochloric acid by 3/6 to find the number of moles of hydrogen gas:

moles of H2 = 0.1071 moles x (3/6)
moles of H2 = 0.0536 moles

Now, we can use the ideal gas law to find the volume of hydrogen gas:

PV = nRT

We know the pressure (0.912 atm), the number of moles (0.0536 moles), the ideal gas constant (0.0821 L•atm/mol•K), and the temperature (35°C = 308.15 K).

Plugging these values into the equation:

V = (nRT) / P
V = (0.0536 moles x 0.0821 L•atm/mol•K x 308.15 K) / 0.912 atm
V = 1.28 L

Therefore, the volume of hydrogen gas collected over water at 35°C and 735 torr will be 1.28 liters.