please help me with gas law problems.?

Question

#1.
A student adds 4.00 g of dry ice (solid CO2) to an empty balloon. What will be the volume of the balloon at STP after all the dry ice sublimes (converts to gaseous CO2)?
#2.
As(s) + O2(g) ----> As2O5(s)
Consider the above unbalanced equation. What mass of arsenic is required to react with 705 mL of O2 at 465 mm Hg and 35.5°C? Use molar masses with at least as many significant digits as the data given.
#3.
C6H6(l) + O2(g)-----> CO2(g) + H2O(g)
Consider the above unbalanced equation. What volume of O2 at 940. mm Hg and 93.5°C is required to produce 125 mL of CO2 at 910. mm Hg and 44.5°C?
#4.
The rate of diffusion of methane gas, CH4, is four times the rate of diffusion of an unknown gas under the same conditions of temperature and pressure. What is the molar mass of the unknown gas? Enter your answer to 3 significant figures.
#5.
A student adds 4.00 g of dry ice (solid CO2) to an empty balloon. What will be the volume of the balloon at STP after all the dry ice sublimes?

Answer 1

These are all variations on the ideal gas law (PV=nRT)
P-Pressure
V-Volume
n-moles
R-Gas Constant (units matching P & V)
T - Temperature (in Kelvin)

#1 & #5
4g / 44 g/mol = 1/11 mol
STP = Standard temperature and pressure (273.15K & 1 atm)

PV=nRT (1 atm)(V)=(1/11 mol)(0.0821 atm-L/K)(273.15 K)
V=2.039 L

I leave the rest to you.

Answer 2

The easy way is to remember that 1 mole of an ideal gas occupies 22.4 L. Since you have 0.25 L, the number of moles must be 0.25 L / 22.4 L/mole = 0.01116 moles. The mass of the gas is 0.190 g so the molar mass (g/mole) = 0.190 g / 0.01116 moles = 17.03 g/mole, same as you. I wonder if the 'element' is incorrect? Ammonia has a MW of 17.03 g/mole