I have 2 problems in chemistry that I cannot figure out.

Question

We started a new section in Chemistry and it doesn't make any sense to me, I've tried them both over 4 times and still can't figure them out.

1. Problem one- A 0.825 g sample of sulfur hexafluoride gas at 15^oC occupies a volume of 0.605 L, what is the pressure in torr?

1. Problem two- What is the difference in volume between A) 6.729 g of carbon monoxide at 320 K and B) the same mass of carbon dioxide at 679 K if they are both at 90.3 kPa?

Answer 1

These aren't that difficult, if you keep a few things in mind. Always use Kelvin as the units of temperature. Be sure to use the correct gas constant with the correct units. Know and understand the relationship between volume, pressure and temperature (and of course, moles).

So, looking at problem 1. We have 0.825 g of SF6 at 15ºC (288K) and it occupies 0.605 L. We are asked to find the pressure (P) in units of torr. So, we use the ideal gas law PV = nRT and solve for P but will then convert from atmospheres to torr, because the R (gas constant) that I'll use is in units of atmospheres.

molar mass SF6 = 146 g

moles (n) = 0.825 g x 1 mol/146 g = 0.00565 moles

P = ?

V = 0.605 L

R = 0.0821 Latm/Kmol (note the units)

T = 288K

P = nRT/V = (0.00565 mol)(0.0821 Latm/Kmol)(288K) / 0.605 L

P = 0.134 atm

Converting to torr, we have 0.134 atm x 760 torr/atm = 102 torr

Problem 2:

moles CO = 6.729 g CO x 1 mol CO/28 g = 0.2403 moles CO

moles CO2 = 6.729 g CO2 x 1 mol CO2/44 g = 0.1529 moles CO2

Since the pressure and temperature are constant, the volume will vary directly with the number of moles. So, the volume of CO will be 1.57 times that the volume of the CO2. If you want the actual values, we can use the ideal gas law:

PV = nRT

V = nRT/P

CO: V = (0.2403)(8.314)(320) / 90.3 = 7.08 L (note R is LkPa/Kmol; this is different than R = 0,0821 Latm/Kmol)

CO2: V = (0.1529)(8.314)(320) / 90.3 = 4.50 L

Difference is 2.58 L