What is the molecular weight of a gas if a 15.0g sample has a pressure of 836 mm Hg at 25.0 degrees C in a 2.00 L flask?
a)167
b)1.35
c)176
d)11.1
e)none of the above
Please explain... I need to understand how to do this. Thanks.
I know that I have to use PV=nRT, then solve for n grams/molar mass
And, I know that:
P=1.1 atm
V=2.00 L
n=?
R= I think .0821
and
T= ?
I have no idea how to change 15.0g to moles, since I do not know what the element is to look at the atomic mass--I know that one mole is avagadro's number, but I'm still really not sure what to do here.
Please help...
Yes this is correct approach
Use PV=nRT
if you are using R=0.082 057 46 then the units are L atm K^−1 mol^−1, so you need to use P in atm which you have done. The tempaeratue needs to be in K rather than deg C.
once you have found n (number of moles)
then the molecular mass (RMM) is found from
n=mass/RMM
n you have calculated above and mass is given in the question.
OK... so I found the temp to be 298, and the moles (n) = .0089921441, but now you are saying to find the molecular mass...what is RMM? and where do I go from here?
Sorry, MM is the molecular mass or 'molecular weight'. Molecular weight tends not to be used as it is a rather old fashioned term. Weight has units of newtons which can be confusing.
In the above if you divide 1.5 g by your figure of 0.0089921441 mole (you forgot the leading zero) you will get the molecular mass (g mol^-1).
To find the molecular weight of the gas, you can use the Ideal Gas Law equation:
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, let's convert the given temperature of 25.0 degrees Celsius to Kelvin. You can do this by adding 273.15 to the Celsius value:
T = 25.0 + 273.15 = 298.15 K
Now, let's rearrange the Ideal Gas Law equation to solve for n:
n = PV / RT
Substitute the given values into the equation:
P = 836 mm Hg (convert to atm by dividing by 760 mm Hg/atm)
P = 836 mm Hg / 760 mm Hg/atm ≈ 1.1 atm
V = 2.00 L
R = 0.0821 L·atm/(mol·K)
T = 298.15 K
Now you have all the values needed to calculate n:
n = (1.1 atm) * (2.00 L) / (0.0821 L·atm/(mol·K) * 298.15 K
n ≈ 0.089 mol
Next, you need to determine the molar mass of the gas. The molar mass is the mass of one mole of the substance. Given that you have a 15.0 g sample, you can use the formula:
molar mass = mass / moles
molar mass = 15.0 g / 0.089 mol
molar mass ≈ 168 g/mol
So, the molecular weight of the gas is approximately 168 g/mol.
Comparing this to the answer choices provided, none of the options given match the calculated value. Therefore, the correct answer is "e) none of the above."