# Chemistry: Gases?

I am completely confused with my chemistry chapter on gases and Laws. Can someone please explain how to figure out the answer to this question?

A 141 mL flask is evacuated, and its mass is measured as 192.227 g. When the flask is filled with 793 torr of helium gas at 25 ∘C, it is found to have a mass of 192.286 g. Is the gas pure Helium?

### 5 Answers

Relevance
• Dr W
Lv 7
2 months ago

helium has a molar mass = 4.00g/mol

convert the mass, P, T, and V given to molar mass.  If it's not 4, it's not helium

**********

starting with these 2 equations

.. PV = nRT.. . . .. . .ideal gas law

.. n = mass / mw... .  .definition of molar mass

subbing and rearranging

.. PV = (mass / mw) RT

.. mw = mass * RT/(PV)

solving

.. .. ... ... ... .. ... ... ... ... ... .. .. .... 0.08206 Latm/molK * 298.15K

mw = (192.286g - 192.227g) * ----- ----- ---- ---- ---- ---- ---- ----- ---- = 9.8 g/mol

.. ... ... .. ... ... ... .. .. ... ... ... ... 793torr * (1 atm / 760 torr) * (0.141L)

not helium!

**********

BillRussell42..He2 molecule?  You wanna rethink that one?

• 2 months ago

n = PV / RT = (793 torr) x (0.141 L) / ((62.36367 L torr/K mol) x (25 + 273) K) =

0.006016 mol

(192.286 g - 192.227 g) / (0.006016 mol) = 9.8 g/mol

The molar mass of He is 4.00 g/mol.  The calculated molar mass is a long way from that, so the helium is not pure, but is contaminated by some heavier gas or gases.

• Anonymous
2 months ago

"the He molecule is 8.006 g/mol"

There is no such thing as the He molecule. This is the correct calculation:

0.059 g / 4.003 g/mol = 0.014739 mol

Use PV = nRT

(793 torr/760 torr/atm) (0.141 L) = (n) (0.08206 L atm / mol K) (298 K)

n = 0.00601632 mol

The conclusion is that the gas is not pure helium. Same conclusion as billrussell42, just with the correct numbers.

• 2 months ago

see if the Ideal gas law applies

mass of gas = 192.286 g – 192.227 g = 0.059 g

Helium is 4.003 g/mol, and the He molecule is 8.006 g/mol

0.059 g / 8.006 g/mol = 0.00737 mol

PV = nRT

793•0.141 = 0.00737•62.36(273.16+25)

112 = 137

NO, error is 20%

if the other numbers are accurate, it should match within 2/59 or 3% worse case.

Ideal gas lawPV = nRTn = number of molesR = gas constant = 0.08206 (atm∙L)/(mol∙K)                   8.314 L∙kPa/mol∙K                   0.08314 L∙bar/mol∙K                   62.36 L∙torr/mol∙K                   62.36 L∙mmHg/mol∙KT = temperature in kelvinsP = absolute pressure in atm, kPa, bar, torrV = volume in liters

• How do you think about the answers? You can sign in to vote the answer.
• KennyB
Lv 7
2 months ago

First, calculate moles from the gas law

PV = nRT  becomes  n = PV/RT

(Remember to put the temperature in degrees K)

Now how much would that many moles of He weigh?  Compare that to the actual weight.

Still have questions? Get your answers by asking now.