# 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

- Dr WLv 72 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?

- Roger the MoleLv 72 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.

- Anonymous2 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.

- billrussell42Lv 72 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

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- KennyBLv 72 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.