The number of moles in 250 l of he gas at stp is mol, round to a whole number,

The number of moles in 250.0 L of He at STP is 11.0 mole.

Explanation:

It is known that 1.0 mole of a gas at STP conditions will occupy 22.7 L.To show this information: STP means that T = 0.0 °C = 273.15 K and P = 1.0 kPa =  (100/101.325) = 0.9869 atm.From the ideal gas law: PV = nRT.Where, P is the pressure in atm (P = 1.0 atm at STP).V is the volume in L.n is the number of moles (n = 1.0 mole).R is the general gas constant (R = 0.0821 L.atm/mol.K).T is the temperature in K (T = 273.15 K at STP).and now we can get the volume of 1.0 mole at STP: V = nRT/PV = (1.0 mole x 0.0821 L.atm/mol.K x 273.15 K) / (0.9869 atm) = 22.7 L.Now, we can get the number of moles of 250.0 L of He at STP:

Using cross multiplication:

1.0 mole → 22.7 L

??? mole → 250.0 L

The number of moles in 250.0 L of He at STP = (250.0 L x 1.0 mole) / (22.7 L) = 11.01 mole ≅ 11.0 mole.