The lowest pressure attainable using the best available vacuum techniques is about 10−12 N/m2. Part A At such a pressure, how many molecules are there per cm3 at 2 ∘C?

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We are given   P = 10⁻²² N/m2 ,

Volume, V = 1 cm3= 10⁻⁶ m³ and

T= 13ºC= 13+273= 286 K

Using gas law,  P*V = n R T,  where n is number of moles and R=8.314JK⁻¹.

n = PV/(RT) =>  n = 10-12*10⁻⁶/ (8.314*286) = 4.205*10₋²² moles

Hence number of molecules = number of moles * Avogadro's number

= 4.205*10-22 moles * 6.023*1023 molecules/mole

= 253

Number of molecules = 250 (upto 2 significant figures)

Number of molecules: N = 263.31

Further explanation

Some of the laws regarding gas can apply to ideal gas (volume expansion does not occur when the gas is heated),:

  • Boyle's law at constant T,

[tex] \displaystyle P = \dfrac {1} {V} [/tex]

  • Charles's law, at constant P,

[tex] \displaystyle V = T [/tex]

  • Avogadro's law, at constant P and T,

[tex] \displaystyle V = n [/tex]

So that the three laws can be combined into a single gas equation, the ideal gas equation

In general, the gas equation can be written

[tex] \large {\boxed {\bold {PV = nRT}}} [/tex]

where

P = pressure, atm , N/m²

V = volume, liter

n = number of moles

R = gas constant = 0.082 l.atm / mol K (P= atm, v= liter),or 8,314 J/mol K (P=Pa or N/m2, v= m³)

T = temperature, Kelvin

n = N / No

n = mole

No = Avogadro number (6.02.10²³)

n = m / m

m = mass

M = relative molecular mass

Known

P = 10−12 N / m2

V = 1 cm3 = 10-6 m3

T = 2 ºC = 2 + 273 = 275 K

R = 8,314 J / mol. K

[tex]\rm n=\dfrac{PV}{RT}\\\\n=\dfrac{10^{-12}\times 10^{-6}}{8.314\times 275}\\\\n=\boxed{\bold{4.374\times 10^{-22}}}[/tex]

then the number of molecules (N):

N = n x No

N = 4,374.10⁻²² x 6.02.10²³

N = 263.31

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