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A 140 mL flask contains 2.1 µg of O2 at 12°C.What is the pressure?Answer in units of Torr.

User Heather Turner
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1 Answer

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Step-by-step explanation:

To calculate the pressure of our oxygen gas sample we will have to use the ideal gas law.

P * V = n * R * T

P = n * R * T/V

If we use the ideal gas constant R in atm*L/(mol*K), P will be the pressure in atm, n the number of moles, T the temperature in K and V the volume in L.

R = 0.082 atm*L/(mol*K)

So we will have to convert the volume in mL to L, the temperature in °C to K and the mass into moles.

T = (273.15 + 12) K

T = 285.15 K

1000 mL = 1 L

V = 140 mL * 1 L/(1000 mL)

V = 0.140 L

To convert the mass into moles we will have to convert from µg to grams and then from grams to moles using the molar mass of O₂.

1 g = 1 *10^6 µg

mass of O₂ = 2.1 µg * 1 g/( 1 *10^6 µg)

mass of O₂ = 2.1 * 10^(-6) g

atomic mass of O = 16.00 amu

molar mass of O₂ = 2 * 16.00 g/mol

molar mass of O₂ = 32.00 g/mol

moles of O₂ = 2.1 *10^(-6) g * 1 mol/(32 g)

moles of O₂ = 6.56 * 10^(-8) moles

n = 6.56 * 10^(-8) moles

Now that we know the different values we can replace them in the ideal gas formula and solve it for P.

P = n * R * T/V

P = 6.56 * 10^(-8) mol * 0.082 atm*L/(mol*K)* 285.15 K/(0.140 L)

P = 1.10 *10^(-5) atm

The pressure is 1.10 *10^(-5) atm but we need it in torr, so to answer our problem we have to convert the pressure from atm to torr.

760 torr = 1 atm

P = 1.10 *10^(-5) atm * 760 torr/(1 atm)

P = 0.00836 torr

Answer: the pressure is 0.00836 torr.

User Terry Kernan
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