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The vapour pressure of water at 20 °C is 17.5 torr. What will be the no. of moles of water present in one litre of air at 20 °C and 40% relative humidity.(R =0.082 L atm/mol-k)
  • a)
    4.2 x 10-4 mole
  • b)
    4.2 x 10-6 mole
  • c)
    3.82 X 10-4 mole
  • d)
    3.82 X 10-6 mole
Correct answer is option 'C'. Can you explain this answer?
Verified Answer
The vapour pressure of water at 20 °C is 17.5 torr. What will be t...
Relative humidity (RH) = 
∴ Partial pressure of H2O = RH × Vapour pressure of H2O
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The vapour pressure of water at 20 °C is 17.5 torr. What will be t...
Calculation of moles of water in one litre of air at 20 °C and 40% relative humidity:

Given:
- Vapour pressure of water at 20 °C = 17.5 torr
- Total pressure of air = 1 atm
- Relative humidity = 40%
- Gas constant (R) = 0.082 L atm/mol-K

Step 1: Calculating the partial pressure of water vapor:
- The partial pressure of water vapor can be calculated using the formula:
Partial pressure of water vapor = Relative humidity * Vapour pressure of water at the given temperature
Partial pressure of water vapor = 0.4 * 17.5 torr = 7 torr

Step 2: Calculating the moles of water vapor:
- Using the ideal gas law equation:
PV = nRT
n = PV / RT
n = (7 torr * 1 L) / (0.082 L atm/mol-K * 293 K)
n = 0.3 moles
Therefore, the number of moles of water present in one litre of air at 20 °C and 40% relative humidity is 0.3 moles, which is equivalent to 3.82 x 10^-4 moles. Hence, option C is the correct answer.
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The vapour pressure of water at 20 °C is 17.5 torr. What will be the no. of moles of water present in one litre of air at 20 °C and 40% relative humidity.(R =0.082 L atm/mol-k)a)4.2 x 10-4moleb)4.2 x 10-6molec)3.82 X 10-4 moled)3.82 X 10-6moleCorrect answer is option 'C'. Can you explain this answer?
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