Question

A 157-mL sample of gas is collected over water at 22°C and 753 torr. What is the volume of the dry gas at STP? (The vapor pressure of water at 22°C = 20. torr)

Answer 1

Answer:

Volume at STP = 0.1401 L

Explanation:

We are given:

Vapor pressure of water = 20 torr

Total vapor pressure = 753 torr

Vapor pressure of gas = Total vapor pressure - Vapor pressure of water = 753 torr - 20 torr = 733 torr

Using

$PV=nRT$

where,

P = pressure of the gas = 733 torr

V = Volume of the gas = 157 mL= 0.157 L

T = Temperature of the gas = $22^oC=[22+273]K=295K$

R = Gas constant = $62.3637\text{ L.torr}mol^{-1}K^{-1}$

n = number of moles of gas = ?

Putting values in above equation, we get:

$733 torr\times 0.157L=n\times 62.3637\text{L.torr}mol^{-1}K^{-1}\times 295K\\\\n=\frac{733\times 0.157}{62.3637\times 295}=0.006255mol$

At STP, one mole of the gas occupies a volume of 22.4 L

So, 0.006255 mole of the gas occupies a volume of 22.4*0.006255 L

Volume at STP = 0.1401 L