Question

Determine the number of moles of air present in 1.35 L at 750 torr and 17.0°C.
Which equation should you use?

N=PV/RT

What is the number of moles present?

⇒ 0.056 mol


A sample of N2 gas occupying 800.0 mL at 20.0°C is chilled on ice to 0.00°C. If the pressure also drops from 1.50 atm to
1.20 atm, what is the final volume of the gas?
Which equation should you use?

V2= P1V1T2/P2T1

What is the final volume of the gas?
⇒ 932 mL


These are the answers

Answer 1

Your answers are correct. I love the Universal formula.

Answer 2

Answer:

For 1: The number of moles of the gas is 0.056 moles.

For 2: The new volume of the gas is 931.74 mL.

Explanation:

• For 1:

To calculate the number of moles of gas, we use the equation given by ideal gas, which is:

$PV=nRT$

where,

P = pressure of the gas = 750 torr

V = volume of the gas = 1.35 L

n = Number of moles of gas = ? mol

R = Gas constant = $62.364\text{L torr }mol^{-1}K^{-1}$

T = temperature of the gas = 17°C = 290 K    (Conversion factor: $T(K)=T(^oC)+273)$

Putting values in above equation, we get:

$750torr\times 1.35L=n\times 62.364\text{L torr }mol^{-1}K^{-1}\times 290K\\\\n=0.056mol$

Hence, the number of moles of the gas is 0.056 mol.

• For 2:

Here, the number of moles of the gas remains the same all the other variables are changing.

To calculate the new volume of the gas, we use the equation given by ideal gas, which is:

$\frac{P_1V_1}{T_1}=\frac{P_2V_2}{T_2}$

where,

$P_1,V_1\text{ and }T_1$ are initial pressure, volume and temperature of the gas

$P_2,V_2\text{ and }T_2$ are final pressure, volume and temperature of the gas

We are given:

$P_1=1.5atm\\V_1=800mL\\T_1=20^oC=293K\\P_2=1.2atm\\V_2=?mL\\T_2=0^oC=273K$

Putting values in above equation, we get:

$\frac{1.5atm\times 800mL}{293}=\frac{1.2atm\times V_2}{273K}\\\\V_2=931.74mL$

Hence, the final volume of the gas will be 931.74 mL.