Question

A gas has density 0.87 g/L at 30oC and 131.2 kPa.

Find density at STP.

*Hint use the density of gases equation.

a
.57 g/L
b
.72 g/L
c
.75 g/L
d
.54 g/L

Answer 1

• Option C. 0.75g/L

Step-by-step explanation:

Using your hint, the density of gases equation is:

`d=(p* M)/(R* T)`

Where:

• d = density of the gas
• M = molar mass of the gas
• R = universal constant of gases
• T = absolute temperature

The conditions of your gas are:

State 1:

• d₁ = 0.87g/L
• T₁ = 30 + 273.15K = 303.15K
• p₁ = 131.2kPa

State 2 (STP):

• d₂ = ?
• T₂ = 0 + 273.15K = 273.15K
• p₂ = 100kPa (in the old definition of STP it was 101.325kPa)

Since M and R are constants, the density of gases equation drives to:

`(d_1* T_1)/(p_1)=(d_2* T_2)/(p_2)`

Now you can substitute and clear d₂

`(0.87g/L* 303.15K)/(131.2kPa)=(d_2* 273.15K)/(100kPa)`

• d₂ = 0.74g/L

If you use 101.325kPa instead of 100kPa it will result 0.75g/L, which is the option C.