A gas is found to occupy a volume of 38.2 mL at 25.0ºC and an unknown
pressure. If this same gas occupies 37.5 mL at STP, determine what its unknown original pressure was in mm Hg.

836 mmHg

Explanation:

Given data:

Initial volume = 38.2 mL

Initial temperature = 25°C (25+273=298 k)

Final volume = 37.5 mL

Final temperature = standard = 273 K

Final pressure = standard = 1 atm

Initial pressure = ?

Solution:

Formula:  

P₁V₁/T₁ = P₂V₂/T₂  

P₁ = Initial pressure

V₁ = Initial volume

T₁ = Initial temperature

P₂ = Final pressure

V₂ = Final volume

T₂ = Final temperature

Solution:

P₁ = P₂V₂T₁ /T₂  V₁

P₁ =  1 atm × 37.5 mL × 298 K / 273 K × 38.2 mL

P₁ =  11175 atm / 10428.6

P₁ = 1.1 atm

atm to mmHg

1.1 atm × 760 mmHg / 1 atm

836 mmHg

the answer is 75.0%.