Answer to Question #184569 in Chemistry for Monique Davis

Question

A 800 mL sample of NH₃ gas is at STP (25 ºC and 1 atm). If the volume changes to 586 mL and the temperature changes to 16 degree C , what will be the new pressure in atmospheres?

Answer 1

P₁ = 1 atm

V₁ = 800 mL

T₁ = 25^oC = 298 K

P₂ = unknown

V₂ = 586 mL

T₂ = 16^oC = 289 K

 

Solution:

The Combined Gas Law can be used:

P₁V₁/T₁ = P₂V₂/T₂

Cross-multiply to clear the fractions:

P₁V₁T₂ = P₂V₂T₁

Divide to isolate P₂:

P₂ = (P₁V₁T₂) / (V₂T₁)

Plug in the numbers and solve for P₂:

P₂ = (1 atm × 800 mL × 289 K) / (586 mL × 298 K) = 1.324 atm

P₂ = 1.324 atm

 

Answer: The new pressure (P₂) will be 1.324 atm.

However, it should be noted that STP is usually defined as a temperature of 273.15 K (0°C) and an absolute pressure of exactly 1 atm (101.325 kPa).