Question

Formaldehyde was sampled in a FEMA trailer at a temperature of 115 degrees F. The lab determined the concentration to be 13 mg/m3 at NTP. What is the concentration in the trailer in ppm and mg/m3 ?

Answer 1

The formaldehyde at NTP is a gas so lets assume it behaves as an ideal gas

ideal gas equation is PV = n R T, p is pressure, V is volume, R is gas constant and T is temperature, the concentration is the relation ship of a mass or moles in a certain volume so lets rearrange the equation

P / RT = n / V, n / V is concentration , to turn moles to mass you have to multiply by the molar mass but since we are dealing with the same compound in all the experiment we can relax about that , so lets say that n/V is C

from this relation ship we can see that if the temperature increases then the concentration decreases, lets say that the pressure is the same always, we only need to worry about the temperature we need to change temperature from F to Kelvin

115 F is 319.261 Kelvin

Normal temperature condition is 20 C or 293 K

so from this values we can make an earlier prediction we can say that the concentration will be lower than 13

so lets get a relationship from state 1 to state 2 using the equatio we got, we can divide this equations like :

, since the Pressures are always the same and R is a constant that does not change so

or

, Rearrange this to get

so:

C2 = C1 * T1 / T2 = 13 * 293 / 319.26 = 11.93 mg / m³

Now we only need to change from mg / m³ to ppm

apply the next formula

Concentration ppm = concentration mg / m³ * molar volume / molar mass

To calculate molar volume apply

Vm = R T / P = 0.082 * 319.26 / 1 = 26.18 L , the molar volume is the volume that is occupied by 1 mole of substance

molar mass formaldehyde is 30 g/gmol

Concentration ppm = 11.93 * 26.18 / 30 = 10.41 ppm