II$.$A A new portable elementary school classroom has raised the concerns of parents about their

children's exposures to formaldehyde $($HCHO$).$ You have been hired as a consultant to advise the

school on the significance of the problem. Instead of immediately collecting air samples in the

classroom you decide to visit the room, collect some relatively easy physical measurements, and then

do some quick "back of the envelope" calculations. You determine the following:

The floor area of the classroom is $75m^2.$ The ceiling height is $1\mathrm{.}8m.$

The classroom contains numerous engineered wood products that emit formaldehyde $($HCHO$).$

You estimate based on a review of the published literature for similar products a collective

HCHO emission rate of $13m\frac{g}{h}r.$

You decide to take some simple carbon dioxide measurements to estimate the air exchange rate

for the classroom. To do this you wait until all students have just left the classroom before

starting measurements. The conditions of the room are assumed to be the same after the children

leave the classroom as when they are in the room, except for the removal of the primary source of

carbon dioxide $($the breath of the children$).$ The following carbon dioxide concentrations were

measured at each of the noted times $($outdoor $C{O}_2=400ppm)$:

Based on your initial analyses, calculate the concentration of HCHO $($in ppb$)$ in classroom air assuming

steady$-$state conditions, an outdoor HCHO concentration of $0ppb,$ and classroom air temp of $22C.$