1 The air in a small room 12 ft by 8 ft by 8 ft is 3 carbon

1. The air in a small room 12 ft by 8 ft by 8 ft is 3% carbon monoxide. Starting at t = 0, fresh air containing no carbon monoxide is blown into the room at a rate of 100 ft^3 /min. If air in the room flows out through a vent at the same rate, when will the air in the room be 0.01% carbon monoxide?

Solution

  Let v(t) be the volume of CO in the room at time t, and let V be the total volume of the room. The volume percent of CO in the room aat a given time is then given by:

p(t) = 100*v(t)/V

Volume percent is the measure of concentration used in this problem. The \"amount\" of CO in the room is then measured in terms of the volume of CO in the room.

Let f be the rate at which fresh air enters the room, which is the same as the rate at which air exits the room. We assume that the air in the room mixes instantantaneously with the air entering the room, so that the concentration of CO is uniform throughout the room.

As you wrote, the rate at which the volume of CO in the room changes with time is given by

dv(t)/dt = 0*f - (f/V)*v(t) = - (f/V)*v(t)

This is a simple first-order equation:

dv/v = -(f/V) dt

ln(v) - ln(c) = -(f/V)*t

where ln(c) is the constant of integration.

ln(v/c) = -(f/V)*t

v(t) = c*exp(-f*t/V)

In terms of volume percent,

p(t) = 100*v(t)/V = (C/V)*exp(-f*t/V)

where C = 100*c/V is just another way of writing the constant.

PLugging in the values for the constants, we get:

p(t) = (C/(768 cu.ft.))* exp(-t/(7.68 min))

Now use the initial condition (p(0) = 3% at t = 0) to solve for C:

3% = C

p(t) = (3%)*exp(-t/(7.68 min))

To find the time when the air in the room reaches a certain value, it is easier to rewrite this solution as:

p(t)/3% = exp(-t/(7.68 min))

t(p) = -(7.68 min)ln(p/3%) = (7.68 min)*ln(3%/p)

The question asks when p(t) = 0.01%. Plugging this into the above equation, we get:

t(0.01%) = (7.68 min)*ln(3/0.01) = 43.8 min