1. easy tank mixing problem

A five gallon tank is full of water. We open a spigot so 1 gal. leaves the tank and
introduce a mixture of 1/2 lb. per gal at 1 gal per minute. Assuming the mixture is
well mixed, what’s the concentration at time t?

i did these a while ago but cant seem to remember how to finish them... based on my notes, i know to do something like
dA/dt= concentration in- concentration out

so, dA/dt= 1/2 - A/5, where A is gallons of water in the tank at time t. also, the initial condition is A(0)=5

I could guess where to go from here, but I'd rather have someone help me with the rest of it before i learn it the wrong way myself.

EDIT: just took a stab at it and got A=5/2 + 5/2 e^-1/5t
i still cant remember how i used to do these, so im not sure if thats right..

2. Originally Posted by twostep08
A five gallon tank is full of water. We open a spigot so 1 gal. leaves the tank and
introduce a mixture of 1/2 lb. per gal at 1 gal per minute. Assuming the mixture is
well mixed, what’s the concentration at time t?

i did these a while ago but cant seem to remember how to finish them... based on my notes, i know to do something like
dA/dt= concentration in- concentration out

so, dA/dt= 1/2 - A/5, where A is gallons of water in the tank at time t. also, the initial condition is A(0)=5

I could guess where to go from here, but I'd rather have someone help me with the rest of it before i learn it the wrong way myself.

EDIT: just took a stab at it and got A=5/2 + 5/2 e^-1/5t
i still cant remember how i used to do these, so im not sure if thats right..
Try working with the mass of the solute (or whatever it is) rather than the concentration, then at any time the concentration is mass/5 lb/gal.

CB