# Differential Equation - Brine Solution Entering Tank

by Onett
Tags: brine, differential, entering, equation, solution, tank
 P: 2 1. The problem statement, all variables and given/known data A tank contains 80 gallons of pure water. A brine solution with 2 lb/gal of salt enters at 2 gal/min, and the well-stirred mixture leaves at the same rate. Find (a) the amount of salt in the tank at any time and (b) the time at which the brine leaving will contain 1 lb/gal of salt. 2. Relevant equations I'm just wondering about (b) really. I know we set S=80 below to solve it, but why? 3. The attempt at a solution The differential equation that gives (a) is S=160 - 160*e^(-t/40) where S is the amount salt in the tank at any time t.
 P: 1,635 If you correctly modeled a diff. eq for this problem and, also correctly solved it to come up with the sol S=160 - 160*e^(-t/40), then part b)is not a problem at all. what it is asking u is that when will S(t)=1, and not 80 as you are saying! remember S(t) is the amount of salt that the tank contains at any time. The diff eq for this problem is dS/dt=Ri*Ci- (S*Ro)/(Vo+(Ri-Ro)t) , where S--- is the amount of salt in the tank, Ri rate in Ro rate out Ci concentration in Vo the initial volume EDIT: You haven't actually showed us what u have done at all, remember one of the forums main policy is that you must first show your work, for after the people here to give you hints!!
 P: 2 Oh, I'm sorry about that. I'll be sure to put up my work soon. Are you sure that what it's asking though? My notes say that I should get somewhere around 28 minutes.
P: 1,635

## Differential Equation - Brine Solution Entering Tank

A tank contains 80 gallons of pure water. A brine solution with 2 lb/gal of salt enters at 2 gal/min, and the well-stirred mixture leaves at the same rate. Find (a) the amount of salt in the tank at any time and (b) the time at which the brine leaving will contain 1 lb/gal of salt.

here it is :

dS/dt=2*2- (S*2)/(80+(2-2)*t)
dS/dt=4-2S/80, just solve this diff eq, if you haven't gone like this.
 P: 1,635 and for the part b) it is just asking you at what time t=? will S(t)=1, like i said. NOTE: Next time show your work if you want to recieve any help!!!!

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