# Homework Help: Solution pumped into a tank problem.

1. Jan 26, 2013

### derail

1. The problem statement, all variables and given/known data
Suppose that a large mixing tank initially holds 400 gallons of water in which 65 pounds of salt have been dissolved. Another brine solution is pumped into the tank at a rate of 6 gal/min, and when the solution is well stirred, it is pumped out at a faster rate of 8 gal/min. If the concentration of the solution entering is 3 lb/gal, determine a differential equation for the amount A(s) of salt in the tank at time t.

2. Relevant equations
Flow rate is Volume times cross sectional area, however, I do not know if that is relevant, it has been a while since I have done something like this.

Last edited: Jan 26, 2013
2. Jan 26, 2013

### Staff: Mentor

Geometry does not matter here.

If the concentration of salt is x and 8 gallons/min flow out, can you determine the rate of salt flowing out?
If 6gallons/min with a salt concentration of 3pounds/gallon (imperial units are weird) flows in, can you determine the rate of salt flowing in?

This should help to get your differential equation.

3. Jan 26, 2013

### derail

So wouldn't the 8 gallons/min be the flow rate?

I would think i can find the rate of salt flowing out by taking the 6 gallons/min and the 3 pounds/gallon and trying to get pounds/minute by multiplying them to get 18 pounds/minute.

4. Jan 26, 2013

### haruspex

There are two flow rates, one in, one out.
You mean the rate of salt flowing in, right? You know the flow rate of water out, so you need the concentration of salt in that. You have the variable A(t) for the amount of salt in the tank at time t, so you need to know how much water that's dissolved in. Can you find an equation for the volume of water in the tank at time t?

5. Jan 26, 2013

### derail

I am not sure how to incorporate the flows of the liquid into the equation when I am ultimately looking for an equation for the amount of salt.

6. Jan 26, 2013

### haruspex

You know the flow rates of water in and out, so how much water is in the tank at time t?

7. Jan 26, 2013

### derail

I see, we should be able to use the same logic to make is so that the units of the A(t) are lbs per minute by using the concentration of salt water. Do you have a suggestion on how to write the equation?

I figure the way we are doing this, we are basically adding two separate equations, one for the flow rate going in and one for the rate going out.

8. Jan 26, 2013

### haruspex

Forget the salt for the moment. There's 6g/m flowing in, 8g/m flowing out. What's the rate of change of volume of water? The tank started with 400 gallons. How many after t minutes?

9. Jan 26, 2013

### derail

So the rate of change is 2g/m flowing out. The tank started with 400 gallons.

10. Jan 26, 2013

### derail

So after t mins we should have, 2*t+400 gallons of water.

11. Jan 26, 2013

### haruspex

Is the volume of water increasing or decreasing?

12. Jan 26, 2013

### derail

Decreasing my bad, so we actually get -2*t+400, now to write that equation.

13. Jan 26, 2013

### derail

So the water flow rate =(-2*t)+400, and we know that 3 pounds/gallon is being pumped in so we should be able to write: =((-2*t)+400)*3 to correct the units for Pounds per minute?

14. Jan 26, 2013

### haruspex

No, that's not a flow rate at all. That's the volume of water in the tank at time t.
If the salt in the tank at time t is A(t), what's the concentration in the tank at time t?

15. Jan 26, 2013

### derail

I am trying to find a differential equation for the amount A(s), meaning the amount of salt. I miss wrote that last equation, you are right. However, I am trying to get the amount of salt, so I figure if I know the amount of water then I can use the concentration to get the amount of salt? like I said in reply #13

Well the concentration of the solution entering is 3lb/g.

16. Jan 26, 2013

### derail

So my thought process was, we can take the flow rate equation and incorporate the concentration of salt to water?

17. Jan 26, 2013

### haruspex

Yes, I know, and I'm trying to lead you to that by asking a series of very simple questions. So please, try to answer them, not a question I didn't ask.
You have calculated the volume of water in the tank at time t. The amount of salt in the tank at time t is A. (Which one writes as A=A(t), or if you want to call the variable As then As=As(t). I don't know why you keep writing A(s). There is no variable s.) So, very simple question, what is the concentration of salt in the tank at time t?