Answer:
(a) 60 kg; (b) 21.6 kg; (c) 0 kg/L
Step-by-step explanation:
(a) Initial amount of salt in tank
The tank initially contains 60 kg of salt.
(b) Amount of salt after 4.5 h
![\text{Let A = mass of salt after t min}\\\text{and }r_{i} = \text{rate of salt coming into tank}\\\text{and }r_{0} =\text{rate of salt going out of tank}](https://tex.z-dn.net/?f=%5Ctext%7BLet%20A%20%3D%20mass%20of%20salt%20after%20t%20min%7D%5C%5C%5Ctext%7Band%20%7Dr_%7Bi%7D%20%3D%20%5Ctext%7Brate%20of%20salt%20coming%20into%20tank%7D%5C%5C%5Ctext%7Band%20%7Dr_%7B0%7D%20%3D%5Ctext%7Brate%20of%20salt%20going%20out%20of%20tank%7D)
(i) Set up an expression for the rate of change of salt concentration.
![\dfrac{\text{d}A}{\text{d}t} = r_{i} - r_{o}\\\\\text{The fresh water is entering with no salt, so}\\ r_{i} = 0\\r_{o} = \dfrac{\text{3 L}}{\text{1 min}} \times \dfrac {A\text{ kg}}{\text{1000 L}} =\dfrac{3A}{1000}\text{ kg/min}\\\\\dfrac{\text{d}A}{\text{d}t} = -0.003A \text{ kg/min}](https://tex.z-dn.net/?f=%5Cdfrac%7B%5Ctext%7Bd%7DA%7D%7B%5Ctext%7Bd%7Dt%7D%20%3D%20r_%7Bi%7D%20-%20r_%7Bo%7D%5C%5C%5C%5C%5Ctext%7BThe%20fresh%20water%20is%20entering%20with%20no%20salt%2C%20so%7D%5C%5C%20r_%7Bi%7D%20%3D%200%5C%5Cr_%7Bo%7D%20%3D%20%5Cdfrac%7B%5Ctext%7B3%20L%7D%7D%7B%5Ctext%7B1%20min%7D%7D%20%5Ctimes%20%5Cdfrac%20%7BA%5Ctext%7B%20kg%7D%7D%7B%5Ctext%7B1000%20L%7D%7D%20%3D%5Cdfrac%7B3A%7D%7B1000%7D%5Ctext%7B%20kg%2Fmin%7D%5C%5C%5C%5C%5Cdfrac%7B%5Ctext%7Bd%7DA%7D%7B%5Ctext%7Bd%7Dt%7D%20%3D%20-0.003A%20%5Ctext%7B%20kg%2Fmin%7D)
(ii) Integrate the expression
![\dfrac{\text{d}A}{\text{d}t} = -0.003A\\\\\dfrac{\text{d}A}{A} = -0.003\text{d}t\\\\\int \dfrac{\text{d}A}{A} = -\int 0.003\text{d}t\\\\\ln A = -0.003t + C](https://tex.z-dn.net/?f=%5Cdfrac%7B%5Ctext%7Bd%7DA%7D%7B%5Ctext%7Bd%7Dt%7D%20%3D%20-0.003A%5C%5C%5C%5C%5Cdfrac%7B%5Ctext%7Bd%7DA%7D%7BA%7D%20%3D%20-0.003%5Ctext%7Bd%7Dt%5C%5C%5C%5C%5Cint%20%5Cdfrac%7B%5Ctext%7Bd%7DA%7D%7BA%7D%20%3D%20-%5Cint%200.003%5Ctext%7Bd%7Dt%5C%5C%5C%5C%5Cln%20A%20%3D%20-0.003t%20%2B%20C)
(iii) Find the constant of integration
![\ln A = -0.003t + C\\\text{At t = 0, A = 60 kg/1000 L = 0.060 kg/L} \\\ln (0.060) = -0.003\times0 + C\\C = \ln(0.060)](https://tex.z-dn.net/?f=%5Cln%20A%20%3D%20-0.003t%20%2B%20C%5C%5C%5Ctext%7BAt%20t%20%3D%200%2C%20A%20%3D%2060%20kg%2F1000%20L%20%3D%200.060%20kg%2FL%7D%20%5C%5C%5Cln%20%280.060%29%20%3D%20-0.003%5Ctimes0%20%2B%20C%5C%5CC%20%3D%20%5Cln%280.060%29)
(iv) Solve for A as a function of time.
![\text{The integrated rate expression is}\\\ln A = -0.003t + \ln(0.060)\\\text{Solve for } A\\A = 0.060e^{-0.003t}](https://tex.z-dn.net/?f=%5Ctext%7BThe%20integrated%20rate%20expression%20is%7D%5C%5C%5Cln%20A%20%3D%20-0.003t%20%2B%20%20%5Cln%280.060%29%5C%5C%5Ctext%7BSolve%20for%20%7D%20A%5C%5CA%20%3D%200.060e%5E%7B-0.003t%7D)
(v) Calculate the amount of salt after 4.5 h
a. Convert hours to minutes
![\text{Time} = \text{4.5 h} \times \dfrac{\text{60 min}}{\text{1h}} = \text{270 min}](https://tex.z-dn.net/?f=%5Ctext%7BTime%7D%20%3D%20%5Ctext%7B4.5%20h%7D%20%5Ctimes%20%5Cdfrac%7B%5Ctext%7B60%20min%7D%7D%7B%5Ctext%7B1h%7D%7D%20%3D%20%5Ctext%7B270%20min%7D)
b.Calculate the concentration
![A = 0.060e^{-0.003t} = 0.060e^{-0.003\times270} = 0.060e^{-0.81} = 0.060 \times 0.445 = \text{0.0267 kg/L}](https://tex.z-dn.net/?f=A%20%3D%200.060e%5E%7B-0.003t%7D%20%3D%200.060e%5E%7B-0.003%5Ctimes270%7D%20%3D%200.060e%5E%7B-0.81%7D%20%3D%200.060%20%5Ctimes%200.445%20%3D%20%5Ctext%7B0.0267%20kg%2FL%7D)
c. Calculate the volume
The tank has been filling at 6 L/min and draining at 3 L/min, so it is filling at a net rate of 3 L/min.
The volume added in 4.5 h is
![\text{Volume added} = \text{270 min} \times \dfrac{\text{3 L}}{\text{1 min}} = \text{810 L}](https://tex.z-dn.net/?f=%5Ctext%7BVolume%20added%7D%20%3D%20%5Ctext%7B270%20min%7D%20%5Ctimes%20%5Cdfrac%7B%5Ctext%7B3%20L%7D%7D%7B%5Ctext%7B1%20min%7D%7D%20%3D%20%5Ctext%7B810%20L%7D)
Total volume in tank = 1000 L + 810 L = 1810 L
d. Calculate the mass of salt in the tank
![\text{Mass of salt in tank } = \text{1810 L} \times \dfrac{\text{0.0267 kg}}{\text{1 L}} = \textbf{21.6 kg}](https://tex.z-dn.net/?f=%5Ctext%7BMass%20of%20salt%20in%20tank%20%7D%20%3D%20%5Ctext%7B1810%20L%7D%20%5Ctimes%20%5Cdfrac%7B%5Ctext%7B0.0267%20kg%7D%7D%7B%5Ctext%7B1%20L%7D%7D%20%3D%20%5Ctextbf%7B21.6%20kg%7D)
(c) Concentration at infinite time
![\text{As t $\longrightarrow \, -\infty,\, e^{-\infty} \longrightarrow \, 0$, so A $\longrightarrow \, 0$.}](https://tex.z-dn.net/?f=%5Ctext%7BAs%20t%20%24%5Clongrightarrow%20%5C%2C%20-%5Cinfty%2C%5C%2C%20e%5E%7B-%5Cinfty%7D%20%5Clongrightarrow%20%5C%2C%200%24%2C%20so%20A%20%24%5Clongrightarrow%20%5C%2C%200%24.%7D)
This makes sense, because the salt is continuously being flushed out by the fresh water coming in.
The graph below shows how the concentration of salt varies with time.