A tank contains 8l of water in which is dissolved 32 g (grams) of chemical. a solution containing 2 g/l. The amount of chemical in the tank after 20 minutes is 98.66 g.
Given that:
rate r1 = 4 L/min
rate r2 = 2 L/min
concentration c1 = 2 g/L
V(0) = 8 L
A(0) = 32 g
the equation is given as :
ΔV = r1 Δt - r2Δt
dV/dt = 2
integrating the condition , we get
V(t) = 2(t+4)
ΔA  = c1r1 Δt - c2r2 Δt
dA/ dt = 8-2c2
c2 = A/ V
dA/dt = 8 - 2A/ V
now by putting the value of V, we get
dA / dt + 1 / t + 4 A = 8
linear equation has integrating factor :
I = e^ ∫ 1/( t + 4) dt = t + 4
d [ ( t + 4)A] / dt = 8 (t + 4)
(t+ 4 )A = 4(t + 4)² + c
A(t) = (1 / t + 4 ) [ 4(t + 4)² + c]
A(0) = 32 means c = 64
A(20) = (1/6) [(24)² + 16]
     = 296 / 3
     = 98.66 g
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