question thread :)

[vintagea]

hehehe thank you! 

a saline sol. Of conc. 0.05kg/L is added to 0.5L of water in a drip bag at constant rate 0.005L/min. The solution is also being drained at a rate of 0.006L/min and the mixture is kept uniform. Find an equation for the rate of change of saline conc. With time.
So from what I have done so far:
The conc of saline added is: 0.00025kg/min to 0.5L water
0.05kg - - - 1 L
X kg - - - - - 0.005L
X = 0.00025kg

So the conc. of saline in the drip bag at time t mins is:
(0.00025 t)/0.5 in kg/L

Overall water change is loss of 0.001L/min

I have no idea what im doing….

dx/dt = inflow - outflow (let x be the amount of saline in sol)
0.005 - (x/0.5)0.006

[vintagea]

someone please help?

[nhmn0301]

someone please help?

Notice that you only need to find the equation for this question because these are the types that VCAA regards as "unsolvable" for Specialist Math, anw,let me know if there is any errors!

[vintagea]

thank you for helping

im just wondering why is it 0.006x on the top line? isn't the 0.006 already taken into consideration in the 0.001 in the bottom line?

[nhmn0301]

thank you for helping

im just wondering why is it 0.006x on the top line? isn't the 0.006 already taken into consideration in the 0.001 in the bottom line?

The "0.5 - 0.001t" does take 0.006 into consideration, but the whole thing is just the "V" term in dx/dV  at"t" time. So, we still need dV/dt as usual because without it, you cannot get dx/dt(out). I hope that makes sense.