[kennykwong]

Here is the question:
A tank contains 10kg of NaOH in 1000 little of water. Distilled water is continuously added to the tank at rate of 5 little/min so that the solution is diluted evenly. At the same time, the solution flows out at the same rate. How long will it take for the molarity of the solution to drop to 0.08mol/dm3?

[Borek]

Look like an engineering mass balance question, not a HS problem.

Please read forum rules. You have to show your attempts at solving the question to receive help. This is a forum policy.

[kennykwong]

Look like an engineering mass balance question, not a HS problem.

Please read forum rules. You have to show your attempts at solving the question to receive help. This is a forum policy.

Haha first and foremost, i want to say that i really read this problem in High School books in the challenging drill questions part.
Secondly,i actually had try to do this here are some of my workings
the final molarity is 0.08 mol/dm3 that means that the concentration is 3.2 g/dm3
the initial is 10000g/1000dm3 that means the initial concentration is 10.0 g/dm3.
then i use M1V1=M2V2 theory
for the first minutes:
(10×1000)/(1005)...(the concentration of the solution after dilution for the first time)
so now the V is equal to 1005 and 5 is flow out so left 1000. that means that
the concentration after 1st minutes is 10×(1000/1005)^2
So i consider the rate of dilution is (1000/1005)^2t where t is the minutes required
so now 3.2=10×(200/201)^2t
and t is 114.28 minutes but the answer given is 228 minutes (2 times longer) that is why i am asking if i am done correctly.
Thanks for your sincere comment just now. Now i apology if i had broken any rule of this forum. Sorry.

[Borek]

the concentration after 1st minutes is 10×(1000/1005)^2

Isn't it after 2^nd minute?

[kennykwong]

the concentration after 1st minutes is 10×(1000/1005)^2

Isn't it after 2^nd minute?

Yeah, thanks a lot, you are right!