[AngeD]

I'm not sure where to start this so any help would be greatly appreciated!

You have 2500 kg of a 3.6% bleach solution.  You also have a stock 10% bleach solution.  How much 10% solution would you need to add to the 3.6% solution to make a 4% solution?

[Borek]

You have to show your attempts at solving the problem to receive help. This is the forum policy.

[Arkcon]

When you're faced with a problem like this one, where you don't know what math to do where, you can change it to simpler numbers.

Say you have 10 kg of 10% solution.  Say you also have 20% solution.  You want a 15% solution.  How do you do that?

Now you've got a bunch of problems to work with -- your measuring your liquids in kg, as you add stringer solution you make more volume, but still, you're just mixing solutions and adding percentages.

[AngeD]

Thanks for your reply, Arkcon!

in the 2500 kg solution at 3.6 %, 90 kg of 10 % solution was added to get there.

90 / 2500 = 0.036 = 3.6 %

to get to 4% is a 0.4% change

the only thing I can think of is not an exact answer but a close answer because the volume is so large and the amount needed would be quite small in comparison, so I change the above equation to reflect an unknown amount of stock solution added to 2500 kg of solution to result in that 0.4%, so:

x / 2500 = 0.4% = 0.004
x = 0.004 * 2500
x = 10 kg of 10 % stock solution

I think that's close, but it's not exact.  What tweaking do I need to do to get to an exact answer?

[AWK]

Completely wrong.
You should do a mass balance of solute (NaOCl)
How do you calculate the mass of NaOCl in 2500 kg of 3.6 % solution?
How do you calculate the mass of NaOCl in x kg of 10 % solution?
How do you calculate the mass of NaOCl in (2500+x) kg of 4.0 % solution?
What does the mass balance mean?

[KungKemi]

The way I would first start is with common mass ratios which express the given and the wanted:

Solution A: 90 kg bleach / 2500 kg solution
Stock: 1 kg bleach / 10 kg solution
Wanted: 1 kg bleach / 25 kg solution

Okay, well what are we doing? We're adding x amount of the stock to Solution A in order to achieve a final ratio of 1 kg bleach / 25 kg solution. The question is, however, what is exactly this x amount? Well, one would need to set up a single system which includes Solution A and x amount of the stock being added to it on one side, and the final goal of 1/25 on the otherside. After this has been done, one would simply have to rearrange to find x. Make sure, however, that when you are setting up the system that you consider the x amount of pure bleach which is being added as well as the x amount of over the stock solution which is not bleach, and, as such, adds to the volume of the final solution.

I get an answer which states that 16 and 2/3 kg of stock solution must be added in order to achieve the final desired concentration of 4.0%.

All the best and I hope that this helps,
KungKemi