[duckyduck]

Situation:

i have oxalic acid that need to be dissolve in 810 litres water to make a stock solution.

Question:

How much i need to dissolve the oxalic acid in 810 litres water, then use a portion to dissolve it in other 1785 liters water so that the strength of the oxalic acid in 1785 liters of water is 0.5%?

[Borek]

Please read the forum rules.

[Arkcon]

When faced with a problem like this, you can simplify it to help yourself.  Suppose you have to make 100 mL of 1%, now how much do you need?  Once you have that, you can make 0.5% of 100 mL, its just half.  You can make 810 or 1785 by multiplying, and then we don't have to guess which one you mean.