This is the actual formula for such:
Step 1:
Volume of stock needed = concentration desired / concentration of stock solution x total volume of solution
? = 0.35 / 0.65 x 500mL
? = 0.538 x 500mL
? = 269mL
This means you need 269mL of 65% stock solution in 500mL of solution to have a 35% concentration.
Step 2:
Subtract the amount of stock solution from the total volume to determine how much water should be present in a 35% solution:
500mL - 269mL = 231mL
dank.Frank