You need to prepare 350ml of 6% solution using 7% and 2% solutions. How many ml of 2% stock solutions do you need for this order?
Let's assume that x ml of 7% solution is needed to prepare the 350ml of 6% solution.
The amount of solute in the 7% solution is 7/100 * x ml = 7x/100
The amount of solute in the 2% solution is 2/100 * (350ml - x ml) = 2(350 - x)/100 = (700 - 2x)/100
To prepare a 6% solution, the total amount of solute needed is 6/100 * 350ml = 21 ml
Since the sum of solutes in the 7% and 2% solutions is equal to the total amount of solute needed, we can write the equation:
7x/100 + (700 - 2x)/100 = 21
7x + 700 - 2x = 21 * 100
5x + 700 = 2,100
5x = 2,100 - 700
5x = 1,400
x = 1,400 / 5
x = <<280=280>>280 ml
Therefore, you need 280 ml of the 7% solution and (350 - 280) = 70 ml of the 2% solution to prepare 350 ml of a 6% solution.