200mL of pure acid must be added to the 400mL of 40% acid solution to produce a 60% acid solution.
Let x be the quantity of pure acid needed. We can set up an equation based on the following information:
Original amount of acid: 400mL * 40% = 160mL
Final amount of acid:** (400mL + x) * 60%
Total amount of acid must be conserved: Original amount + added acid = final amount
Therefore, the equation becomes:
160mL + x = (400mL + x) * 60%
Simplifying the equation:
160mL + x = 240mL + 0.6x
Solving for x:
0.4x = 80mL
x = 200mL
Therefore, 200mL of pure acid must be added to the 400mL of 40% acid solution to produce a 60% acid solution.