Final answer:
To make 20 ml of 65% acid from 50% acid and pure acid, 14 ml of 50% acid should be added to 6 ml of pure acid. The closest answer from the given options would be 15 ml (option C), even though this is not the exact answer.
Step-by-step explanation:
How to Calculate the Volume of 50% Acid Needed
The question asks how many ml of 50% acid should be added to pure acid to make 20 ml of 65% acid. The calculation involves a simple yet important concept in solution chemistry known as mixing solutions of different concentrations.
To begin with, we know that the final solution must have 65% acid in 20 ml. Let's assume the volume of 50% acid needed is 'V' ml. The pure acid is effectively 100% concentrated. To create the final mixture, the amount of acid from the pure acid plus the amount from the 50% acid should equal the total amount of acid in the 20 ml of 65% acid solution.
For the 50% solution:
Acid from 50% solution = 0.50 * V
For the pure acid, which is 100%:
Acid from pure acid = 1.00 * (20 - V)
Thus, the total acid content in the final solution should be:
Total acid content = 0.65 * 20
Now we set up our equation:
0.50 * V + 1.00 * (20 - V) = 0.65 * 20
0.50V + 20 - V = 13
-0.50V = -7
V = 14
Therefore, 14 ml of 50% acid should be added to 6 ml of pure acid to get 20 ml of 65% acid.
We can see that none of the provided multiple choice answers (A: 5 ml, B: 10 ml, C: 15 ml, D: 20 ml) actually match our calculated amount; thus, if forced to select from these options, none would be correct. However, since this could be a typographical error in the question, we'll go with the closest answer, which would be option C.