Imagine that you are in chemistry lab and need to make 1.00 Lof a solution with
ID: 952601 • Letter: I
Question
Imagine that you are in chemistry lab and need to make 1.00 Lof a solution with a pH of 2.40.
You have in front of you
100 mL of 6.00×102M HCl,
100 mL of 5.00×102M NaOH, and
plenty of distilled water.
You start to add HCl to a beaker of water when someone asks you a question. When you return to your dilution, you accidentally grab the wrong cylinder and add some NaOH. Once you realize your error, you assess the situation. You have 85.0 mL of HCland 86.0 mL of NaOH left in their original containers.
Assuming the final solution will be diluted to 1.00 L , how much more HCl should you add to achieve the desired pH?
Express your answer to three significant figures and include the appropriate units.
Explanation / Answer
For a pH = 2.40 in a 1 L solution,
[H+] = 3.981 x 10^-3 M is required
Present solution has,
moles of HCl = 6 x 10^-2 M x 85 ml = 5.1 mmol
moles of NaOH = 5 x 10^-2 M x 86 ml = 4.3 mmol
excess [H+] = 5.1 - 4.3 = 0.8 mmol
In 1 L the concentration [H+] = 0.8/1000 = 8 x 10^-4 M
we want an additional [H+] = 3.981 x 10^-3 - 8 x 10^-4 = 3.181 x 10^-3 M
we have to add = 3.181 x 10^-3 x 1 L/6 x 10^-2 = 53.017 ml of HCl stock solution