Imagine that you are in chemistry lab and need to make 1.00 L of a solution with
ID: 1003927 • Letter: I
Question
Imagine that you are in chemistry lab and need to make 1.00 L of a solution with a pH of 2.70. You have in front of you 100 mL of 7.00×102 M HCl, 100 mL of 5.00×102 M 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 82.0 mL of HCl and 87.0 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
1) how much HCl is in your solution
100mL(amount started)-82mL(amount left)=18mL of HCl
2) how much NaOH is in your solution
100mL(amount started)-87mL(amount left)=13mL of NaOH
3)convert volume into moles
18/1000*7*10-2M=1.26*10-3 moles Of HCl
13/1000*5*10-2M=6.5*10-4 moles Of NaOH
4)HCl have reacted with NaOH after reacting .Amount of HCl will be neutralised by NaOH calculate remaing HCl
1.26*10-3moles Of HCl-6.5*10-4 moles Of NaOH=6.1*10-4 moles Of HCl-
5)determine the total amount of HCl need to reach pH 2.7
pH=-log[H+]
2.7=-log[H+]
10-2.7=[H+]
1.995*10-3=[H+]
1.995*10-3 M is concentration need to have pH 2.7
6) convert desired concentartion in moles
molarity=moles/lit
moles=molarity*lit
moles=1.995*10-3 mol HCl is need to have pH 2.7
7)determine the amount of HCl you need to your soln to recah pH 2.7
(Total moles of HCl needed)-(moles of HCl in soln)=moles need to add
1.995*10-3-6.1*10-4mol=1.385*10-3mol of HCl
8) convert moles of HCl to ml of HCl
molarity=moles/lit
1.385*10-3/7*10-2=1.97*10-2 lit=19.7 mL of HCl
9) 19.7 mL HCl need to added to your soln to have pH 2.7 .The total soln need to be 1 lit this is achived by adding water to your soln.the dilution with water will noit effect the pH level