Some Acid and Bases Questions

[squance]

My sister is stuck on some of these MC questions from past exams.

When 50 mL of 2.00 M HCl, 100 mL of 1.00 M HCL and 100 mL of 0.500 M HCl are mixed together, the resulting HCl concentration of the solution is:

A. 0.25M
B. 1.00 M
C. 3.50 M
D. 6.25 M


20 mL of 0.10 M HCl is added to 20 ml of an unknown solution. The pH of the resulting mixture is measured and found to be 2.0. THe unknown solution could have been:

a. 0.20 M NaOH
B. 0.10 M KOH
C. 0.04 M Ca(OH)2
D. 0.0010 M HCl

The answer to the first one is B and the second one is C but how and why??

[ed_saifa]

solution attached

[Dark Horse]

With second question:
1) Calculate the number of moles of HCL initially
2) At the end, the pH is 2. Therefore, the concentration of HCl is 0.01M. Calculate the number of moles of HCL at the end of the reaction. (note, at the end of the reaction, the volume is 40ml)
3) Minus the number of mole of HCL at the end from the number of mole of HCL at the start. I got .0016 mol
Hence, you know that .0016 mol of hydroxide ions reacted. As the volume of base is 20ml, the concentration of hydroxide ions is 0.08M. Then you look at the multiple choice answers, and the answer must therefore be C

Hope that helped

[Dark Horse]

lol, sorry, didnt see the answers had been posted up, my bad

[squance]

solution attached

With second question:
1) Calculate the number of moles of HCL initially
2) At the end, the pH is 2. Therefore, the concentration of HCl is 0.01M. Calculate the number of moles of HCL at the end of the reaction. (note, at the end of the reaction, the volume is 40ml)
3) Minus the number of mole of HCL at the end from the number of mole of HCL at the start. I got .0016 mol
Hence, you know that .0016 mol of hydroxide ions reacted. As the volume of base is 20ml, the concentration of hydroxide ions is 0.08M. Then you look at the multiple choice answers, and the answer must therefore be C

Hope that helped

Thanks you two
My sister appreciates the help

[squance]

Another Acid and base MC..
Help would be muchly appreciated.

A student mixes 25.0 mL of 0.230 M NaOH with 25.0 mL of 0.100 M HCl and then dilutes the mixtue to 100.0 mL in a volumetric flask. What is the final concentration of hydroxide ions?

A. 0.750 M
B. 0.325 M
C. 0.188 M
D. 0.0325 M

[TrueTears]

n(NaOH) =  0.00575 mol

n(HCl) = 0.0025 mol

n(NaOH) : n(HCl) = 1:1

We have 0.00575 : 0.0025

Therefore NaOH is in excess.

So we have 0.00575 - 0.0025 = 0.00325 mol of NaOH left unreacted.

unreacted mol

M

Hence D

[squance]

Thanks so much Truetears.
My sis had D as her answer too but she just wasn't sure if her working was right.

THANKS AGAIN!