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VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Chemistry => Topic started by: squance on June 08, 2009, 11:25:35 am

Title: Some Acid and Bases Questions
Post by: squance on June 08, 2009, 11:25:35 am: 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??
Title: Re: Some Acid and Bases Questions
Post by: ed_saifa on June 08, 2009, 11:39:20 am: solution attached
Title: Re: Some Acid and Bases Questions
Post by: Dark Horse on June 08, 2009, 11:48:36 am: 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 :)
Title: Re: Some Acid and Bases Questions
Post by: Dark Horse on June 08, 2009, 11:49:19 am: lol, sorry, didnt see the answers had been posted up, my bad
Title: Re: Some Acid and Bases Questions
Post by: squance on June 08, 2009, 11:51:37 am: Quote from: ed_saifa on June 08, 2009, 11:39:20 am
solution attached
Quote from: Dark Horse on June 08, 2009, 11:48:36 am
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 :)
Title: Re: Some Acid and Bases Questions
Post by: squance on June 09, 2009, 05:02:56 pm: 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
Title: Re: Some Acid and Bases Questions
Post by: TrueTears on June 09, 2009, 05:08:26 pm: $NaOH + HCl \to NaCl + H_2O$

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 $n(OH^{-}) = n(NaOH) = 0.00325$ mol

$[OH^{-}] = \frac{0.00325}{100 \times 10^{-3}} = 0.0325$ M

Hence D
Title: Re: Some Acid and Bases Questions
Post by: squance on June 09, 2009, 05:11:42 pm: 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!