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Chemistry doubt

Respected members of Brilliant !

I have a doubt in inorganic chemistry.

What is the pH of the resulting solution when 40 ml of HCl reacts with 16ml of NaOH ? (pH of HCl is 1 and pH of NaOH is 14 )

Please post the answer and solution.

Thanking you in advance

Note by Manish Dash
2 years, 6 months ago

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Conc. of HCl = 0.1M Conc. of NaOH = 1M No. of moles of HCl= 0.004 No. of moles of NaOH= 0.016 Therefore, 0.004 moles HCl and NaOH will neutralise each other and 0.016-0.004=0.012 moles of NaOH will remain. conc. of NaOH=0.012/0.056=0.2M pOH= -log(0.2)=0.7 ph=13.3

Arpita Karkera - 2 years, 6 months ago

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@Manish Dash

The given pH of NaOH is 14, hence it's molarity is \(1\). Similarly, the molarity of HCl is \(10^{-1}\). Now, we have \(16\)ml NaOH and \(40\)ml HCl, which means:

Millimoles of NaOH= \(16\)

Millimoles of HCl= \(40*10^{-1}=4\)

Now, HCl is in deficit, hence it acts as limiting reagent. \(4\) millimoles of HCl will react with \(4\) millimoles of NaOH to form \(4\) millimoles of neutral compound \(NaCl\). Note that \(12\) millimoles of NaOH is still in solution.

Now, after adding both solutions, the new volume is \(16+40=56\text{ml}\). In this \(56\)ml, there is \(12\) millimoles of NaOH, hence, molarity of this solution is:

\(M=\frac {12 \times 10^{-3}} {56} \times 1000 = \frac {12} {56}= \frac {3} {14}=[OH^-]\)

Now,

\(pOH= -\log {[OH^-]}=-\log{\frac {3} {14}}=0.669\) (approx. . logarithm is in base \(10\))

Therefore:

\(pH=14-pOH=13.33\)

Raghav Vaidyanathan - 2 years, 6 months ago

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Thank you very much @Raghav Vaidyanathan

Manish Dash - 2 years, 6 months ago

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Comment deleted Apr 15, 2015

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Why do you think its a buffer solution? A mixture of strong acid \((HCl)\) and a salt of strong acid and strong base \((NaCl)\) is not a buffer solution.

Pranjal Jain - 2 years, 6 months ago

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I apologise.

My approach would be to calculate the number of moles of the acid or the base that still remains and then work on it's pH

Agnishom Chattopadhyay - 2 years, 6 months ago

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Any method to support your statement?

Manish Dash - 2 years, 6 months ago

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Hey manish, could I mention a site for Your help?

Swapnil Das - 2 years, 6 months ago

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Yes please !!!

Manish Dash - 2 years, 6 months ago

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Stack Exchange Chemistry, post your question here, U would be answered at the earliest!

Swapnil Das - 2 years, 6 months ago

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@Swapnil Das Thank you very much

Manish Dash - 2 years, 6 months ago

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@Manish Dash My pleasure! Feel free to ask me anything or mentioning me!

Swapnil Das - 2 years, 6 months ago

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Comment deleted Apr 15, 2015

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Comment deleted Apr 15, 2015

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Hi Manish, I am sorry to have deleted what I posted. I found that there was a big mistake. The answer to your question is : it is 1 m because it is by the definition of Ph. By definition of PH , PH of 0.1 M HCL is 1 and 1M NaOH is 1.

Edit : (I shall read up a bit on the dissociation values and try the problem again, I guess I messed up there)

Krishna Ar - 2 years, 6 months ago

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@Krishna Ar Yep. Considering they are ideal bases.

Agnishom Chattopadhyay - 2 years, 6 months ago

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@Agnishom Chattopadhyay Sorry, but what are ideal bases? Even a google search isn't of use in this regard. :/

Krishna Ar - 2 years, 6 months ago

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@Krishna Ar I obviously can't tell you what he meant, but my guess would be a base which disassociates (almost) completely is known as an ideal base. (almost) because no base will dissociate completely in water.

Siddhartha Srivastava - 2 years, 6 months ago

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@Krishna Ar But pH of 1M NaOH is 14 and pOH of 1M NaOH is 0

Manish Dash - 2 years, 6 months ago

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@Manish Dash Absolutely.

Krishna Ar - 2 years, 6 months ago

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Hi manish, thanks for mentioning me! But I guess U don't know that I am the weakest in Biology and Chemistry! But for your help, I would surely ask my coaching friends!

Swapnil Das - 2 years, 6 months ago

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Thank you

Manish Dash - 2 years, 6 months ago

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Your most welcome ! I hope I could answer you at the earliest.

Swapnil Das - 2 years, 6 months ago

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Sorry Manish but I have not studied pH & all that stuffs so I cannot help you :(

Harsh Shrivastava - 2 years, 6 months ago

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No problem. Thanks for replying

Manish Dash - 2 years, 6 months ago

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1, i guess??

Abhineet Nayyar - 2 years, 6 months ago

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Why do you think so ?

Manish Dash - 2 years, 6 months ago

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See, HCl and NaOH being strong acid and base will completely neutralize equal volumes of each other giving us a neutral solution. But Since the volume of HCl solution is more, so we will be left with HCl solution of pH =1. What do you think??

Abhineet Nayyar - 2 years, 6 months ago

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@Abhineet Nayyar Equal moles will react and not equal volumes. It depends upon concentration of \(HCl\) and \(NaOH\)

Pranjal Jain - 2 years, 6 months ago

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@Abhineet Nayyar Is there any thing to do about normality and concentration of solution ?

Manish Dash - 2 years, 6 months ago

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@Manish Dash Do u have the answer????

Abhineet Nayyar - 2 years, 6 months ago

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@Abhineet Nayyar I don't have the answer

Manish Dash - 2 years, 6 months ago

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