[bioguy2]

okay so i am having trouble trying to figure out how to do a titration curve for this particular problem.  if 30.00 mL of .2000 M NaOH and 30.00 mL of .300 M NH3 is titrated with .400 M HCl, sketch the curve while showing work for each point on the curve.  So i understand how to do this problem if it was a diprotic acid, but i am not sure how to start this problem with both .300 NH3 and .2000 M NaOH.  i was thinking i would calculate the moles of NaOH and NH3, i think this is a buffer but i am not sure.  any help is a appreciated.

thanks,
Bioguy

[bioguy2]

so i calculated the moles of NaoH and moles of NH3
i got
.200 mol/L NaOH x .0300 L = .006 moles NaOH
.300 mol/L NH3 x .0300 L = .009 moles NH3

i just am not sure where to go from this point

[stewie griffin]

Are you sure the question isn't asking you to sketch two different titration curves. one with HCl and NaOH, and the other with HCl and NH₃?
If the problem indeed is saying that you titrate the mixture of NaOH and NH₃ with HCl then that's not a variant of titration curves that I've encountered. I'm not sure exactly what you would do but think that conceptually you would say that the HCl is going to react first with the NaOH (since that's the stronger base compared to NH₃). Once all of the NaOH has been consumed, then move on and allow the HCl that's added to react with the NH₃.

[bioguy2]

yah the question i have is the exact same question i wrote above.  that is exactly why i am confused i though it should be two titration curves buy it says sketch the titration curve, while showing the work for each point on the curve and identify which species are present in solution at each point.
thanks,
Bioguy

[Borek]

Follow Stewie advice - let the stronger base react first. Once NaOH is neutralized you have just solution of ammonia in NaCl.

[bioguy2]

okay so i got it should take 15 mL of HCl to neutralize the NaOH.  then from there with just NH3 it should take another 22.5 mL to Neutralize the NH3 i think if i am doing this right.

.200 mol/ L NaOH x 1/1L x 30.00 mL/1 x 1mol HCl/1mol NaOH x 1L/.4000 mol HCl x 1000ml/L = 15 mL

.300 mol NH3/1L x  1/1L x 30.00 mL/1 x 1mol HCl/1mol NaOH x 1L/.4000 mol HCl x 1000ml/L = 22.5 mL

so would this mean a total of 37.5 mL of HCl is required for the entire basic solution, and also would the titration curve be a single line with the pH for NaOH equaling the initial pH of NH3.

so i think i did that right but how would i make the titration curve.  so if i wanted to determine the Inititial pH, then say the pH after 2 mL, 5 mL, 10 mL and 15 mL of HCl was added how would i do that. 

[Borek]

http://www.titrations.info/titration-curve-calculation

http://www.titrations.info/acid-base-titration-curve-calculation

Once again, assume NaOH gets completely neutralized before NH₃.

[bioguy2]

okay so it will take 15 mL of the HCl to completely neutralize all of the NaOH before NH3.  so what i need to do is calculate the initial pH of the entire solution before any HCl was added  in order to make a titration curve.  so wouldn't i have to calc the pH of the solution which includes both the .200 M NaOH and .3000 M NH3, then i would completely react all the .200 m NaOH with HCl and only Na and NH3 would be in solution.  then i would have to calculate the initial pH of a solution only contain 30.00 mL of .300 M NH3, determine the total amount of HCl required to completely react with all the NH3 which is the endpoint.  i think that's right.  so the endpoint of the NaOH reaction would be equivalent to the NH3 pH in order to have only one line on the titration curve.  i don't if this is making sense but this is what i think i would have to do. 

[Borek]

You are mostly right (with one problem - see below). You have lost me around having one line on the titration curve - no idea what you refer to.

Please remember that your initial concentrations are not 0.200M and 0.300M - both solutions were diluted when mixed. They are diluted further by the addition of titrant.

[bioguy2]

okay so i was the problem says construct the titration curve on pH vs HCl added.  so would there be a separate curve for both NaOH and NH3 or can you combine the curves into one curve, if this makes sense how i am saying it. 
and my question is how can you calculate the initial pH of the basic solution.   i can do it for an acid but i don't know how to do it for a basic solution

[savy2020]

...So would there be a separate curve for both NaOH and NH3 or can you combine the curves into one curve...

What do you mean by seperate curve?.
Titration curve is a plot between the the volume of the titrant and pH. So as you add titrant pH gradually changes but at equivalent point, rapid change in pH is observed. So ANY titration curve ought to be a smooth one with no sharp points or breaks in it.

and my question is how can you calculate the initial pH of the basic solution.

Well it is a good approximation to neglect the contribution of NH₃{a weak base} and consider only the [OH^-] due to NaOH to find the initial pH.

[bioguy2]

okay what i mean is there will be two separate curves on one graph for the NaOH and NH3 titration right

[savy2020]

okay what i mean is there will be two separate curves on one graph for the NaOH and NH3 titration right

Though you draw them seperately- one after the other - you need to join them.

[bioguy2]

okay thats what i was confused about

[Borek]

There will be one, continuous titration curve. Its calculation can be a little bit tricky at the end of NaOH neutralization - when 99.9% of NaOH is neutralized you may expect pH to go fast down (inflection point) - but it can't, as curve must be continuous. Here ammonia starts to play important role, even if NaOH is not yet 100% neutralized.