Chemistry 3/4 2013 Thread

[FlorianK]

Can someone please explain to me how on a titration curve do you identify the end point and equivalence point preferably with a diagram please.

The equivalence point is the only point of inflection of the titration curve, which is at pH=7, if you are tirtating a strong acid.
If you a titrating weak acids or bases it'll be the second point of inflection from the left. The first point of inflection is the half-equivalence point and the pH is euqivalent to the pKa of the chemical you are titrating against.
The end point depends on the indicator used.

[Limista]

One litre of an aqueous solution of potassium hydroxide (KOH) has a pH of 12.0 at 25 degrees celsius. The amount of pure HCl gas, in mol, that must be added to the solution to lower the pH from pH 12.0 to 2.0 would be?

Please help & thanks in advance 

I solved it! 

[Edward21]

Does anyone use the Heinamann textbook and have any idea on how to do question 24 a in the chapter review of spectroscopy? I can't post the question because it has has a specific spectra image for AAS that is very different from your usual one with zero absorbance equal to x, the ppm of an unknown, then successive points on the calibration curve with x + 10, ect.. I just don't know what to do!

[Snorlax]

Does anyone use the Heinamann textbook and have any idea on how to do question 24 a in the chapter review of spectroscopy? I can't post the question because it has has a specific spectra image for AAS that is very different from your usual one with zero absorbance equal to x, the ppm of an unknown, then successive points on the calibration curve with x + 10, ect.. I just don't know what to do!

Heinamann has answers at the back? Don't they?

[Edward21]

The mineral cobaltite is mined for the production of cobalt.
Ore containing cobaltite may also contain trace quantities of
nickel. A sample of ore was analysed by AAS to determine
the concentration of nickel present. 5.0 g of the ore was
dissolved in 25 mL concentrated nitric acid, then diluted to
100 mL. A concentrated stock solution containing 1000 ppm
nickel was also prepared. 10 mL of the ore sample solution
was pipetted separately into four 100 mL fl asks and 1, 2, 4
and 6 mL of the concentrated stock nickel solution was added
to the fl asks. All five flasks were then made up to the mark.
A ‘standard addition’ calibration curve was prepared using
10, 20, 40 and 60 ppm added nickel, giving the standards
listed in the table.

Concentration of nickel
(ppm)
Absorbance
Standard 1 x + 10 0.25
Standard 2 x + 20 0.34
Standard 3 x + 30 0.44
Standard 4 x + 40 0.53
Sample x 0.15

[Edward21]

Heinamann has answers at the back? Don't they?

They do...FOR EVERY OTHER QUESTION OTHER THAN THIS!   

[Mr Keshy]

They do...FOR EVERY OTHER QUESTION OTHER THAN THIS!   

I hate it when that happens..

[Edward21]

I hate it when that happens..

My frustration on the matter is exemplified even more by the lengthy answers they give the simple questions, compared to the lack of explanation on the hard questions that could potentially crop up 

[hardworker]

Can anyone please explain the impact of this if you titrate a standard solution of 2.75 g of sodium carbonate against hydrochloric acid but what is the difference if you titrate a solution of 5.30 g of sodium carbonate in the standard solution with the hydrochloric acid. Some one please answer

[clıppy]

Can anyone please explain the impact of this if you titrate a standard solution of 2.75 g of sodium carbonate against hydrochloric acid but what is the difference if you titrate a solution of 5.30 g of sodium carbonate in the standard solution with the hydrochloric acid. Some one please answer

If you have more mass of sodium carbonate you have more mols of sodium carbonate and more mols requires a greater titre (of the same concentration HCl) to neutralise so I would believe the solution would become basic again due to the higher amount of sodium carbonate over hydrochloric acid.

I'm not exactly sure about this, so if anyone else wants to have a shot

[justsoundslikeaworn-outcliche]

If you have more mass of sodium carbonate you have more mols of sodium carbonate and more mols requires a greater titre (of the same concentration HCl) to neutralise so I would believe the solution would become basic again due to the higher amount of sodium carbonate over hydrochloric acid.

I'm not exactly sure about this, so if anyone else wants to have a shot

i think that sounds right

can anyone help me with these prac question im not sure on a few things:
1)  find amount of sodium carbonate, in mol, present in each flask when :  20ml aliquots of standard solution sodium carbonate of 0.1M are added to 100ml conical flasks  you use n=cv right so v=0.02  and c=? 0.1?
]sorry if really dumb sometimes the easy questions are the ones i struggle at most

[clıppy]

can anyone help me with these prac question im not sure on a few things:
1)  find amount of sodium carbonate, in mol, present in each flask when :  20ml aliquots of standard solution sodium carbonate of 0.1M are added to 100ml conical flasks  you use n=cv right so v=0.02  and c=? 0.1?
]sorry if really dumb sometimes the easy questions are the ones i struggle at most

Taking a 20ml aliquot of standard solution 0.1M
n=cv
n = 0.1 * 0.02
n = 0.002 mols of sodium carbonate in the 20ml aliquot

Yeah thats all correct. The question asks about the amount of mol in the 100ml flasks, so keep in mind if there was any sodium carbonate in the flask that would need to be taken into account

[illuminati]

Can anyone please explain the impact of this if you titrate a standard solution of 2.75 g of sodium carbonate against hydrochloric acid but what is the difference if you titrate a solution of 5.30 g of sodium carbonate in the standard solution with the hydrochloric acid. Some one please answer

In this case the standard is the sodium carbonate - standard means that the concentration can be accurately determined by its mass and known volume. Given the two masses of sodium carbonate in both standards, one should know both concentrations.
Let's assume that both masses of sodium carbonate are dissolved in the same known volume so that the one with 5.30g is more concentrated. The effect on the titration is that this solution will require a larger titre of HCl to neutralise to its equivalence point.
The ultimate goal of titration is that it is a technique for quantitative analysis - both standard solutions of sodium carbonate will give the same concentration of HCl after titration is complete.

[hardworker]

so today was the first part of my back titration sac and we were investigating Lemonade i was just wondering what acid is present within it it. In the first equation we're reacting it with the base known as the Na2CO3. Cause i thought it might be carbonic acid

[Daenerys Targaryen]

Citric acid?
HNO3?