does adding solid NaCl increase the pH of a acidic solution such as HCl? i mean the final product of the mixture of NaCl and HCl. I asked my tutor this and he said that the pH value will not change. But i thought that since NaCl has a pH of 7 while HCl has a pH of 2 the final product formed will be a pH of around 4.5? I asked this before but im not sure if UltimaOnline was talking about that.
Thanks so much for the help
yes
NaCl will disociate fully into Na + and Cl -
in strong acids like HCl, it will not matter but pH will rise as pH is determined by the concentration of H+ which will decrease as NaCl solution will add volume to the reaction mixture. Hence, if 50cc of NaCl and 50cc of HCl mix together, the pH will be affected as previously, 50 moles of H+ is present assuming HCl solution is 1M/cc over 50/1000 dm3 but now is 50 moles of H+ is present over 100/1000 dm3, (dilution factor = 1/2)
(Hence, pH is not a good acidity indicator, Ka is better - A levels)
To TS, you are O level student right? If A levels, do consider common ion effect, :)
EDIT: SOLID NaCl will not as NaCl is a neutral salt which will undergo only hydration and not hydrolysis as Na charge density is too low to have any polarising power. But if it is solution, my previous explanation applies
im an O level student so i dont really get it, i thought that since NaCl is neutral and HCl is around 2, so there will be a change in pH? um so how do i increase the pH value? if possible pls explain using O level sylabus cos i dont know anyhing about the upper level stuff
In 'O' level terms, the pH of your hydrochloric acid solution will not change unless you change the concentration or add a base or alkali to react with it. Since you're only adding solid sodium chloride (which is only slightly acidic in solution, regarded as neutral at your level), the concentration doesn't change.
Originally posted by Kuali Baba:In 'O' level terms, the pH of your hydrochloric acid solution will not change unless you change the concentration or add a base or alkali to react with it. Since you're only adding solid sodium chloride (which is only slightly acidic in solution, regarded as neutral at your level), the concentration doesn't change.
No. Solid NaCl is neutral and its ions, Na+ and Cl- have no effect on the pH of a solution.
Acidity/basicity is determined by the concentration of H+ and OH- ions in a solution. When SOLID NaCl is added, the volume of the solution remains constant, hence concentration of H+ ions remain the same, and pH remains constant.
However, if aqueous NaCl (or just plain water) is added to the acid solution, volume of the solution increases while the AMOUNT of H+ ions remain the same. Hence, concentration of H+ decreases, and the pH increases (less acidic due to lower conc. of H+ -> higher pH)
The previous thread on this question has been answered at a level for higher learning.
Solid NaCl does not undergo hydrolysis to dissociate into Na+ and Cl- ions. This will have no effect on the acidity and basicity of the resultant.
but i thought if i put solid NaCl the volume with increase by like super little but still increase so like eg HCl has 5 H+ ions and 1 OH- ion then when i put NaCl there will be like eg 3Na+ n 3Cl- so at first the conc of H+ is 5/6 X 100=83.3% but after it will be 5/12 X 100=41.7%? so the conc of H+ has decreased?
EDIT: i thought NaCl will dissociate in water so the total volume of water will increase?
Originally posted by SBST390:Solid NaCl does not undergo hydrolysis to dissociate into Na+ and Cl- ions. This will have no effect on the acidity and basicity of the resultant.
It does undergo hydration to form Na+ and Cl- ions (in A levels), and in O levels, you should know mobile charge carriers of Na+ and Cl- is present when aqueous NaCl.
Originally posted by SgStudentStressed:EDIT: i thought NaCl will dissociate in water so the total volume of water will increase?
Solid NaCl will dissociate in water , that's true, but remember put powder in water volume of water will not increase (fact)
Originally posted by Darkness_hacker99:The previous thread on this question has been answered at a level for higher learning.
Eh, I use quite a lot of O levels stuff, sorry for the unclear explanation tho'
Originally posted by SgStudentStressed:but i thought if i put solid NaCl the volume with increase by like super little but still increase so like eg HCl has 5 H+ ions and 1 OH- ion then when i put NaCl there will be like eg 3Na+ n 3Cl- so at first the conc of H+ is 5/6 X 100=83.3% but after it will be 5/12 X 100=41.7%? so the conc of H+ has decreased?
Nope
Concentration of H+ ions formulae
(Number of H+ / total volume of H20)
Originally posted by SBS n SMRT:Eh, I use quite a lot of O levels stuff, sorry for the unclear explanation tho'
Was refering to this
http://sgforums.com/forums/2297/topics/439413
oh ok that clears things up :) thanks for thr help
wait bt if i put 10cm3 of salt the volume will increawe hy 10 so the percentage of H+ will eecrease due to the increawee volume?
At O Level:
If you add in the salt NaCl, the change in volume will not result any change in the pH. The number of moles of H+ ions will still remain the same.
bt why does it work like this?
i thought like if the volume increase the conc of H+ ions decrease so pH increase? why wont the pH increaee?
okok so r these principles right
1) pH will only increase when the substance added into the acidic solution is in aqueous form ( NaCl (aq)) or in liquid form (H2O)
2) pH will NOT increase when the substance added into the acidic solution is a solid (NaCl (s)) despite it being a neutral salt.
Hence, solid stuff doesnt do anything only liquid or aqeuous stuff?
thanks so much n y did my 3 posts get deleted i checked the rules n didnt break any except the salts one (so sorry but cos it was like not updated)
Originally posted by StudentQns:okok so r these principles right
1) pH will only increase when the substance added into the acidic solution is in aqueous form ( NaCl (aq)) or in liquid form (H2O)
2) pH will NOT increase when the substance added into the acidic solution is a solid (NaCl (s)) despite it being a neutral salt.
Hence, solid stuff doesnt do anything only liquid or aqeuous stuff?
Yes.
um but why does it work like that? to aid my understanding, when calculating pH is it comparing the hyrogen ions against the hyroxide ions only or if there are Na+ or Cl- ions present it is also compared with? is its only the hydroxide ions then my understanding is: adding solid NaCl will not increase the OH- in the solution thus the conc of H+ ions dont change hence the pH does not increase.
Originally posted by StudentQns:um but why does it work like that? to aid my understanding, when calculating pH is it comparing the hyrogen ions against the hyroxide ions only or if there are Na+ or Cl- ions present it is also compared with? is its only the hydroxide ions then my understanding is: adding solid NaCl will not increase the OH- in the solution thus the conc of H+ ions dont change hence the pH does not increase.
At O level, you will only need to know it's the H+ and OH- ions that determines the acidity or alkalinity of a solution. Since solid NaCl doesn't have any H+ ions or OH- ions, pH remains the same.
When it goes higher up at A level above, you will learn about physical and other conditions that affect pH at an indepth level.