08 exam help!

[martin1106]

 Hi. Can someone help me with this question?? I read the answer in the checkpoints but I still can't figure out that why the resulting pH will not be 3.1 but between 2.1 and 3.1?

- vcaa 08 Q3) e)

[daniel034]

Acid 1 will increase in pH more than Acid 4. This occurs because Acid 1 is fully ionised, (-log0.1=1) meaning that when it is diluted by a factor of 10 the pH will increase by 1, resulting a pH of 2.

Acid 4 however does not completely ionise! Shown by it's pH being greater than 1. The question now turns into an equilibrium problem. The generic reaction for an Acid is HA + H2O <-> HA- + H3O+, as you have added water (through the dilution) the equilibrium will shift to the forward reaction as the system will try to oppose the change. This in turn increases the yield of H3O+, as H30+ increases the pH decreases.

Now the pH of the solution will not return to 2.1 as this will change the Equilibrium constant which cannot occur due to dilution (only by change in temp). Hence the pH will increase to a point between 2.1 and 3.1.

[martin1106]

Acid 1 will increase in pH more than Acid 4. This occurs because Acid 1 is fully ionised, (-log0.1=1) meaning that when it is diluted by a factor of 10 the pH will increase by 1, resulting a pH of 2.

Acid 4 however does not completely ionise! Shown by it's pH being greater than 1. The question now turns into an equilibrium problem. The generic reaction for an Acid is HA + H2O <-> HA- + H3O+, as you have added water (through the dilution) the equilibrium will shift to the forward reaction as the system will try to oppose the change. This in turn increases the yield of H3O+, as H30+ increases the pH decreases.

Now the pH of the solution will not return to 2.1 as this will change the Equilibrium constant which cannot occur due to dilution (only by change in temp). Hence the pH will increase to a point between 2.1 and 3.1.

Thank you  

[rebeccab26]

Hi. Can someone help me with this question?? I read the answer in the checkpoints but I still can't figure out that why the resulting pH will not be 3.1 but between 2.1 and 3.1?

- vcaa 08 Q3) e)

greater for acid I than IV because dilution has a greater effect on stronger acids [lower pH]