Acids And Bases Question

[tdubthebassist]

Which compound, when dissolved in aqueous solution, has the highest pH?

A. NaCl

B. Na2CO3

C. NH4Cl

D. NH4NO3

I am really weak at these kind of question.

Probably it is just my pure disgust and abortion towards Acids and bases

[LaFin]

In this type of questions, you need to focus on how much H+ or OH- involved in rxn. For calculating pH level, you need to calculate how many hydrogen ions are in the solution&rxn.

[HeyItsHales]

Well, a high pH means it's a strong base. So, if you formulate reaction equations with all of the choices and water, you'll come up with some sort of product. Whichever product is the strongest base would be your answer.

[soadquake981]

Wouldn't it be choice B? NaCl in water is just salt water -- no pH change. Na2CO3 will dissociate into Na(+) and CO3(2-)... and carbonate is a Bronsted-Lowry base when dissolved in water because it accepts a proton to become bicarbonate, right?

Both C and D have ammonium ions, which will donate protons.

[master135]

It should be B. Just thinking about how the salt was made (for ex. strong base + weak acid so the salt will result in a more basic solution ie. high pH) is the easiest and quickest way to answer salt hydrolysis questions.

[x___x]

for questions like these, you should try to write the equations for the ions in solution:

A. NaCl, this would have the following equation with water molecule:

NaCl + H2O --> NaOH + HCl

the neutral solution that will result from adding NaCl to water is due to the neutralisation of NaOH with HCl. thus, pH would be 7 exactly.

B. Na2CO3, the equation for this one:

Na2CO3 + 2H2O --> 2NaOH + H2CO3

H2CO3 is a weak acid, while NaOH is a strong base, thus, the overall pH would be above 7.

C. NH4Cl:

NH4Cl + H2O --> HCl + NH3 + H2O

HCl is a strong acid, while NH3 is a weak base, thus, the overall pH is less than7.

D. NH4NO3:

NH4NO3 + H2O --> HNO3 + NH3 + H2O

HNO3 is a strong acid, while NH3 is a weak base, thus the overall pH is less than 7.

now that everything is clear, it's easy to spot that the Na2CO3 would produce a solution with the higher pH, since it's most basic.

note, though, that, technically speaking, the equations above are wrong, a salt does not react with water, it only forms ions. but you can use this method to help you decide which salt will ionize into the highest/ lowest solution.

good luck, hope you got it.

Edited May 15, 2010 by x___x