gen chem question

[drgummybear]

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Which of the following salts has a pH>7?

KBr
Na2SO4
CsI
RbNO3

I narrowed it down to Na2So4 and RbNO3. The answer is Na2SO4. However, I'm not sure why RbNO3 wouldn't be correct as well?

 

[TexasDDS]

on this problem you want to see which one is a base

so when you do
KBr + H2O --> KOH + HBr which is netural so ph = 7
Na2SO4 + 2H2O --> 2NaOH + H2SO4.. so you have 2 strong bases and 1 strong acid
CsI + H2O --> CsOH + HI neutral so ph = 7
RbNO3 + H2O --> RbOH + HNO3 neutral so ph = 7


I believe that should be the reason behind it.. where did you get this question from?

 

[DoCt0rPeTe]

Which of the following salts has a pH>7?

KBr
Na2SO4
CsI
RbNO3

I narrowed it down to Na2So4 and RbNO3. The answer is Na2SO4. However, I'm not sure why RbNO3 wouldn't be correct as well?

2 moles of Na thus 2 moles of NaOH for a mole of acid.

 

[mightyrat]

First before you can tackle this problem, make sure you know your strong acids and bases!

There is a little trick that works all the time (with an exception to one compound) which makes problems like these a joke.

Write your compound, in this case you have Na[2]SO[4], then directly under that write water in this form HOH. Then you just criss cross the elements, the OH part of water goes to the Na, and the H goes to the SO[4]. You have now formed 2NaOH (a super strong base) and HSO[4] (an ehhh acid, the better form would have been the H[2]SO[4] )

You now have a strong base with an okay acid which would give you a pH above 7, not too basic though like 11, but above 7, like 9 or so.

The reason RbNO[3] would not give you a pH above 7 because it is a neutral salt. Use the trick i used above and you will get RbOH, a super strong base, and HNO[3], a super strong acid. In that case you will get a neutral salt around the pH of 7

if your still having trouble, look at problem 117 in the 2010 Destroyer, in it a list of strong acids/bases are given and this trick is done more visually!

Hope this helps 😀

 

[DoCt0rPeTe]

First before you can tackle this problem, make sure you know your strong acids and bases!

There is a little trick that works all the time (with an exception to one compound) which makes problems like these a joke.

Write your compound, in this case you have Na[2]SO[4], then directly under that write water in this form HOH. Then you just criss cross the elements, the OH part of water goes to the Na, and the H goes to the SO[4]. You have now formed 2NaOH (a super strong base) and HSO[4] (an ehhh acid, the better form would have been the H[2]SO[4] )

You now have a strong base with an okay acid which would give you a pH above 7, not too basic though like 11, but above 7, like 9 or so.

The reason RbNO[3] would not give you a pH above 7 because it is a neutral salt. Use the trick i used above and you will get RbOH, a super strong base, and HNO[3], a super strong acid. In that case you will get a neutral salt around the pH of 7

if your still having trouble, look at problem 117 in the 2010 Destroyer, in it a list of strong acids/bases are given and this trick is done more visually!

Hope this helps 😀

yo mightyrat shouldn't you be studying? I wanna see those 30's next week bro!

 

[mightyrat]

yo mightyrat shouldn't you be studying? I wanna see those 30's next week bro!

as you can see im extremely bored and fed up with this crap!! 😡 i was productive though i did a math destroyer exam and did destroyer oh and some PAT so i decided to show off a bit on this forum 😛 if i cant show off like you with those 30s then i might as well get a head start lol

 

[drgummybear]

ahhh i see! thanks so much for the replies everyone!
mightyrat, thanks esp for ur explanation 🙂

texasDDS, i got this question from chad's videos. they do provide a reasoning but you have to get the answer wrong for it to show up. i guessed na2so4 and got it right lol but i didn't know why it was right lolsss.

 

[DQLEUCD]

Which of the following salts has a pH>7?

KBr
Na2SO4
CsI
RbNO3

I narrowed it down to Na2So4 and RbNO3. The answer is Na2SO4. However, I'm not sure why RbNO3 wouldn't be correct as well?

You will have to go through the solubility rules and see which is soluble and neutral. All nitrates are soluble therefore it will be neutral in the compound. Also all strong acids have a very weak base, so weak that it's negligible. As you can see, NO3- is the conjugate base of the strong acid HNO3, HI to I-, and HBr Br-. HSO4 - is the weak base of H2SO4 Strong acid. Thus SO4- is a STRONGER conjugate base (does not mean it's a strong conjugate base, you will need to know your strong base rules for this one) then it's weak acid HSO4- thus the pH will be greater then 7. Also all group 1 and part of group 2 metals are soluble thus will not contribute anything to the pH.