[INeedSerotonin]

A pH indicator gives us blue for basic; green for neutral; and yellow for acid.

Considering that the reagents in aqueous solution (aq) react in equal amounts of volume and have the same concentrations in mol per liter, point out the alternative that contains reagents that form products whose final solution is green-colored in the presence of this indicator.

(A) Na2O + H2O --->
(B) Mg(OH)2(aq) + H2SO4(aq) --->
(C) SO2 + H2O --->
(D) Al2(SO4)3 + H2O --->
(E) H3PO4(aq) + KOH(aq) --->

(I marked the correct answer above)

Could you guys help me with this one? I think that it should be a result of a strong base and a strong acid, or perhaps, also, a weak base and a weak acid.

So it cannot be (A) (as it makes a strong base), nor (C) (as it makes an acid). So we have either (B), (D), or (E).

How to proceed?

Thank you

[hypervalent_iodine]

One of the key things in the question is the point about equal amounts. IOW, the same number of moles of each would be reacting. Given this, what do you suppose the products would be of each reaction? Are those products acidic, basic or neutral?

[AWK]

http://www.chembuddy.com/?left=pH-calculation&right=pH-amphiprotic-salt

[INeedSerotonin]

One of the key things in the question is the point about equal amounts. IOW, the same number of moles of each would be reacting. Given this, what do you suppose the products would be of each reaction? Are those products acidic, basic or neutral?

I think that the products would be these:

(A) Na₂O + H₂O ---> NaOH, basic

(B) Mg(OH)₂(aq) + H₂SO₄(aq) ---> 2 H₂O + MgSO₄; it comes from a weak base and a strong acid, so I think it should be acidic. However, somehow, Google tells me it is neutral.

(C) SO₂ + H₂O ---> H₂SO₃, acidic.

(D) Al₂(SO4)₃ + H₂O ---> no idea; I think it just dissociates, right? 2 Al⁺ + 3 SO₄^2-. I think that the cations get close to the OH^-, and the anions get close to the H⁺, so probably there is some weak base being formed and strong acid being formed. So I guess, acidic?

(E) H₃PO₄(aq) + KOH(aq) ---> H₂O + H₂PO₄, which, I suppose, is acidic

So I think the only options that could be the answer are (B) or (D).

http://www.chembuddy.com/?left=pH-calculation&right=pH-amphiprotic-salt

Thanks for the link, but I don't know how to apply it in the exercise, as I wasn't given any value of concentration, pH or Ka.

[AWK]

(D) Al2(SO4)3 + H2O

Hydrolysis of Al³⁺ (Al³⁺ in water is a bit more acidic than acetic acid)

B. search in Google  "pH of MgSO4"

(E) H3PO4(aq) + KOH(aq) ---> H2O + H2PO4, which, I suppose, is acidic

wrong reaction, the link given by me informs on the acidity of this salt.

[INeedSerotonin]

(D) Al2(SO4)3 + H2O

Hydrolysis of Al³⁺ (Al³⁺ in water is a bit more acidic than acetic acid)

B. search in Google  "pH of MgSO4"

(E) H3PO4(aq) + KOH(aq) ---> H2O + H2PO4, which, I suppose, is acidic

wrong reaction, the link given by me informs on the acidity of this salt.

Thank you. I had a lapse of attention here, but I think that (E) has to be an incomplete reaction, as the exercise tells me all of them have the same concentration/molarity.

So perhaps it should be (E) H3PO4(aq) + KOH(aq) ---> H2O + KH2PO4 ?

I know that the complete reaction is (E) H3PO4(aq) + 3 KOH(aq) ---> 3 H2O + K3PO4, but the uttering tells me they have the same concentration, so I think it is not possible.

But how can I find which one is closest to pH = 7 without any values for concentration, pH or Ka? This is what confuses me the most.

[AWK]

See the link I gave. Some salts show an almost constant pH regardless of concentration. And so it is in the E reaction - provided that you write it correctly.

[INeedSerotonin]

I think I got it now. It took me some time, but I got it. Thank you!