Dec 14, 2009
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Pre-Dental
I always get wrong a question like

"When dissolved in water, which compound produce acid or base?

Please explain me in details. I am so frustrating for this part.

One of the examples is
Which of the following substances would produce a basic solution when dissolved in water
a) CO2
b) H3AsO4
c) Hcl
d) Na2O
e) SO3

answer is d
 
Last edited:

UndergradGuy7

10+ Year Member
Jun 23, 2007
897
2
241
Status
Pre-Dental
I always get wrong a question like

"When dissolved in water, which compound produce acid or base?

Please explain me in details. I am so frustrating for this part.

One of the examples is
Which of the following substances would produce a basic solution when dissolved in water
a) CO2
b) H3AsO4
c) Hcl
d) Na2O
e) SO3

answer is d
CO2 + H2O makes H2CO3 which turns into HCO3- and H+. This is just like in respiration in blood.

H3AsO4 it clearly has 3 H's so it will make H3O+.
HCl... another acid makes H3O+

Next two aren't as easy... but you should know SO3 + h2o = h2so4

so this leaves na2o + h2o if we split it apart we get 2na+ and O^2- + h2O
if you try pairing the h from h2o with anything it obviously will pair with the o2- since it is a minus charge and not na+. This makes oh-.
 

loveoforganic

-Account Deactivated-
10+ Year Member
Jan 30, 2009
4,222
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0
Status
I always get wrong a question like

"When dissolved in water, which compound produce acid or base?

Please explain me in details. I am so frustrating for this part.

One of the examples is
Which of the following substances would produce a basic solution when dissolved in water
a) CO2
b) H3AsO4
c) Hcl
d) Na2O
e) SO3

answer is d
The key is recognizing what these structures look like.

CO2 is a diketone. That means the carbon is very electron deficient and will act as a lewis acid.

H3AsO4 is the arsenic analogue of phosphoric acid (H3PO4). You should recognize this due to arsenic appearing in the same group as phosphorus.

HCl - hydrogen bound to a halogen. Chloride is a very weak conjugate base (stable anion), you should recognize this.

Na2O - you have 2 Na+ (stable cations, don't accept OH- from water). O2- on the other hand is a very strong base. You should recognize this since OH- is a strong base.

SO3 - 6 oxygen bonds to sulfur. Oxygen, being highly electronegative, is going to withdraw electron density, making the sulfur center act as a lewis acid.
 

Cool Beans

7+ Year Member
Apr 5, 2010
83
7
151
Status
I always get wrong a question like

"When dissolved in water, which compound produce acid or base?

Please explain me in details. I am so frustrating for this part.

One of the examples is
Which of the following substances would produce a basic solution when dissolved in water
a) CO2
b) H3AsO4
c) Hcl
d) Na2O
e) SO3

answer is d
Let's simplify this some. You should recognize HCl and H3AsO4 as acids right off the bat. After that it comes down to a simple rule:

Non-metal oxides (like CO2 and SO3) are acidic.
Metal oxides (like Na2O) are basic.

Others spoke of the reasons why so I won't be redundant but the important part is being able to recognize which compound when dissolved in water is acidic or basic.