DAT question dilemma! - Please help!!!

[fleur531]

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hi you guys!! i can't seem to figure out one particular question type that i notice i keep missing every time during practice tests... here is one example of the question---

Which of the following substances would produce a basic solution when dissolved in water?
A. CO>2
B. H>3AsO>4
C. HCl
D. Na>2O
5. SO>3

The answer is D. How do you go about solving these types of questions? They also ask about which substance will produce an acidic solution when dissolved in water..... What's the general approach one should take to solve these questions??...I can't seem to understand what concept this question is testing....Any references would be appreciated as well!!.. Your help is very much appreciated!!!!

Thanks a bunch in advance! Best wishes and happy studying to all!!!!

 

[MattH25]

Unfortunately this looks like it may be a time trap question. if you don't see the answer right off you could easily waste a few minutes solving it. You will need to write out a reaction for each of the answer (or write them out till you get a reaction that produces a base). Since you are mixing these in an aqueous solution the listed substance and water will be your reactants.

A.) CO2 + H2O ----> H2CO3 (carbonic acid...so it would be a acidic)
B.) H3AsO4 + H20 -> H3O+ + H2AsO4 (arsenic acid would donate proton to water giving hydronium)
C.) HCl + H2O ---> H3O+ (should be pretty clear..hydrochloric acid...definitely acidic)
D.) Na2O + H2O ---> 2 OH- (there's our base so it would give a basic soln)
E.) SO3 + H2O ---> H2SO4 (sulfuric acid so acidic solution)

 

[fleur531]

that explains it perfectly.... thanks so much Matt!!!