Quick chem Q

[portlander]

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Sorry for posting so many DAT questions, but hopefully the discussions are helping those of us who need it!!

Anyways, here it goes:

The formula of the compound formed by elements Al and S is most likely:

A. Al3S2
B. AlS
C. Al2S3
D. Al3S2
E. None of these

The key says 'C'. I've done these types of problems before (mostly involving halogens) but is there an easy way to do this? Any chem tricks I should be aware of??

PS I took a full length topscore today and got 22 PAT/19 Ac. I am STRESSED about my sceinces and math! I got so many random invertabrate questions today!! Who the heck cares about a trematoda, rotifers, and snails!!!

 

[bres]

Find out the # of valence electrons of those atoms first.

For example, S=6 need 2 more electrons Al=3 lose 3 electrons, when forming ionic bonds.
S2- + Al3+-->Al2S3

 

[UBTom]

Yep that's easy.

It's all about valence electrons dude.

If you take a look at Aluminum on the Periodic table, you will see it's a Group IIIA element-- It has 3 valence electrons to give. And Sulphur is a Group VIA element-- It has 6 valence electrons (needs 2 more valence electrons to complete its outer shell).

So you need two Aluminums to give up a total of 6 valence electrons to complete the outer shells of 3 sulfurs.

So anytime you see a problem like that, look at the elements' groups on the periodic table, and think valence electrons!

Good luck!

edit: Whoa, bres there beat me to the "Post message" button by 60 seconds! I must be getting old..

 

[hockeydentist]

Just wondering if you get the element chart on the dat?

that would be a hard question if you didn't get the chart. Otherwise you would have to remeber the charges, and who can remeber all the charges and #$$%

later hockey dentist

 

[UBTom]

I would think that a standard reference source like the periodic table should be available on an exam if needed.

Shee-oot, it's been such a long time since I took the DATs I can't remember if I saw one in the tests..

 

[ItsGavinC]

The periodic table is available on the DAT.