TBR CBT#1 -- Formal Charge of Cis-Platin

[Majik]

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What is the formal charge of platinum in cis-platin?

A. 0
B. +2
C. +4
D. -2

I'm confused as to how we're expected to find the formal charge of platinum. TBR explains: B is the best answer. Platinum in cis-platin has a +2 charge, because the two chlorine atoms have negative charges and the two ammonias are each neutral. For the overall compound to be neutral, platinum must carry a +2 charge, and thus have a formal charge of +2. The best answer is B.

I dug up older threads, but I'm still not really understanding this. Are we considering the charges on each ligand (ammonia and chlorine) before their bound to Platinum? Do we sum up the charges on all the ligands to figure out what the charge has to be on Platinum (in order for the total charge to be zero)? In otherwords, ammonia is neutral and each chlorine originally has a -1 charge (-2 total). Therefore, Platinum must have a +2 charge to balance this out?

I'm so confused.

 

[docntrainin]

No worries man, I missed this one too. When I first read the answer, I thought that they had meant to say oxidation state instead of formal charge... but they didn't. So I think this is a special type of question because it is dealing with a complex as opposed to a molecule with covalent bonds. So I think when you are dealing with complexes, take each of the atoms involved as if they are separate i.e. Cl is usually found as an anion with a -1 charge (2x-1 = -2) and ammonia is usually neutral (0), hence the Pt must have a charge of 2+ to cancel out. Here is a site I found that might help as well! G luck studying... TBR4 for me tomorrow! http://www.chemguide.co.uk/inorganic/complexions/shapes.html

 

[Majik]

No worries man, I missed this one too. When I first read the answer, I thought that they had meant to say oxidation state instead of formal charge... but they didn't. So I think this is a special type of question because it is dealing with a complex as opposed to a molecule with covalent bonds. So I think when you are dealing with complexes, take each of the atoms involved as if they are separate i.e. Cl is usually found as an anion with a -1 charge (2x-1 = -2) and ammonia is usually neutral (0), hence the Pt must have a charge of 2+ to cancel out. Here is a site I found that might help as well! G luck studying... TBR4 for me tomorrow! http://www.chemguide.co.uk/inorganic/complexions/shapes.html

Thanks! Their CBT's are so confusing. How've you been doing on them? I missed 14 questions on the PS section alone lol. I still have to finish the verbal & bio section. I'm gonna sit down for the rest as if I'm taking the real deal.

 

[docntrainin]

I have been taking them as full lengths. I have been scoring around 29/30-31 for each of them. I am thankful for the extra problems in addition to the AAMC's. I'm retaking the MCAT so I needed new material.