[JBake010]

Hello all,

I am almost done with my General Chem I class, and a requirement of the class is that I pass a nomenclature exam. The exam will have 30 compounds on it, 15 with the formula (requiring me to write the name), and 15 with the name (requiring me to write the formula). I have tried to study for it a little bit, but I seem to be struggling with it. I believe I am struggling the most with the use of the roman numerals in certain compounds. My professor has tried to explain it, but I still can't seem to get a grasp on it.

If anyone has a good explanation of the usage of roman numerals, as well as any tips/good websites to study for the nomenclature, I would really appreciate it. I am going to try some more practice worksheets this week, and I will post which ones I got wrong, and maybe someone can help me understand why.

Thanks for any help you guys can offer,

Jordan

[XGen]

Compounds containing roman numerals are ionic compounds. The roman numeral exists to indicate the charge on the cation. To write the formula with a name containing roman numerals, simply balance the anion charge with the roman numeral.

For example, lead(II) oxide is PbO, and lead(IV) oxide is PbO₂. In lead(II) oxide, the roman numeral is two. That means that the cation, lead, exists with charge 2+, leaving the Pb²⁺ ion. Since oxygen is 2-, 2+ and 2- equal zero and the formula is PbO. The same process can be followed for lead(IV) oxide, except lead(IV) is now Pb⁴⁺ and requires two oxygens to balance the charge.

[JBake010]

Hey XGen, thanks for the tips! I used the information you gave me, as well as some from some YouTube videos I found, and now I'm beginning to get a grasp on the nomenclature.

I did a practice quiz today with 30 problems on it, and so far from looking online, I've gotten most right. But there are a few I got wrong and had questions about, so I will post them below and maybe one of you can offer some insight.

Nickel (II) bromide hexahydrate -  I didn't know where to start on this one. How would you write out the chemical formula with something that had 3 parts?

Ammonium phosphate - I wrote down the chemical formula as (NH₄)₃PO₄, but a website I went to said that it's H₁₂N₃O₄P?

Potassium dichromate - Again, I wrote this down as K(Cr₂O₇), but a website I went to said that it's K₂(Cr₂O₇)? Both K and Cr₂O₇ have positive charges, so how would I know what sub number to put on the K in order to balance them out?

KC₂H₃O₂ - Again, how would I go about naming something that has 3 parts?

FeSO₄ · 7H₂O - No idea where to start on this one; what does the · mean in regards to chemistry?

H₃PO₄ - I had this written down has hydrogen phosphate, but I've seen everyone else call it phosphoric acid. Why is that?

And then two chemical formulas that I had answers to, but was unable to find answers to on the web (to check mine) were Pb(NO₃₎)₃ which I had as lead (III) nitrate, and Pb₃N₂, which I had as trilead dinitride. Are these correct?

Thanks to any help you all can offer.

[fledarmus]

Nickel (II) bromide hexahydrate -  I didn't know where to start on this one. How would you write out the chemical formula with something that had 3 parts?

A hydrate means that it has associated waters - that is usually indicated in a formula by adding a dot and then the water. In your case, NiBr₂^.6H₂O

Ammonium phosphate - I wrote down the chemical formula as (NH₄)₃PO₄, but a website I went to said that it's H₁₂N₃O₄P?

I prefer your formula, because it shows the structure of the compound. The formula from the website you looked at gave only the empirical formula.

Potassium dichromate - Again, I wrote this down as K(Cr₂O₇), but a website I went to said that it's K₂(Cr₂O₇)? Both K and Cr₂O₇ have positive charges, so how would I know what sub number to put on the K in order to balance them out?

At this level, probably easiest just to memorize that the dichromate ion has a charge of -2  (Cr₂O₇^2-). Later you will learn the structures of complex anions like phosphates, chromates, sulfates, and chlorates, and this will make more sense.

KC₂H₃O₂ - Again, how would I go about naming something that has 3 parts?

Again, this is a structural thing that for now you will have to memorize. The "C₂H₃O₂ is the acetate anion, and has a charge of -1. The name for this compound would be potassium acetate. See http://en.wikipedia.org/wiki/Acetate

FeSO₄ · 7H₂O - No idea where to start on this one; what does the · mean in regards to chemistry?

See the previous explanation - that dot and the H₂O's mean associated water, and the compound would be named as a -hydrate, with a prefix (in this case "hepta") indicating how many associated waters - Iron(II) sulfate heptahydrate

H₃PO₄ - I had this written down has hydrogen phosphate, but I've seen everyone else call it phosphoric acid. Why is that?

Because when hydrogen is associated with complex anions or with halogens, it is acidic. Another quirk about naming. If there are other metals present, this isn't the case, but dihydrogen sulfate (H₂SO₄) is called sulfuric acid, dihydrogen carbonate (H₂CO₃) is called carbonic acid, hydrogen chloride (HCl) is called hydrochloric acid. If there is another metal present, the name usually reverts back to hydrogen - for example, K₂HPO₄ would be dipotassium hydrogen phosphate.

I hope some of this is useful - good luck.

[JBake010]

Thank you to everyone who helped out! I took my nomenclature exam last Thursday and got a 29/30.

The only mistake I made was forgetting to put the correct subscript on one of my formulas. Now that I've passed this I'm on to the chemical reactions exam and some Lewis structures.

Thanks again!