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### Question E.9 - Writing Molecular Formula + Mole Calculations

Posted: Sat Sep 28, 2019 6:09 pm
Hi! I'm having trouble with question E.9 part a) from the textbook on page F45:
Epsom salts consist of magnesium sulfate heptahydrate. Write its formula.

Figuring out the correct molecular formula for a given name of a compound was unfortunately not my strong suit in HS chem as my teacher kind of glossed over it and didn't spend too much time teaching us how. Would any of you guys be able to refresh me on the basics and give any tips/tricks you have on deriving a molecular formula from a compound's name?

BTW the answer I got was MgSO4 7H2O but i'm not sure about that formula. Thank you in advance for the help!

### Re: Question E.9 - Writing Molecular Formula + Mole Calculations

Posted: Sat Sep 28, 2019 6:29 pm
Your answer is correct as magnesium and sulfate both have a -2 charge, and thus forms MgSO4. Heptahydrate simply means 7 water molecules, so the entire formula would be MgSO4 · 7H2O.

(Also for tips I would say refresh on common polyatomic ions plus the prefixes).

### Re: Question E.9 - Writing Molecular Formula + Mole Calculations

Posted: Sat Sep 28, 2019 8:29 pm
Thank you! Conversely, if anyone has any refresher tips for deriving the name of the compound from the molecular formula that would be much appreciated. For exmaple, being given NH₄NO₃ and having to figure out the proper suffixes/prefixes.

### Re: Question E.9 - Writing Molecular Formula + Mole Calculations

Posted: Sun Sep 29, 2019 9:20 am
The example NH4NO3 is made up of two polyatomic ions. One is NH4+ and the other is NO3-. Because the charges balance out, you'll see that there is only one NH4+ and one NO3- ion. I think there is a list somewhere of good polyatomic ions to memorize by name, but NH4+ is ammonium ion and NO3- is a nitrate ion. Thus, putting the two together gives you ammonium nitrate. As far as proper suffixes, the reason nitrate has an -ate is because the central atom is bonded to 3 O, as opposed to bonded with less (2 Oxygens) which would give the suffix -ite.

### Re: Question E.9 - Writing Molecular Formula + Mole Calculations

Posted: Sun Sep 29, 2019 12:51 pm
For deriving the names of compounds from molecular formulas, memorizing the charges of common ions and polyatomic ions will go a long way in assigning the correct suffixes. Just for more practical advice, flash cards with all the common ions and charges for constant review helped me! :)

### Re: Question E.9 - Writing Molecular Formula + Mole Calculations

Posted: Mon Sep 30, 2019 12:05 pm
Does anyone know if we will have to memorize the basic polyatomic ions for tests?

### Re: Question E.9 - Writing Molecular Formula + Mole Calculations

Posted: Mon Sep 30, 2019 12:19 pm
Whenever you are naming a polyatomic compound such as this epsom salt start by identifying the cation and the anion. The cation for magnesium sulfate heptahydrate would be magnesium because it has a +2 charge. The anion in this instance would be the sulfate ion with a -2 charge. Now to write the formula for such a compound start with the cation and then name the anion. So the first part of this salt's formula would be the cation: magnesium (Mg) and the second the anion: (Sulfate) SO4. The second part of the salt is the heptahydrate. If you notice this is a hydrated salt. The "hepta" is simply a greek numerical prefix which represents the quantity 7. So heptahydrate means we have 7 molecules of H2O. Also, I do recommend memorizing the names of common polyatomic ions and numerical prefixes.