Writing Compound Formulas

Moderators: Chem_Mod, Chem_Admin

Jasmine 2C
Posts: 125
Joined: Wed Sep 18, 2019 12:18 am

Writing Compound Formulas

Postby Jasmine 2C » Mon Sep 30, 2019 8:23 pm

Honestly, I don't know which is the correct place for this topic but this seems to be the best place. Anyways, I was wondering how to figure out what the formulas for compounds are through their names. For example, question E9, says "Epsom salts consist of magnesium sulfate heptahydrate. Write its formula." How exactly are we supposed to figure out the formula with just this? By the way, I have a very weak high school chemistry background so please bear with me! :(
And along the lines of this topic, would we need to be able to write out the proper name of a formula, and vise versa, for this course? For example, C02 is carbon dioxide and carbon dioxide is C02.

Jainam Shah 4I
Posts: 79
Joined: Fri Aug 30, 2019 12:16 am

Re: Writing Compound Formulas

Postby Jainam Shah 4I » Mon Sep 30, 2019 8:31 pm

To answer your question, we will have to be able to name compounds and polyatomic compounds as it was included in the fundamentals. I first recommend you memorize the names of common polyatomic ions like sulfite, sulfate, permanganate, and many more. This will help you in writing the formulas given a compound's name. Now to answer your initial query about magnesium sulfate hepta hydrate, the first step is to recognize this is a polyatomic compound surrounded by water aka hydrated by water molecules. Let's break down this compound by parts. The first part is magnesium sulfate. The cation in this compound is Magnesium because it has a net +2 charge and the anion is sulfate which has a net -2 charge. We name compounds by the cation first. So the formula would be MgSO4. Now the heptahyrdate part is simply a numerical prefix. The greek numerical prefix heat means 7. Heptahydrate indicates we have 7 molecules of H2O. Thus, the formula would be MgSO4 7H20.

Zaynab Hashm 2I
Posts: 76
Joined: Wed Sep 11, 2019 12:15 am

Re: Writing Compound Formulas

Postby Zaynab Hashm 2I » Mon Sep 30, 2019 8:37 pm

For this certain example you mentioned (Magnesium Sulfate Heptahydrate), you need to at least know the symbols for certain elements from the periodic table. Magnesium is Mg, so you write that down first as it comes first in the name, then you add sulfate which is SO4(2-), for this one you should recognize from the name that sulfate contains sulfur (S) and -ate indicates that the compound has one more oxygen; so basically 4 instead of 3, thus, SO4 and not SO3. Lastly, hepta is seven in Greek prefixes, and hydrate refers to water. If you add all that up you get; MgSO4.7H2O. Does that make sense?

Andrew Jang 4D
Posts: 50
Joined: Sat Aug 17, 2019 12:18 am

Re: Writing Compound Formulas

Postby Andrew Jang 4D » Tue Oct 01, 2019 12:21 am

It’s very easy to panic when you’re asked to write the formula of some compound that has a long and unfamiliar name; this was my Achilles heel in high school chemistry and I struggled due to not being able to write these out. However, it’s not as bad as I initially thought! Like the previous replies mentioned, if you try to break down the compound name into parts you’ll have a better sense of what you’re trying to write out. Realizing that Mg has a +2 charge and that sulfate (SO4) has a -2 charge makes it make more sense, and as mentioned before hepta is 7. I also recommend that you really make an effort to familiarize yourself with these parts because if you encounter sulfate in another compound and realize it’s SO4 with a -2 charge it’ll make your life a little less stressful :)


Return to “SI Units, Unit Conversions”

Who is online

Users browsing this forum: No registered users and 1 guest