Exercise E.9

[Molly Posta 1H]

Question E.9 states: "Epsom salts consist of magnesium sulfate heptahydrate. Write its formula". I wrote MgSO4 as the formula for magnesium sulfate heptahydrate, but the answer key has MgSO4*7H20. Where did they get this? Why is water included in the formula and how do I know it should be written with it?

[Mahnoor_Wani_1I]

Heptahydrate means 7 water molecules. SO 7H20
Hepta= Seven
Hydrates= a compound, typically a crystalline one like this epsom salt, in which water molecules are chemically bound to another compound (MgS04) or an element

Basically anytime you see a hydrate you know that H20 is going to be in the formula

[Aliya 1H]

Hey! So if you look at "magnesium sulfate heptahydrate", you correctly recognized that magnesium sulfate is MgSO4. Then, you have to include the heptahydrate part now. Hydrate is, as you would think, H2O. Taking into account the prefix "hepta", which means 7, the heptahydrate part comes out to 7H2O and you include that in the total formula to get MgSO4*7H2O. (I apologize for not putting the numbers here in subscript bc I haven't figured that out yet)

[allyssa bradley 1H]

My friend made this same mistake on that problem, and a tip that I have is to double-check your Google search. So if you type in the molecular name INCLUDING heptahydrate, the Google pop-up (like the most common answer) doesn't include it. If you look at the bottom of the answer box, it does not say heptahydrate. As sort of a chem newb (or at least being rusty), I definitely was not immediately recognizing heptahydrate as meaning 7 waters when I saw this problem for the first time, but I was able to scroll down and see the Wikipedia page or something that properly wrote the full molecular name and, therefore, correct formula (including 7H2O).

[IanWheeler3F]

To answer the part of your question pertaining to why it is hydrated is: a lot of (if not all) metals often form these "coordination complexes" where they bond with certain elements and molecules and ions (most notably water, ammonia, halides, etc.) and since water is extra "sticky" magnesium sulfate forms these types of bonds with water in the air, hydrating it (please correct me if any of this is wrong, its been a while)

[ZachMLB963]

Hiya!

A "hydrate" has water molecules included in the compound. Because this is a "heptahydrate," there are 7 water molecules added.

[Thomas Gimeno]

The key is the last part of the name: heptahydrate. When hydrate is at the end of the name of a chemical it means water molecules are incorporated into the structure of that chemical. In this case there are seven molecules of water because the prefix hepta means seven. When it comes to writing this, you just write MgSO4*7H2O (i'm pretty sue the star is supposed to be a little dot right in between them but I couldn't find that symbol)