writing the formula of a molecule

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Jackie Ngo 1C
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Joined: Fri Sep 29, 2017 6:06 am

writing the formula of a molecule

Postby Jackie Ngo 1C » Thu Oct 04, 2018 10:30 pm

hi! I'm stuck on question E9 of the Review of Chemical & Physical Principles. it lists that I need to write magnesium sulfate heptahydrate as a formula. how would I do that/read it? thanks so much for your time!

Henri_de_Guzman_3L
Posts: 88
Joined: Thu Sep 27, 2018 11:25 pm

Re: writing the formula of a molecule

Postby Henri_de_Guzman_3L » Thu Oct 04, 2018 10:35 pm

A magnesium ion is Mg2+
Sulfate is SO4 2-
Together that makes MgSO4

The heptahydrate basically just adds H2O at the end. Hepta- is a prefix for 7.

So the official formula is MgSO4 . 7H2O
In chemistry, it's standard to add a little dot between the compound and the hydrate

maldonadojs
Posts: 40
Joined: Thu Sep 27, 2018 11:27 pm

Re: writing the formula of a molecule

Postby maldonadojs » Thu Oct 04, 2018 10:39 pm

I began this problem writing Magnesium Sulfate as MgSo4. Then the prefix hepta- refers to the number 7. Hydrate also refers to just water so it would result in MgSo4 7H2O.

Jovian Cheung 1K
Posts: 48
Joined: Thu Sep 27, 2018 11:16 pm

Re: writing the formula of a molecule

Postby Jovian Cheung 1K » Thu Oct 04, 2018 10:40 pm

Magnesium is Mg. It forms the ion Mg2+.
Sulfate refers to the sulfate ion, which is a polyatomic ion with formula SO42-.
As their charges have the same absolute value, which is 2, their ratio in an ionic compound is 1:1.
Thus Magnesium Sulfate is MgSO4. (As you see, the 2+ and 2- cancel each other out.)

As for heptahydrate, hepta- refers to seven, and -hydrate refers to water/H2O.
Thus the heptahydrate means 7H2O.

Combining the two, magnesium heptahydrate is MgSO4 • 7H2O
I'm guessing that sections about ionic bonding and polyatomic ions (Dr Lavelle said we'd eventually memorize the frequently seen ones) would be related to this problem. Can also check out hydrated ionic compounds / hydrates, as this is an example of one. Hope this helps! :0)

Ruiting Jia 4D
Posts: 46
Joined: Thu Sep 27, 2018 11:27 pm

Re: writing the formula of a molecule

Postby Ruiting Jia 4D » Thu Oct 04, 2018 10:53 pm

Henri_de_Guzman_3L wrote:A magnesium ion is Mg2+
Sulfate is SO4 2-
Together that makes MgSO4

The heptahydrate basically just adds H2O at the end. Hepta- is a prefix for 7.

So the official formula is MgSO4 . 7H2O
In chemistry, it's standard to add a little dot between the compound and the hydrate


For quizzes/tests, would we have to automatically know how to add hydrates to compounds? I understand where the + 7H2O comes from, but I was wondering if we had to memorize prefixes for numbers such as hepta- in order to compose a chemical equation or formula.

Henri_de_Guzman_3L
Posts: 88
Joined: Thu Sep 27, 2018 11:25 pm

Re: writing the formula of a molecule

Postby Henri_de_Guzman_3L » Tue Oct 09, 2018 12:54 pm

While Dr. Lavelle didn't explicitly state to do so, I think it would generally be a good idea in chem to know these anyway. They're not hard! I pasted them below, they're pretty intuitive:

1: mono-
2: di-
3: tri-
4: tetra-
5: penta-
6: hexa-
7: hepta-
8: octa-
9: nona-
10: deca-


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