# Chemical Formulas

1. Oct 7, 2004

Hello

I am currently enrolled in General Chem. We are learning how to write chemical formulas. Can someone please explain to me how you write a chemical formula given the name of the compound? Like for example, how would you write the chemical forumla for calcium hydrogen carbonate?

Any help is greatly appreciated!

Thanks

2. Oct 7, 2004

### ShawnD

Just write it the way you hear it.
$CaHCO_3$

Just remember to write the cations (positive ion) first. Writing something like $CO_3HCa$ would probably be marked incorrect by your instructor.

Last edited: Oct 7, 2004
3. Oct 7, 2004

### jreason

take the charge on the cation, say $$Ca^+^2$$ and then the charge on the anion hydrogen carbonate $$HCO_3^-$$ and you will criss cross the charges to form $$Ca(HCO_3)_2$$

other examples:

$$Fe^+^3$$ and $$ClO_4^-$$ forms $$Fe(ClO_4)_3$$

$$Ca^+^2$$ and $$Cl^-$$ forms $$CaCl_2$$

4. Oct 8, 2004

### chem_tr

Some more complex examples will come from me:

$Ca_3(PO_4)_2$ is made from $Ca^{2+}$ and $(PO_4)^{3-}$.

There are some ions like hexacyanoferrate(3-), a.k.a. $[Fe(CN)_6]^{3-}$. If you mix this solution with an $Fe^{2+}$ solution, you'll get the famous "Berlin Blue". This compound is, therefore, $Fe_3[Fe(CN)_6]_2$

Well, it is possible that my reply is too complex for your needs, but you might benefit this message later.

Last edited: Oct 8, 2004
5. Oct 18, 2004

How do you know the charges of the cations and anions? Do you have to look them up?

6. Oct 18, 2004

### chem_tr

It is somewhat close to what you're implying, just be patient and go on studying these ions to "memorize" them.

7. Jan 7, 2007

### Michelle2007

formulas

Hi,

I'm in HS Chemistry and am working on a review and didn't bring my book home....I'm trying to make sure I'm doing this right. I need to write the correct formula for quite a few elements. One example is for:

a. magnesium and oxygen

Can anyone tell me how to write the formula?

Thanks

8. Jan 7, 2007

### Gokul43201

Staff Emeritus
1. Next time you've got a question, please start a new thread rather than posting in an older one. Also, next time, a question like this would belong in the Homework & Coursework subforum.

2. By the rules of this forum, we can not help you unless you first show us your effort. You say you want to make sure you are doing this right. Show us what you've done so far, and we'll tell you how you're doing and help you if you are getting stuck on something.

3. Make sure you write down the original question EXACTLY as it was given to you. What you've written here has errors in it, but that's possibly because you're rewriting it from memory rather than copying down exactly what was given to you.

Last edited: Jan 7, 2007
9. Jan 7, 2007

### Michelle2007

Courswork question

Hi,

Sorry, I am not sure how this works. Since you said next time I'm assuming that it is ok to continue here....if not I'll start a new thread.

The question is written:

1. Write the correct formula for the following (a-f)

a. magnesium & oxygen My answer is Mg6O2

b. calcium & nitrogen My answer is CA5N2

Thanks,

10. Jan 7, 2007

### cristo

Staff Emeritus
Your answers are not correct. You need to know the charges on the individual ions, and then ensure that when they form a compound together that the charge on the compound is zero.

For 1. Do you know the charges on Mg and O ions?

11. Jan 7, 2007

### Michelle2007

No, I don't know. We just have to look at the periodic table and then write the formula for magnesium & oxygen and so on.

12. Jan 7, 2007

### Gokul43201

Staff Emeritus
As stated, the question is only asking for the formulas for the elements and not their reaction products - but it's funny that the elements be paired up that way. It's not clear, however, if the question does want you to write down the formula for the reaction product. We might be able to guess better if you wrote down parts c,d,e,and f as well.

Do you understand the difference between writing the formula for the elements separately and writing the formula for the compound they will form when they react with each other?

It's best to clarify with the teacher what the question really wants.

13. Jan 7, 2007

### Michelle2007

I can ask my teacher tomorrow. I need to write the formula for the compound they will form.
a. magnesium & oxygen
c. calcium & nitrogen
d. lithium & sulfur
e. sodium & sulfate
f. barium & phospate

14. Jan 19, 2007

### rooparul

I m sorry but i m new to this forum; how do i ask a question? my guess is tat the space i m using is for posting replies and not asking questions so my apologies ut i need help

15. Jan 19, 2007

### rooparul

Do I have to "memorize" the valencies, variable valencies and the valencies of common radicals or is there a "method' of deducting thoese?

16. Jan 19, 2007

### gravenewworld

The periodic table is your best friend!!!!!!!!!!!

17. Jan 19, 2007

### Gokul43201

Staff Emeritus
Correct.

If your question is related to standard textbook chemistry or is from assigned coursework/homework, you must start a new thread in the Other Sciences sub-forum of the Homework & Coursework forum (near the top of the main forum page). To start a thread, enter the desired forum, and above the list of threads, you will see a button titled "New Topic" (or New Thread) - click that. If you are starting a thread under the Homework forum, you will see a template appear when you click the 'New Topic' button. Use this template to correctly format your post.

18. Jan 19, 2007

### rooparul

Then I am not sure how to read valencies from there i can see only rows and columns with their atomic numbers