Understanding Chemical Formulas: How to Write Them for General Chemistry

In summary: You can also look up the charges on the individual ions on the internet. Also, there is a "method" of deducting the valencies of common radicals.
  • #1
courtrigrad
1,236
2
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
 
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  • #2
courtrigrad said:
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?

Just write it the way you hear it.
[itex]CaHCO_3[/itex]

Just remember to write the cations (positive ion) first. Writing something like [itex]CO_3HCa[/itex] would probably be marked incorrect by your instructor.
 
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  • #3
take the charge on the cation, say [tex]Ca^+^2[/tex] and then the charge on the anion hydrogen carbonate [tex]HCO_3^-[/tex] and you will criss cross the charges to form [tex]Ca(HCO_3)_2[/tex]

other examples:

[tex]Fe^+^3[/tex] and [tex]ClO_4^-[/tex] forms [tex]Fe(ClO_4)_3[/tex]

[tex]Ca^+^2[/tex] and [tex]Cl^-[/tex] forms [tex]CaCl_2[/tex]
 
  • #4
Some more complex examples will come from me:

[itex]Ca_3(PO_4)_2[/itex] is made from [itex]Ca^{2+}[/itex] and [itex](PO_4)^{3-}[/itex].

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

Well, it is possible that my reply is too complex for your needs, but you might benefit this message later.
 
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  • #5
How do you know the charges of the cations and anions? Do you have to look them up?
 
  • #6
It is somewhat close to what you're implying, just be patient and go on studying these ions to "memorize" them.
 
  • #7
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

b. lead (II) and clorine

Can anyone tell me how to write the formula?

Thanks
 
  • #8
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.
 
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  • #9
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
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
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
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
I can ask my teacher tomorrow. I need to write the formula for the compound they will form.
a. magnesium & oxygen
b. lead (II) & chlorine
c. calcium & nitrogen
d. lithium & sulfur
e. sodium & sulfate
f. barium & phospate
 
  • #14
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
Do I have to "memorize" the valencies, variable valencies and the valencies of common radicals or is there a "method' of deducting thoese?
 
  • #16
rooparul said:
Do I have to "memorize" the valencies, variable valencies and the valencies of common radicals or is there a "method' of deducting thoese?



The periodic table is your best friend!
 
  • #17
rooparul said:
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
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
gravenewworld said:
The periodic table is your best friend!

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

1. What is a chemical formula?

A chemical formula is a notation used to represent the type and number of atoms present in a molecule or compound. It includes the chemical symbols of the elements present and the ratio of their atoms in the compound.

2. How do I write a chemical formula?

To write a chemical formula, you need to first identify the elements present in the compound. Then, use the element's chemical symbol and subscript numbers to indicate the ratio of atoms in the compound. The subscript number indicates the number of atoms of that element in the compound.

3. What is the difference between a molecular formula and an empirical formula?

A molecular formula gives the exact number of atoms of each element present in a molecule, while an empirical formula gives the simplest whole-number ratio of the elements present. For example, the molecular formula of glucose is C6H12O6, while its empirical formula is CH2O.

4. What are some common mistakes to avoid when writing chemical formulas?

Some common mistakes to avoid when writing chemical formulas include forgetting to use subscripts, using incorrect chemical symbols, or not simplifying the ratios in the formula. It is also important to double-check the formula for accuracy and balance.

5. How can I determine the charge of an ion from its chemical formula?

The charge of an ion can be determined by looking at the symbols and subscripts in the chemical formula. The sum of the positive charges (from cations) should be equal to the sum of the negative charges (from anions). If they are not, the difference between the two will be the charge of the ion.

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