Question

For enthalpy of formation/combustion questions, how would one know the chemical formula of the compound given in the question if the compound is given in words?

Answer 1

Simply use IUPAC rules or use common sense

Identify the cation and write its formula and charge. Then identify the anion and write its formula and charge. Combine to produce an electrically neutral compound. .

Explanation:

There is no way around it. You MUST MEMORIZE the formulas and charges of the ions.

The rules to follow are:

1. In the name of the compound, the name of the cation comes first.
2. In the formula of the compound, the symbol of the cation comes first.
3. Althoughionic compoundsare composed of both positively and negatively charged ions, the overall compound and its formula are electrically neutral. In other words: total positive charge from cation = total negative charge from anion

To write the empirical formula for an ionic compound:

1. Identify the cation. The cation is written first in the name of the compound.
2. Write the correct formula and charge for the cation.
3. Identify the anion. The anion is written last in the name of the compound.
4. Write the correct formula and charge for the anion.
5. Combine the cation and anion to produce an electrically neutral compound. --> If the charges on the cation and anion are equal in magnitude (i.e., +1/-1, +2/-2, +3/-3), combine the cation and anion in a 1:1 ratio. --> If the charges on the cation and anion are NOT equal in magnitude, use the charge on the cation as the subscript for the anion. Use the charge on the anion (omitting the negative sign) as the subscript for the cation.
6. Place parentheses around a polyatomic ion if you need more than one of them in the final formula.
7. Do not show the charges of the ions when you write the final formula for the compound.
8. Make sure that the subscripts for the cation and anion are the smallest whole number ratio. Do not use “1” as a subscript. A missing subscript is automatically assumed to be 1.

Let’s apply these rules to generate some formulas.

EXAMPLE 1: Write the formula for sodium carbonate.

1. Identify the cation: sodium ion = Na⁺
2. Identify the anion: carbonate ion = CO₃²⁻
3. Combine the ions to form an electrically neutral compound: Since the charges are not equal in magnitude, the charge on the sodium ion (+1) becomes the subscript for the carbonate ion. The charge on the carbonate ion (2-) becomes the subscript for the sodium ion.
4. The final formula is Na₂CO₃.

Also notice that we do not write the charges of the ions when we write the final formula.

EXAMPLE 2: Write the formula for aluminum sulfate.

1. Write the cation first: aluminum ion (Al³⁺)
2. Write the anion last: sulfate ion (SO₄²⁻)
3. Combine them to form an electrically neutral compound: Since the charges are not equal in magnitude, the charge on the aluminum ion (+3) becomes the subscript for the sulfate ion. The charge on the sulfate ion (-2) becomes the subscript for the aluminum ion.
4. The correct formula is Al₂(SO₄)₃.