Question

counting atoms
the formula for a compound indicates the elements that make up the compound and the number of atoms of each element present in the compound. these numbers of atoms are indicated by the use of small numbers called subscripts. sometimes groups of atoms act as a single atom. such a group of atoms is called a polyatomic ion. if a polyatomic ion is used in a formula more than once, it is put in parentheses and the subscript appears outside the parentheses. when a subscript appears outside the parentheses, it indicates that all the elements inside the parentheses should be multiplied by that subscript. for example, the formula fe(oh)₃ indicates the combination of one atom of iron, fe, three atoms of oxygen, o, and three atoms of hydrogen, h.
in the following examples, list each element in the compound and the number of atoms of each element present. the first example has been done for you. you may already be familiar with some of the compounds.

name	use	formula	atoms in formula
aspirin	pain reliever	c₉h₈o₄
magnesium hydroxide	found in milk of magnesia	mg(oh)₂
paradichlorobenzene	moth crystals	c₆h₄cl₂
acetic acid	found in vinegar	c₂h₄o₂
trinitrotoluene (tnt)	explosive	c₇h₅(no₂)₃
calcium dihydrogen phosphate	fertilizer	ca(h₂po₄)₂
pyrite	fool’s gold	fes₂

Explanation:

Step1: Analyze aspirin formula

For $\text{C}_9\text{H}_8\text{O}_4$, subscripts give atom counts directly.

Step2: Analyze magnesium hydroxide

For $\text{Mg(OH)}_2$: Mg=1; O and H are multiplied by 2, so O=2, H=2.

Step3: Analyze paradichlorobenzene

For $\text{C}_6\text{H}_4\text{Cl}_2$, subscripts give atom counts directly.

Step4: Analyze acetic acid

For $\text{C}_2\text{H}_4\text{O}_2$, subscripts give atom counts directly.

Step5: Analyze TNT formula

For $\text{C}_7\text{H}_5(\text{NO}_2)_3$: C=7, H=5; N and O are multiplied by 3, so N=3, O=$2\times3=6$.

Step6: Analyze calcium dihydrogen phosphate

For $\text{Ca(H}_2\text{PO}_4\text{)}_2$: Ca=1; H, P, O are multiplied by 2, so H=$2\times2=4$, P=$1\times2=2$, O=$4\times2=8$.

Step7: Analyze pyrite formula

For $\text{FeS}_2$, subscripts give atom counts directly.

Answer:

1. Aspirin: C = carbon 9, H = hydrogen 8, O = oxygen 4
2. Magnesium hydroxide: Mg = magnesium 1, O = oxygen 2, H = hydrogen 2
3. Paradichlorobenzene: C = carbon 6, H = hydrogen 4, Cl = chlorine 2
4. Acetic acid: C = carbon 2, H = hydrogen 4, O = oxygen 2
5. Trinitrotoluene (TNT): C = carbon 7, H = hydrogen 5, N = nitrogen 3, O = oxygen 6
6. Calcium dihydrogen phosphates: Ca = calcium 1, H = hydrogen 4, P = phosphorus 2, O = oxygen 8
7. Pyrite: Fe = iron 1, S = sulfur 2