Question

Explain how `4"Ca"_3("PO"_4)_2` contains `52` total atoms?

Ca=?
P=?
O=?

(I'm getting 22 total and not quite understanding what I've done wrong)

Answer 1

Here's how I do it.

Explanation:

Assume you have one formula unit `"Ca"_3("PO"_4)_2`.

The subscripts represent the number of atoms or groups immediately before them.

Thus,

`"Ca"_3` means that you have three `"Ca"` atoms.

`("PO"_4)_2` means that you have two `"PO"_4` groups.

One `"PO"_4` group contains one `"P"` atom and four `"O"` atoms.

∴ Two `"PO"_4` group contain two `"P"` atoms and eight `"O"` atoms.

Let's count the number of atoms in one formula unit.

`"Ca" color(white)(m)= color(white)(l)"3 atoms"`
`"P"color(white)(mll) =color(white)(l) "2 atoms"`
`ul("O" color(white)(mll)= color(white)(l)"8 atoms")`
`"Total = 13 atoms"`

If 1 formula unit contains 13 atoms, then
`color(white)(ll)` 4 formula units contains `4 × 13 = 52` atoms.