Answer:
The first choice. The rule is go left. Those subscript numbers always talk about the element to the left of them.
Explanation:
The straight forward answer is the first one.
You should always answer that when reading the chemical make up of molecules. Let's look at some examples.
C8 H18
Consider the 8. It must be describing how many carbons there are. If it was meant for the hydrogen, then what does the 18 mean? You wouldn't know.
Now look at something a lot more complicated.
Mg3(PO4)2
Use the going left rule. That satisfies Mg doesn't it? There are 3 of them and 3 is correct.
The two at the end is also straightforward if you think about it. It must be telling you how many molecules are in the brackets. If that were not so, then you wouldn't be able to tell what it did. So there are 2 PO4
But you still have the 4 to deal with. Use the going left rule again. 4 must be telling you how many oxygens you have inside the brackets. There are 4 of them.
