Question #4cdbc

I edited your question to make it more understandable. Maybe now you can see that there are indeed 7 oxygen atoms?

Lets first see how much of each element is in #2CO_2# :
you should read this as: # 2*(C + O_2)#
which is similar to: # 2*C + 2* (O+O)#
so there are #2# carbon atoms (#C#) and #2 * 2 = 4# oxygen (#O#) atoms.

Than the elements in the second part #3H_2O# :
this you should read as: #3* (H_2 + O)#
which is similar to: #3 * (H + H) + 3 * O#
so there are #6# hydrogen (#H#) atoms and #3# oxygen (#O#) atoms

In total: #4 * O# in #2CO_2# and #3 * O# in #3H_2O# = #7# oxygen atoms

The numbers in front of the chemical formula indicate how many lots of that compund you have. The numbers apply to each element in the compound.

The example states:
#2CO_2+3H_2O#

If the numbers apply to each element, and there is a small subscript number next to it, then you multiply them together.

For example, #2CO_2#. Here there are 2 carbon atoms, as there is no number, which means that you only have one, and since there are 2 lots of #CO_2#, then you will have 2 carbon atoms and 4 oxygen atoms.

If we then apply this to the water molecules, we find that there are a total of 2 carbon atoms, 7 oxygen atoms, and 6 hydrogen atoms.