# Which of the following have the same molecular formula as the molecule shown in the model? This model has 3 hydrogens connected the the first and last carbon and 2 hydrogens connected to the other carbons

Finding the same molecular formula as the alkane model I have to check all that apply I know the last two options are not correct, because they don't have the same amount of carbons, but when it comes to the first two I'm confused because there is only two carbons attached to 3 hydrogens yet in these options there are 3 different carbons attached to 3 hydrogens.

Would they both be correct or does it matter if it's connected to the second bond instead of the third?

Im assuming since they are isomers that they are equal to eachother.

• Please see the site's homework policy. You'll need to share your thoughts as you work towards an answer, otherwise your question might get closed. – Todd Minehardt Sep 8 '15 at 18:52
• As you correctly observed, the model has two $\ce{CH_3}$ groups, while each of the options has 3 of them. Would you expect 2 to turn into 3 if you rotate picture left, or right, or maybe upside down, or do anything else with it? – Ivan Neretin Sep 8 '15 at 18:55

## 1 Answer

Remember they are asking for the molecular formulas which are the same, not the structural formulas. The structural formulas are shown in the question, but for example the first option, if you count up the number of carbons and hydrogens you get the molecular formula $\ce{C7H16}$, so you'll need to check if the molecule shown has the same formula.

So for molecular formulas, it doesn't matter how the atoms are arranged; all isomers have the same molecular formula. Only in structural formulas do you get information on the arrangement of atoms in the molecule.

Note there is also condensed structural formulas which look similar to the molecular formula but is still a structural formula. For the first option the condensed structural formula would be $\ce{CH3CH(CH3)CH2CH2CH2CH3}$.