Molar Ratio

[DPTinthemaking15]

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I realized my weakness in Gen Chem. Sadly, it is stoichiometry/compounds. Long story short, I have been watching videos to correct a few issues, but I am having troubles with this problem. "Which of the following statements are true of C02?" I. It is a molecule. II. It is a compound. III. 7 moles of it contains 3.5 moles of oxygen. IV. When mixed with water, one mole of it yields 3 ions.

Here is the answer: This statement reverses the actual ratio. 7 moles of carbon dioxide consists of 14 moles of oxygen atoms and 7 moles of carbon.

These all make sense to me, but the molar ratio is throwing me off. If we aren't given a formula, do we just assume that we multiply each element by the 7 moles given? For instance, 7 moles of C02= 7 moles x C= 7 moles of carbon and 7 moles of 02= 14 moles of O? I'm furious that this is what is giving me issues on the Chemistry/Physics section, but I cant wrap my head around this concept for some reason.

 

[AdaptPrep]

Hi, DPTinthemaking15--

You take the moles multiplied by the subscripts of the known formula (if you don't know the formula, you might be in trouble!). It might help to follow this line of thinking:

One molecule of the compound CO2 has one atom of carbon and 2 atoms of oxygen (the subscripts are the numbers used)
One mole of CO2 then has one mole of carbon atoms and 2 moles of oxygen atoms.
Therefore, seven moles of CO2 would then have 7 moles of carbon atoms and 14 moles of oxygen atoms.

If you like to see it done with math where units cancel:

(7 mole CO2) (1 mole C / 1 mole CO2) = 7 mole C
(7 mole CO2) (2 mole O / 1 mole CO2) = 14 mole O

Have fun!

 

[DPTinthemaking15]

Hi, DPTinthemaking15--

You take the moles multiplied by the subscripts of the known formula (if you don't know the formula, you might be in trouble!). It might help to follow this line of thinking:

One molecule of the compound CO2 has one atom of carbon and 2 atoms of oxygen (the subscripts are the numbers used)
One mole of CO2 then has one mole of carbon atoms and 2 moles of oxygen atoms.
Therefore, seven moles of CO2 would then have 7 moles of carbon atoms and 14 moles of oxygen atoms.

If you like to see it done with math where units cancel:

(7 mole CO2) (1 mole C / 1 mole CO2) = 7 mole C
(7 mole CO2) (2 mole O / 1 mole CO2) = 14 mole O

Have fun!

You alone will be the reason I do well on the MCAT. Thank you for the clear explanation on such a simple subject.