[Robbie91]

Hey guys!....I'm so excited to fine a page like this! ...no one here in my house has any idea how to do this problem...

Here is my problem I'm trying to work on:

"Given the equation 2CO + O2 ---> 2CO2, how many grams of carbon dioxide form if 17 grams of carbon monoxide are combined with excess oxygen?"

So I know that CO2 (carbon dioxide) weighs 44 grams, the equation has two - so that makes it 88 grams.  So I just subtracted 17 from 88 and got 71.  One of my answers I can choose from is "72.5", my question is - does the difference matter...and where did my other 1 and a half grams go?

Thanks....any help is really appreciated!

[xiankai]

stoichiometric reactions deal with molarity, so the values given are in moles, not mass.

eg. 2 moles of CO are oxidised with 1 mole of oxygen to produce 2 moles of carbon dioxide, not grams.

now the molar mass of CO2 indeed is 44, so u have to find the moles of CO2. but first of couse, u'll have to find the moles of CO. this can be done by dividing 17 by the Mr of CO.

the value that u have obtained is also the moles of CO2, because in the reaction they have the same number of moles, and there's no reason why it should be different. now u can find the mass of CO2 by multiplying the moles by its Mr.

[Borek]

stoichiometric reactions deal with molarity, so the values given are in moles, not mass.

I think Robbie refers to the proportional reasoning which is another approach to stoichiometry calculations:

2CO + O₂ -> 2CO₂
56g + 32g -> 88g
17g + x     -> y

56:17 = 88:y (solve for y)

or

56:17 = 32:x (solve for x)

[lemonoman]

Good explanation Borek...it's being able to think like that that can make highschool chemistry a breeze...and everybody else is thinking, "How do you do this" and the people getting good marks just say, "Think about it" when they're looking for a method they can memorize.   It's quicker to do and you feel better if you know what you're doing.

[xiankai]

in which case, subtraction/addition should not be present as it would not fit in the ratios.

[Robbie91]

Thanks Borek!---the ratios make so much sense!!!