[waycoo]

Alright, i've got this problem here which i'm having trouble understanding:

Liquid methanol is poured into a 500ml bottle then ignited by a spark between electrodes

For the equation 2CH3OH + 3O2 --->  2CO2 + 4H2O

Calculate the number of moles of oxygen and methanol in a 500 mililiter bottle and identify the limiting reagent for the combustion reaction in the bottle. (The vapor pressure of methanol at 25 degrees Celsius is is 125 mmHg.  Assume that air is 20% oxygen,the atmospheric pressure is 760 mmHg and the temperature is 25 degrees Celsius). 

 

I know how to find the limiting reagent.  However, my problem is finding the number of moles of each in the given situation. For the methanol, it seems like I can just divide the given mass by the molar mass to find the amount of moles.  The amount of oxygen would of course be 3/2 that amount.  Can someone explain to me why my teacher included the vapor pressure of methanol, the percent of oxygen in air, the current pressure, and the current temperature?  What role does this play in my calculations.  It seems like i can make those calculations without it.

[lemonoman]

my problem is finding the number of moles of each in the given situation. For the methanol, it seems like I can just divide the given mass by the molar mass to find the amount of moles.  The amount of oxygen would of course be 3/2 that amount.  Can someone explain to me why my teacher included the vapor pressure of methanol, the percent of oxygen in air, the current pressure, and the current temperature?  What role does this play in my calculations.  It seems like i can make those calculations without it

The amount of methanol, if he gave you a mass like you say here, is calculated like you said.

The amount of oxygen available is calculated with Pv=nRT where P n is the variable to find.  That's where that information about the atmosphere comes in.

The amount of oxygen you NEED (to completely use up the methanol) is 3/2 the amount of methanol.

A comparison of those two numbers tells you the limiting reagent.

So to answer your question, you will need some of that info he gave.

[Borek]

I don't see methanol mass here, although I am ready to calculate its amount from the vapour pressure

The amount of oxygen available is calculated with Pv=nRT where P is the variable to find.  That's where that information about the atmosphere comes in.

n is the variable. P is given. Probably typo on your side, but it could confuse already confused waycoo

So, in short - it is mostly about ideal gas equation plus some simple stoichiometry.

[lemonoman]

The amount n is the variable. P is given. Probably typo on your side, but it could confuse already confused waycoo

Oops.  Corrected.  Thanks, B.