# Chemical Forums

## Chemistry Forums for Students => High School Chemistry Forum => Topic started by: xrebelangel on September 13, 2019, 07:56:50 PM

Title: When converting moles to an element, how do you determine...
Post by: xrebelangel on September 13, 2019, 07:56:50 PM
When converting moles to an element, say water, how do you determine which element gets divided by which when trying to determine the number of moles?

I'm working on mastering chemistry and trying to understand how to do it, and the question was:
Calculate the mass of water produced when 6.59 g of butane reacts with excess oxygen.

I understand first you convert the mass of butane to moles, than you calculate the number of moles to water by looking at the coefficients in the balanced equation.

So essentially its: 0.113mole C4H10 × 10 moles H2O divided by 2 moles C4H10 = 0.565 mole

But how do you determine which element gets divided by which?
Like how do I know I'm not supposed to divide butane by H2O instead?

Sorry it's been a while so this is all new to me.
Title: Re: When converting moles to an element, how do you determine...
Post by: Borek on September 14, 2019, 01:11:21 PM
You don't divide elements.

Let's say you have cookies and kids. When calculating how many cookies per kid do you divide number of cookies by number of kids, or do you divide number of kids by number of cookies?

Title: Re: When converting moles to an element, how do you determine...
Post by: Vidya on September 18, 2019, 01:21:34 AM
When converting moles to an element, say water, how do you determine which element gets divided by which when trying to determine the number of moles?

I'm working on mastering chemistry and trying to understand how to do it, and the question was:
Calculate the mass of water produced when 6.59 g of butane reacts with excess oxygen.

I understand first you convert the mass of butane to moles, than you calculate the number of moles to water by looking at the coefficients in the balanced equation.

So essentially its: 0.113mole C4H10 × 10 moles H2O divided by 2 moles C4H10 = 0.565 mole

But how do you determine which element gets divided by which?
Like how do I know I'm not supposed to divide butane by H2O instead?

Sorry it's been a while so this is all new to me.
For this you must master dimensional analysis.In dimensional analysis we use those units in denominator which we need to cancel and units in numerator which we need to retain.You want your answer in moles of water so keep moles of water in numerator and moles of butane in denominator.
Title: Re: When converting moles to an element, how do you determine...
Post by: Fudjsk on September 22, 2019, 06:51:24 PM
The balanced chemical equation is 2C4H10+9O2 yields 4CO2+10H2O. You find the number of moles of butane by dividing 6.59g by the molar mass. It should equal 0.133moles. From there you divide it by two since it is 2C4H10. Then you multiply by 10 since there are 10 molecules of water. You don't divide the 6.59 grams to find the individual hydrogen's inside the molecule. You divide/multiply something they all have in common which is the number of moles.