Basic Calculatons Question

[Tetra]

Hi, I'm a little new to the field of chemistry, so if this question seems a little too basic, then sorry for that.

 

The questions is: a compound has the formula X (subscript 2) O (subscript 5)

where X is the unknown element. The compund is 44% oxygen by mass. What is the identity of elemnt X?

 

What I thought of doing was that, by converting it into grams, 44% O is 80g, but wasn't sure how to work from there (If I'm even suposed to do that.)

 

Could you also please explain the (proper) procedure? Thanks

[hermanntrude]

OK the method for this type of question is similar in many ways to the type of question where you obtain the empirical formula of a compound. ever done that?

 

first you assume that you have 100g of the compound. that makes the numbers easy. Then figure out how many grams of oxygen there are in your 100g sample. Then figure out how many grams of the other element you have. Bear in mind there are 100g total,and only two elements present.

 

then you can calculate how many moles of oxygen you have. use moles = mass/molar mass. once you've done that you can calculate how many moles of element X you have, by dividing the number of moles of oxygen by 5 and multiplying by 2. Now you know the mass and moles of element X and you can find the molar mass.

 

any further explanation you require, just ask :0)

[Tetra]

This is my interpretation of the method. Is it correct?

 

If you divide the number of oxygen moles by 5 (in this case, 2.75mol) then that means that each mole of the compound would be 0.55 mol? And we multiply by 2 to get the mass of element X?

 

Then, to find the molar mass, we divide the mass by the number of moles (2?) to get the molar mass?

 

The the closest answer would be hydrogen...I don't think I did that right...

[Fuzzwood]

You have .55 moles of compound, it contains double that of X and 56g of remaining mass is X. So if 2 X weighs 56g, how much does 1 X weigh in grams? Thats .55 moles of X, how much would 1 mole weigh?