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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: johnnyjohn993 on November 22, 2014, 05:12:38 AM

Title: finding the atomic mass unit
Post by: johnnyjohn993 on November 22, 2014, 05:12:38 AM
A 2.077 g sample of an element, which has an atomic mass between 40amu and 55amu,  reacts with oxygen to form 3.708g of an oxide. Determine the formula of the oxide (and identify the element)

My attempt:
mass of oxygen= 3.708- 2.077 = 1.631g
(I'm using the moles of each substance)

2.077  +  1.631     3.708
------      ------  = --------
  x            32        x + 16
the value of x is an imaginary number (doing the calculations) plz help me ,this is so confusing X(
Title: Re: finding the atomic mass unit
Post by: Borek on November 22, 2014, 06:33:22 AM
Adding number of moles of the element to the number of moles of oxygen doesn't yield number of moles of the oxide.

You know mass of oxygen. How many moles is that?

Assuming formula of oxide to be XO, how many moles of the element? What would be its aomic mass?

And what if the formula of the oxide is X2O?

What other formulas of oxides can you think of?

Each formula yields a possible atomic mass for the element, just compare them to all six possible elements.
Title: Re: finding the atomic mass unit
Post by: johnnyjohn993 on November 22, 2014, 08:12:30 AM
Quote from: Borek on November 22, 2014, 06:33:22 AM
Adding number of moles of the element to the number of moles of oxygen doesn't yield number of moles of the oxide.

You know mass of oxygen. How many moles is that?

Assuming formula of oxide to be XO, how many moles of the element? What would be its aomic mass?

And what if the formula of the oxide is X2O?

What other formulas of oxides can you think of?

Each formula yields a possible atomic mass for the element, just compare them to all six possible elements.

At the back of my book it said to be Vanadium is the element. I don't know how to solve this problem, except adding the moles of the reactants to have the same values of moles on the product side because I used it before and able to get a right answer :(

Title: Re: finding the atomic mass unit
Post by: Borek on November 22, 2014, 08:54:43 AM
Follow what I wrote, I asked you questions, try to answer them one by one, in the same order I asked them.
Title: Re: finding the atomic mass unit
Post by: johnnyjohn993 on November 23, 2014, 01:00:55 AM
Quote from: Borek on November 22, 2014, 08:54:43 AM
Follow what I wrote, I asked you questions, try to answer them one by one, in the same order I asked them.

the following oxides would come up are:  XO , X2O3 , XO2 , X2O5 , XO3 , X2O7

moles :

moles of oxigen: 0.05097 mol

XO = (3.708/x+16 ) mol
X2O3= (3.708/2x +48)mol
XO2=  (3.708/x +32)mol
X2O5=  (3.708/2x +80)mol
 XO3 = (3.708/x +48)mol
 X2O7= (3.708/2x +112)mol

I don't know how to go further anymore  , just please tell me how you did it ? T_T






Title: Re: finding the atomic mass unit
Post by: Borek on November 23, 2014, 03:29:42 AM
Quote from: johnnyjohn993 on November 23, 2014, 01:00:55 AM
XO = (3.708/x+16 ) mol

No, that's not what I asked about.

If the formula of the oxide is XO, number of moles of the metal is identical to number of moles of the oxygen.

If the formula of the oxide is X2O, number of moles of the metal is twice the number of moles of the oxygen.

If the formula of the oxide is XO2, number of moles of the metal is half the number of moles of the oxygen.

For each formula calculate number of moles of the metal, and find its atomic mass.