[m3thlab]

Hi guys, I have a chemistry question I can't seem to figure out...
14.555g of an unknown metal (M) react with oxygen to form 25.98g of M2O5. What is the molar mass of element M?
Thanks so much!

[MNIO]

nice variation of a common textbook problem.  you have this reaction
  __ M + __ O2 ----> __ M2O5

tossing in a mass balance gives
   14.555g M +(25.98g - 14.555g) O2 ---> 25.98g M2O5

we can readily calculate moles O2
then moles M2O5
then moles M
them molar mass M
right?

**********
the rest of the problem
  moles O2 = (25.98g - 14.555g) * (1mol / 32.00g) = 0.35703 mol O2
  moles M2O5 = 0.3570 moles O2 * (2mol O /  1mol O2) * (1 mol M2O5 / 5 mol O) = 0.14281
  moles M = 0.14281 mol M2O5 * (2 mol M / 1 mol M2O5) = 0.28562
  molar mass M = 14.555g / 0.28562mol = 50.96 g/mol

your metal is probably vanadium

[m3thlab]

Thank you for your reply. I am confused as to how you calculated the molar mass of M2O5. I see what you did with the oxygens but I do not understand it at all. I guess the fact that we dont know the molar mass of M2O5 is making it very difficult for me to understand how you calculated it using oxygen.

[MNIO]

I didn't calculate the molar mass of M2O5

watch again
   (1) calculate mass O2 consumed
             mass O2 consumed = mass M2O5 produced - mass Metal consumed
                                          = 25.98g M2O5 - 14.555g M2 = 11.425g O2
   (2) convert mass O2 to moles O2
             moles O2 = 11.425g O2 * (1 mol / 32.00g) = 0.35073 mol O2

           you can do both those calcs in 1 dimensional analysis equation
               (25.98 - 14.555)g O2     1 mol O2
             -------------------------- x ------------- = 0.35073 mol O2
                          1                       32.00g O2

   (3) convert moles O2 consumed to moles M2O5 produced

           again via dimensional analysis equation
               0.35073 mol O2     2 mol O      1 mol M2O5
             ------------------- x ------------ x -------------- = 0.14281 mol M2O5
                         1               1 mol O2         5 mol O

  (4) convert moles M2O5 to moles M

              0.14281 mol M2O5        2 mol M
            ------------------------ x -------------- = 0.28262 mol M
                           1                  1 mol M2O5

  (5) molar mass M = mass M / moles M = 14.555g / 0.28562mol = 50.96 g/mol

[Borek]

your metal is probably vanadium

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