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Chem prelim question

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  • Maith
    Maith's Avatar
    1,827 posts since Dec '07
    • 16 Jul `08, 2:00PM

      Just had my exam..everything can do la..except for one question.Think its on the mole concept de.Anyway here's the question:

      1)Tungsten metal,W,is manufactured by reducing tungsten(III)oxide,WO3,with carbon.

      WO3+3C----->3CO+W

      Calculate

      i)the mass of carbon needed to reduce 116g of tungsten(III)oxide,

      ii)the maximum mass of tungsten that can be formed.

       

      That's all. Hope u guys can help. (:

  • Moderator
    ^tamago^
    我又郁闷了...
    ^tamago^'s Avatar
    49,242 posts since Sep '03
    • 16 Jul `08, 2:44PM

      Mass of C needed
      = 116g/[(183.84+16×3)g/mol of WO3] × 3mol of C/mol of WO3 × 12g/mol of C
      = 18.0g of C

      Maximum mass of W
      = [183.84g/mol of W]/[(183.84+16×3)g/mol of WO3] × 116g
      = 92.0g of W

      Edited by ^tamago^ 17 Jul `08, 3:35PM
  • secretliker
    secretliker's Avatar
    174 posts since Dec '06
    • 16 Jul `08, 6:40PM

      (i)

      No. of moles of WO3 = 116 / (184 + 3*16) = 0.5 mol

      No of moles of C needed = 3 * 0.5 = 1.5 mol

      Mass of C required = 12 * 1.5 = 18g

      (ii)

      Max no. of moles of W = no. of moles of WO3 = 0.5

      Max mass of W = 0.5 * 184 = 92g

  • Moderator
    UltimaOnline
    UltimaOnline's Avatar
    536 posts since May '05
  • Maith
    Maith's Avatar
    1,827 posts since Dec '07
    • 16 Jul `08, 8:15PM

      ooooh i see i see...so its just making use of the formula no. of mol=mass/molar mass ar...thanks you guys! ^^

  • Moderator
    Darkness_hacker99
    Darkness_hacker99's Avatar
    31,701 posts since Jun '05
    • 16 Jul `08, 9:47PM

      Yup!

      Steps:

      1) Find the amount of the given sample(X) reacted.
      2) Find the mol of X.
      3) Using the balance equation, you know the proportion. In this case W03:C = 1:3
      4) Find the what you're suppose to find.

      Tada!

  • Moderator
    Darkness_hacker99
    Darkness_hacker99's Avatar
    31,701 posts since Jun '05
    • 16 Jul `08, 9:49PM

      For titration, it's also similar. The concept is there.. to find the number of mols reacted to determine the unknown figures.

  • Moderator
    eagle
    eagle's Avatar
    16,241 posts since Aug '01
    • 17 Jul `08, 10:20AM

      I like mole concept the most in chem last time because it was the most maths part of chemistry.

  • seanchua7
    seanchua7's Avatar
    1 post since Dec '07
    • 09 Aug `08, 2:27AM

      Many students find Mole Calculations a tough topic to handle in GCE O Level Chemistry Examinations. But in fact, it is not really that tough like you imagined.

       

      Just remember that you need to convert the information into MOLES and then compared with the stoichiometries in the balanced chemical equations. And that is the reason why this topic is known as "Mole Calculations".

       

      And for reaction that involves all gases, you can compare directly using the information that are given in volumes (i,e. dm3 or cm3) to get the answers.

       

      Hope that helps.

       

      Sean

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