Moles problems

• hoay

  4 Jun 15, 20:14

  1.15 g of a metallic element reacts with 300 cm3 of oxygen at 298 K and 1 atm pressure, to form an oxide which contains O2– ions.

  What could be the identity of the metal?

  A calcium       B magnesium         C potassium        D sodium

  Ans.  M   +   O2.........MxO

  The no of moles of O2 is 300/24000 = 0.0125

  Do not know what to do next....?

   

• UltimaOnline

  4 Jun 15, 22:33

  'A' Level H2 Chemistry Qn
  
  1.15 g of a metallic element reacts with 300 cm3 of oxygen at 298 K and 1 atm pressure, to form an oxide which contains dinegative O2– anions. What could be the identity of the metal?
  
  MCQ options : A calcium - B magnesium - C potassium - D sodium.
  
  
  
  My BedokFunland JC Solution :
  
  Let the molar mass of the metal be y grams
  
  No of moles of metal present = 1.15 / y
  
  Using PV=nRT, molar volume of gas = 24.45 dm3.
  
  Hence moles of gas present = 12.27 x 10^-2 mol.
  
  By stoichiometry, moles of dinegative O2- anions generated = 24.54 x 10^-2 mol
  
  Assuming the metal cation has a unipositive charge :
  
  moles of metal cation = 2 x moles of oxide anion
  
  Hence 1.15 / y = 49.08 x 10^-2, and y = 23.43 g
  
  Assuming the metal cation has a dipositive charge :
  
  moles of metal cation = moles of oxide anion
  
  Hence 1.15 / y = 24.54 x 10^-2, and y = 46.86 g.
  
  Conclusion : Comparing against the molar masses and ionic charges of the 4 metals listed in the options, the closest match and hence the best answer, is Na (sodium).
  
  Commentary : if we had used 24dm3 as the molar volume of gas, as might have been intended by the question author, we would obtain molar masses of 23.0g and 46.0g for unipositive and dipositive ionic charges respectively, making Na (sodium) a more obvious answer. However, 24dm3 is actually the molar volume of a gas to 3 sig fig when room temperature is taken to refer to 20 deg Celsius, rather than 25 deg Celsius (both may be described as "room temperature", itself an ambiguous term). Alternatively, recalcitrant folks might argue that 24dm3 is still correct for 25 deg Celsius, if rounded off to 2 sig figs. Yeah well, 2 sig fig is just bad math, particularly when the question itself gave the sample mass of the metal to 3 sig figs. This is commonly misunderstood (even by school teachers), but will usually not be much of a problem in the actual A levels, as Cambridge will usually specify along the lines of"You may assume the molar volume of a gas under these conditions to be 24dm3" *or* explicitly require you to use PV=nRT.