Question

Determine the number of atoms in each sample.

Answer 1

So there are three formulas that you must be aware of hen attempting these questions (by technicality, only two because the third is a transposition of the second).

These formulas are :

   mole x L =  # of atoms   (where L = Avogadro's Constant)

   mass / Mr  = mole  (where Mr = Molar Mass)

   Mole  x  Mr  = mass 

So no we can easily ace these questions

1)  a.  98.3 g  of  Hg

             mass of Hg  =  98.3g

             Mr      "   "    =  201 g / mol

                   mole =  $\frac{98.3 g}{201 g / mol}$

                           =  0.489 mol

       ∴ # of atoms  = 0.489 mol  *  (6.03 * 10²³)

                             = 2.95  *  10²³  atoms

   

      c.  10.7g of Li

            mass of Li  =  10.7 g

            Mr      "   "  =   7 g / mol

               thus  mole =  $\frac{10.7g}{7 g / mol}$

                                 = 1.529 mol

           # of atoms  =  1.529 mol  *  (6.03 * 10²³)

                              =   9.22  *  10²³

2)  a.   6.84 g   of    C₁₂H₂₂O₁₁

          mass of sucrose  =  6.84 g

          Mr      "     "  =  ∑ (Mr of each element  *  amount of each element's atom)

                              =  ∑ [(12 * 12)+(1 * 12)+(16 * 11)]  g / mol

                              =  342 g / mol

          ∴   mole of sucrose  =  $\frac{6.84 g}{342 g / mol}$

                                           =  0.02 mol

    c)  68.0g    of  NH₃

               mass of NH₃   =   68.0 g

               Mr       "   "       =   [(14 * 1) + ( 1 * 3)] g / mol

         ∴  mole  of  NH₃    =   $\frac{68.0 g }{17 g / mol}$

                                       =    4  mol

3)     a.   3.52 mol of Si

                   mole of Si  =  3.52 mol

                   Mr     "   "   =  28 g / mol

               Mass =  3.52 mol  *  28 g / mol

                        =   98.56 g

        c.   0.550  mol  of F₂

              mol  of F₂   =  0.55 mol

               Mr   "  "     =  (19  *  2 )  g / mol

          Mass =  0.55 mol  *  38 g / mol

                   =  20.9 g

Hope I as clear in my answers and that they helped