Question

1. Using average atomic masses calculate the number of atoms present in each of the following samples.

(a) 1.27 g of silver, Ag

(b) 0.0010 mol of copper, Cu

(c) 0.0010 g of copper, Cu

(d) 1.76 kg of magnesium, Mg

(e) 2.45 oz of calcium, Ca

(f) 2.45 g of calcium, Ca

(g) 2.45 mol of calcium, Ca

Answer 1

Remember:

Number of atoms = avogadro's number * number of moles
                                  = (6.022*10^23) *mass/molar mass
(a) 1.27 g of silver, Ag
mass= 1.27 gm
molar mass = 108 gm
number of atoms = (6.022*10^23) *mass/molar mass
                                      = (6.022*10^23) *1.27/108
                                      = 7.08*10^21 atoms

(b) 0.0010 mol of copper, Cu
Number of atoms = avogadro's number * number of moles
                                  = (6.022*10^23) * 0.001
                                 = 6.022*10^20 atoms
(c) 0.0010 g of copper, Cu
mass= 0.0010 gm
molar mass = 63.5 gm
number of atoms = (6.022*10^23) *mass/molar mass
                                      = (6.022*10^23) *0.0010/ 63.5
                                      = 9.48*10^18 atoms
(d) 1.76 kg of magnesium, Mg
mass= 1.76 Kg = 1760 gm
molar mass = 24.3 gm
number of atoms = (6.022*10^23) *mass/molar mass
                                      = (6.022*10^23) *1760/ 24.3
                                      = 4.36*10^25 atoms
(e) 2.45 oz of calcium, Ca
1 Oz= 28.35 gm
mass= 2.45 oz = 2.45*28.35 gm = 69.46 gm
molar mass = 40 gm
number of atoms = (6.022*10^23) *mass/molar mass
                                      = (6.022*10^23) *69.46/ 40
                                      = 1.0*10^24 atoms
(f) 2.45 g of calcium, Ca
mass= 2.45 gm
molar mass = 40 gm
number of atoms = (6.022*10^23) *mass/molar mass
                                      = (6.022*10^23) *2.45/ 40
                                      = 3.69*10^22 atoms
(g) 2.45 mol of calcium, Ca
Number of atoms = avogadro's number * number of moles
                                  = (6.022*10^23) * 2.45
                                 = 1.48*10^24 atoms