Question

How many total moles of ions are released when each of the following samples dissolves completely in water? (a) 0.421 mol of K2HPO4 (b) 7.31 g of MgSO47 H2O . (c) 5.947 x10^21 formula units of NiCl2

Answer 1

1)

K2HPO4 dissociates as:

K2HPO4 —> 2 K+ + HPO4-

moles of ions formed = 3*number of moles of K2HPO4

= 3*0.421 moles

= 1.263 moles

number of ions = moles of ions formed * Avogadro’s number

= 1.263 moles * 6.022*10^23 ions/moles

= 7.61*10^23 ions

Answer: 7.61*10^23 ions

2)

Molar mass of MgSO4.7H2O,

MM = 1*MM(Mg) + 1*MM(S) + 11*MM(O) + 14*MM(H)

= 1*24.31 + 1*32.07 + 11*16.0 + 14*1.008

= 246.492 g/mol

mass(MgSO4.7H2O)= 7.31 g

number of mol of MgSO4.7H2O,

n = mass of MgSO4.7H2O / molar mass of MgSO4.7H2O

=(7.31 g)/(246.492 g/mol)

= 2.966*10^-2 mol

MgSO4.7H2O dissociates as:

MgSO4.7H2O —> Mg2+ + SO42- + 7 H2O

moles of ions formed = 2*number of moles of MgSO4.7H2O

= 2*2.966*10^-2 mol

= 0.05932 moles

number of ions = moles of ions formed * Avogadro’s number

= 0.05932 moles * 6.022*10^23 ions/moles

= 3.57*10^22 ions

Answer: 3.57*10^22 ions

3)

NiCl2 dissociates as:

NiCl2 —> Ni2+ + 2 Cl-

number of ions formed = 3*number of unit of NiCl2

= 3*5.947*10^21

= 1.78*10^22 ions

Answer: 1.78*10^22 ions