Question

In the laboratory you dissolve 21.4 g of cobalt(II) acetate in a volumetric flask and add water to a total volume of 500 mL.  

What is the molarity of the solution?   M.

What is the concentration of the cobalt(II) cation?   M.

What is the concentration of the acetate anion?   M.

Answer 1

The mass of cobalt(II) acetate (i.e. Co(OAc)₂) dissolved in water = 21.4 g

The total volume of water = 500 mL = (500/1000) L = 0.5 L

The molar mass of Co(OAc)₂ = 177.021 g/mol

Therefore, the no. of moles of Co(OAc)₂ = 21.4 g/177.021 g mol^-1 = 0.121 mol

Now, the molar concentration of solution of Co(OAc)₂ = [Co(OAc)₂] = 0.121 mol/0.5 L = 0.242 M

Since 1 Co(OAc)₂ molecule contains 1 Co(II) ion, the con, the molarity of Co(OAc)₂ and that of Co(II) ion will be the same, whereas 1 Co(OAc)₂ molecule contains 2 OAc^- ions, the molarity of OAc^- ion will be twice that of Co(OAc)₂.

Therefore, [Co(II)] = 0.242 M

And [OAc^-] = 2*0.242 = 0.484 M