Question

In an experiment, a student was given 2.0 g of an unknown salt either CoCl₂ or CoS-2H₂O. During the experiment, the student isolated 0.93g of Co. Which salt did he start with? Why?

Answer 1

1) Lets calculate the mass of Co is salt was CoCl2

Molar mass of CoCl2,
MM = 1*MM(Co) + 2*MM(Cl)
= 1*58.93 + 2*35.45
= 129.83 g/mol


mass(CoCl2)= 2.0 g

use:
number of mol of CoCl2,
n = mass of CoCl2/molar mass of CoCl2
=(2.0 g)/(129.83 g/mol)
= 1.54*10^-2 mol
This is number of moles of CoCl2

one mole of CoCl2 contains 1 moles of Co
use:
number of moles of Co = 1 * number of moles of CoCl2
= 1 * 1.54*10^-2
= 1.54*10^-2

Molar mass of Co = 58.93 g/mol

use:
mass of Co,
m = number of mol * molar mass
= 1.54*10^-2 mol * 58.93 g/mol
= 0.91 g

2) Lets calculate the mass of Co is salt was CoS(H2O)2,

Molar mass of CoS(H2O)2,
MM = 1*MM(Co) + 1*MM(S) + 4*MM(H) + 2*MM(O)
= 1*58.93 + 1*32.07 + 4*1.008 + 2*16.0
= 127.032 g/mol


mass(CoS(H2O)2)= 2.0 g

use:
number of mol of CoS(H2O)2,
n = mass of CoS(H2O)2/molar mass of CoS(H2O)2
=(2.0 g)/(127.032 g/mol)
= 1.574*10^-2 mol
This is number of moles of CoS(H2O)2

one mole of CoS(H2O)2 contains 1 moles of Co
use:
number of moles of Co = 1 * number of moles of CoS(H2O)2
= 1 * 1.574*10^-2
= 1.574*10^-2

Molar mass of Co = 58.93 g/mol

use:
mass of Co,
m = number of mol * molar mass
= 1.574*10^-2 mol * 58.93 g/mol
= 0.93 g

this is correct salt which he started with

Answer: CoS-2H2O.