1000g aqueous solution of CaCO3 contain 10g of calcium carbonate concentration of the solution is :

Molar mass of CaCO₃ = 100g
​Mass of CaCO₃ = 10g
Moles of CaCO₃ = Mass of solute/ Molar mass of solute = 10/100 = 0.1mol
Mass of solvent = Mass of solution - Mass of solute = 1000 - 10 = 990g or 0.99 kg

Molality = Number of moles of solute / Mass of solvent in Kg
Molality = 0.1/0.99 =  0.1010 m