A chemist makes a solution of Ca (NO3) 2 by dissolving 21.3 g Ca (NO3) 2 in water to make 100.0 mL of solution. What is the concentration of NO3 - ions in the solution? Assume that Ca (NO3) 2 is the only solute in the solution. The molar mass of Ca (NO3) 2 is 164.10 g/mol.

[NO₃⁻ ] = 2.596 M

Explanation:

Ca (NO₃) ₂ dissolves in water according to the following equation:

Ca (NO₃) ₂ ⇒ Ca²⁺ + 2NO₃⁻

The moles of Ca (NO₃) ₂ that dissolve is found as followed:

(21.3 g) / (164.10 g/mol) = 0.1298 ... mol

The number of NO₃⁻ ions are related to the above quantity by the molar ratio:

(0.1298 mol Ca (NO₃) ₂) (2NO₃⁻/Ca (NO₃) ₂) = 0.2596 ... mol NO₃⁻

The concentration of the nitrate ions is then calculated:

[NO₃⁻ ] = (0.2596 ... mol) / (100.0ml) x (1000mL/L) = 2.596 M