Three solutions 39.0 mL of 0.2 M KNO₃; 59.0 mL of 0.9 M Ca (NO₃) ₂; and 63.0 mL of 0.345 M KCl were poured together to make one new solution. What is the concentration of Cl⁻ after mixing? Answer in units of mol/L.

[Cl⁻] = 0.135 M

Explanation:

The concentration of Cl⁻ is equal to the total moles of Cl⁻ divided by the total volume:

C = n/V

All of the moles of Cl⁻ come from dissociation of the KCl:

KCl ⇒ K⁺ + Cl⁻

We will calculate the moles of Cl⁻ based on the amount of KCl added by using the molar ratio of 1:1.

(0.345 mol/L) (63.0 mL) = (21.735 mmol KCl) (Cl⁻/KCl) = 21.735 mmol Cl⁻

Extra significant figure are kept to avoid round-off errors.

Now we can calculate the total volume, which is the sum of the volume of the three solutions added:

(39.0 mL) + (59.0 mL) + (63.0 mL) = 161.0 mL

Finally, we calculate the concentration of Cl⁻

[Cl⁻] = (21.735 mmol) / (161.0 mL) = 0.135 M