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Concentration and Molarity Worked Example Problem

Finding Concentration of Ions in an Aqueous Solution

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Molarity is one of the most used measurement of concentration. Molarity is measured in number of moles of a substance per unit volume. This worked example problem illustrates the steps necessary to calculate the concentration of ions in an aqueous solution.

Question

a. State the concentration, in moles per liter, of each ion in 1.0 mol Al(NO3)3.
b. State the concentration, in moles per liter, of each ion in 0.20 mol K2CrO4.

Solution

Part a.) Dissolving 1 mol of Al(NO3)3 in water dissociates into 1 mol Al3+ and 3 mol NO3- by the reaction:

Al(NO3)3(s) → Al3+(aq) + 3 NO3-(aq)

Therefore:

concentration of Al3+ = 1.0 M
concentration of NO3- = 3.0 M

Part b.) K2CrO4 dissociates in water by the reaction:

K2CrO4 → 2 K+(aq) + CrO42-

One mol of K2CrO4 produces 2 mol of K+ and 1 mol of CrO42-. Therefore, for a 0.20 M solution:

concentration of CrO42- = 0.20 M
concentration of K+ = 2×(0.20 M) = 0.40 M

Answer

Part a).
Concentration of Al3+ = 1.0 M
Concentration of NO3- = 3.0 M

Part b.)
Concentration of CrO42- = 0.20 M
Concentration of K+ = 0.40 M

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