Question

Measuring the electrical conductivity of a solution. Ammonia solution is added from a burette to 50.0 cm3 of hydrochloric acid in a beaker. When the NH3 is added to the solution, the conductivity decreases until 20 cm3 of NH3 is added. The ammonia solution has a density of 0.075 g cm-3 and contains 40% by mass of ammonia. Find the molarity of the hydrochloric acid.

Answer 1

Conductivity decreases due to the following reaction:

NH₃ + HCl NH₄Cl

The decrease in conductivity stops at the point where all of the HCl is consumed by added NH₃

Now, mass of NH₃ added = Volume of solution * density * mass% NH₃ / 100 = 20 * 0.075 * 40/100 = 0.6 g

Moles of NH₃ = mass / molar mass = 0.6 / 17 =0.0353 mol

From the equation, mole sof NH₃ = moles of HCl present in solution = 0.0353 mol

MOlarity of HCl = moles of HCl / Volume of solution in Liters = 0.0353 / (50* 10^-3) = 0.706 M