A sample of 61.8 g of H3BO3, a weak acid is dissolved in 1,000 g of water to make a 1.0-molal solution.?

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Which of the following would be the best procedure to determine to molarity of the solution? (Assume no additional information is available.)

(A) Titration of the solution with standard acid

(B) Measurement of the pH with a pH meter

(C) Determination of the boiling point of the solution

(D) Measurement of the total volume of the solution

(E) Measurement of the specific heat of the solution

Please could you explain the answer as well.

1 Answer

  • While your question refers to boric acid as H3BO3, a better formula is B(OH)3. Granted this makes it look more like a base than an acid, but the only reason that B(OH)3 is acidic is due to the H+ which comes from water when B(OH)4^- is formed. NONE of the hydrogen atoms in B(OH)3 ionize as hydrogen ions, H+.

    B(OH)3(aq) + HOH(l) <==> B(OH)4^- + H+

    Now, on to your question. Molarity and molality are different ways of expressing concentration and are not equal to each other. Molality is the concentration expressed in moles of solute per kilogram of solvent. Molarity is the concentration expressed in moles of solute per liter of solution. The volume of solution will be different than the mass of the solvent. If the density of the solution is known then the molarity can be computed from the molality.

    Titrating the boric acid solution with a standardized base would easily give you the molarity, but not titration with an acid. Therefore, titration with an acid is of no use.

    A pH meter will give you the hydrogen ion concentration, but since boric acid is a very weak acid, and the Ka of boric acid is not known, then the molarity cannot be calculated. Therefore, the pH is of no use.

    The boiling point of the solution is a colligative property and depends on the molality, which can already be calculated and the van’t Hoff factor (i) which we don’t know. Therefore, boiling point is of no use.

    If we measure the volume of the solution then molarity CAN be determined since molarity is the moles of solute per liter of solution.

    Measuring the specific heat of the solution will be of little use since the specific heat will depend on the mass of the solution a temperature change, but no other information is available.

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