Answer to Question #152384 in General Chemistry for Michael Gordon

Question

Two solutes are dissolved in distilled water. The resulting solutions are placed in a container and they are separated by a semipermeable membrane. The initial volumes are equal.

Solute A: Boric acid in water to form a 9.00% (w/v) solution
Solute B: Phosphoric acid in water to form a 1.40 mol/L solution

a) Is the solution A isotonic, hypotonic or hypertonic compared to solution B?
b) Is there one side of the container that will have a net volume increase?
c) Which solute between A and B is the strongest acid?

Answer 1

9.0(w/v)% solution of boric acid = 9.0 g of boric acid per 100 mL of solution

Molarity of boric acid solution  $=\frac{(9.0g)}{(61.83g mol^{-1})(0.1L)}$

Molarity of boric acid solution = 1.4556 M

Concentration of phosphoric acid solution = 1.40 M

Since the phosphoric acid is a strong acid it will undergo a greater extent of ionization. So the number of particles of solute will be more in aqueous phosphoric acid solution than in aqueous boric acid solution. So the osmotic pressure of phosphoric acid solution remains greater than the boric acid solution.

1. The boric acid solution(A) is hypotonic than the phosphoric acid solution (B)

So the solvent particles flow from boric acid solution(A) to phosphoric acid solution(B).

2. So the volume of phosphoric acid solution (B) increases.

3. Boric acid (A) is a weak monobasic acid because of its polymeric stricture through intermolecular hydrogen bonds, it does not dissociate completely to give H⁺ ions but it can form metaborate by accepting OH^- ions from water.

For oxoacids with different numbers of O atoms, acid strength increases with a number of O atoms (or O.N. of central nonmetal). The electronegative O atoms pull electron density away from the central atom which makes the O—H bond more polar. So H3PO4 is a stronger acid