Question

Place the following solutions in order of increasing osmotic pressure. I. 0.15 M C2H6O2. II. 0.15 M MgCl2, III. 0.15 M NaCl, A. III < I < II B. II < III < I C. I < II < III D. I < III < II the answer is D but how do you know?

Answer 1

Answer:

D. I < III < II

Explanation:

• The osmotic pressure (π) is given by the relation:

π = iMRT.

where, π is the osmotic pressure.

i is van 't Hoff factor.

M is the molarity of the solution.

R is the general gas constant.

T is the temperature.

M, R and T are constant for all solutions.

So, the osmotic pressure depends on the van 't Hoff factor.

• The van 't Hoff factor is the ratio between the actual concentration of particles produced when the substance is dissolved and the concentration of a substance as calculated from its mass.

• For most non-electrolytes dissolved in water, the van 't Hoff factor is essentially 1.

• For most ionic compounds dissolved in water, the van 't Hoff factor is equal to the number of discrete ions in a formula unit of the substance.

For C₂H₆O₂ (non-electrolyte solute): i = 1.

For MgCl₂: i = 3.

It dissociates to give (Mg²⁺ + 2Cl⁻).

For NaCl: i = 2.

It dissociates to give (Na⁺ + Cl⁻).

So, the solute that has the highest osmotic pressure is  II. 0.15 M MgCl₂, then III. 0.15 M NaCl, then I. 0.15 M C₂H₆O₂.

• So, the answer is:

D. I < III < II.

Answer 2

D. I < III < II

Further explanation

Given:

(I). 0.15 M C₂H₆O₂

(II) 0.15 M MgCl₂

(III) 0.15 M NaCl

Question:

Place the following solutions in order of increasing osmotic pressure assuming the complete dissociation of ionic compounds.

The Process:

The osmotic pressure of a nonelectrolyte solution is calculated as follows:

$\boxed{ \ \pi = MRT \ }$

The osmotic pressure of an electrolyte solution is calculated as follows:

$\boxed{ \ \pi = MRTi \ }$  

The van't Hoff factor is i = 1 + (n - 1)α, with  

• n = number of ions, and  
• α = degree of dissociation.

In our problem, assuming the complete dissociation of ionic compounds results in α = 100% and i = n.

From the information above, each type of solution can be prepared as follows:

• C₂H₆O₂ (ethylene glycol) is non-electrolyte solutions.
• MgCl₂ and NaCl are strong electrolyte solutions.
• $\boxed{ \ MgCl_2 \rightarrow Mg^{2+} + 2Cl^- \ } \rightarrow \boxed{ \ i = n = 3 \ ions \ }$
• $\boxed{ \ NaCl \rightarrow Na^{+} + Cl^- \ } \rightarrow \boxed{ \ i = n = 2 \ ions \ }$

Now we compare the amount of osmotic pressure from each solution.

• 0.15 M C₂H₆O₂ ⇒ $\boxed{ \ \pi = 0.15 \times RT \ } \rightarrow \boxed{ \ \pi = 0.15RT \ }$ in atm.
• 0.15 M MgCl₂ ⇒ $\boxed{ \ \pi = 0.15 \times RT \times 3 \ } \rightarrow \boxed{ \ \pi = 0.45RT \ }$ in atm.
• 0.15 M NaCl ⇒ $\boxed{ \ \pi = 0.15 \times RT \times 2 \ } \rightarrow \boxed{ \ \pi = 0.30RT \ }$ in atm.

From the previous results, it can be observed that 0.15 M MgCl₂ delivers the most considerable osmotic pressure while 0.15 M C₂H₆O₂ has the smallest.

Thus, the rank of the solutions according to their respective osmotic pressures in increasing orders is 0.15 M C₂H₆O₂ < 0.15 M NaCl < 0.15 M MgCl₂.

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Notes:

• Colligative properties are physical properties of a solution that depend on the amount of solute expressed as concentration. One of the colligative properties is osmotic pressure (π).
• Osmosis is a spontaneous process in which a solvent molecule passes through a semipermeable membrane from a dilute solution (lower solute concentration) to a more concentrated solution (higher solute concentration).
• The pressure that causes the osmosis process to stop is considered osmotic pressure. We can also observe osmotic pressure as the external pressure needed to prevent the osmosis process. The required external pressure is the same as the osmotic pressure of the solution.

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