Question

A 22 % NaCl solution (w/v) will have \(a_w\) of -

• A. 0.72
• B. 0.86
• C. 0.92
• D. 0.96

Hint:

Remember that saturated salt solutions are used to calibrate water activity meters.
A saturated NaCl solution maintains a constant relative humidity/water activity of \(0.75\) (or \(75\%\) ERH) at \(25^\circ\text{C}\). \[\begin{aligned} 10\% \text{ NaCl} &\approx 0.93\ a_w
15\% \text{ NaCl} &\approx 0.89\ a_w
20\% \text{ NaCl} &\approx 0.84\ a_w
22\% \text{ NaCl} &\approx 0.86\ a_w
26.4\% \text{ Saturated NaCl} &\approx 0.75\ a_w \end{aligned}\]

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Water activity (\(a_w\)) is a key thermodynamic parameter in food preservation.
Adding highly soluble ionic solutes like sodium chloride (NaCl) lowers the water activity of a solution by binding water molecules via hydration forces, reducing the amount of free water available for microbial growth.

Step 2: Detailed Explanation:
The water activity of sodium chloride solutions decreases non-linearly with increasing concentration:
- A \(10\%\) w/v NaCl solution has an \(a_w\) of approximately \(0.93\text{--}0.94\).
- A \(15\%\) w/v NaCl solution has an \(a_w\) of approximately \(0.89\text{--}0.90\).
- A \(20\%\) w/v NaCl solution has an \(a_w\) of approximately \(0.84\text{--}0.85\).
- A \(22\%\) w/v NaCl solution has an \(a_w\) of approximately \(0.86\) (specifically around \(0.85\text{--}0.86\)).
- A fully saturated NaCl solution (\(\approx 26.4\%\) w/v) has a thermodynamic water activity limit of \(0.75\).

Step 3: Final Answer:
Therefore, a \(22\%\) w/v NaCl solution has a water activity of 0.86, which corresponds to option (B).