I'm doing a lab about osmosis/ diffusion. We put a 2 molar solution of NaCl into a dialysis bag and submerse the bag in water, weighing the bag after 15 minute increments for an hour. I need help explaining why the weight of the bag would fluctuate (EX. 6 grams -> 7 grams -> 6 grams) rather than just increasing in weight or remaining the same.
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The fluctuation in the weight of the dialysis bag during the osmosis/diffusion lab can be attributed to the movement of solvent molecules (in this case, water) across a semi-permeable membrane. In this experiment, the dialysis bag acts as the semi-permeable membrane, allowing the passage of water molecules but not the larger NaCl molecules.
When the bag is placed in water, there is a concentration gradient between the inside and outside of the bag. The NaCl concentration inside the bag is higher (2 molar) compared to the lower concentration of NaCl in the water. As a result, water molecules are inclined to move from an area of lower solute concentration (water outside the bag) to an area of higher solute concentration (water inside the bag) in an effort to equalize the solute concentration on both sides of the membrane. This movement of water molecules is known as osmosis.
During the initial stages of the lab, the dialysis bag absorbs water, causing an increase in weight. This is due to the water molecules diffusing from an area of lower solute concentration (outside the bag - water) to an area of higher solute concentration (inside the bag - NaCl solution), as explained earlier.
However, as the osmosis process continues over time, the concentration of NaCl inside the bag becomes more similar to the concentration of NaCl in the surrounding water. As a result, the net movement of water molecules into the bag slows down. Eventually, the net movement of water becomes negligible and the weight of the bag stabilizes.
The subsequent decrease in weight observed at certain time intervals could be due to factors such as evaporation or specific characteristics of the dialysis bag itself, which may have caused some molecules to escape or be released from the bag. These factors can result in a slight loss of mass.
In summary, the fluctuation in weight observed during the osmosis/diffusion lab is primarily due to the initial increase in weight as the bag absorbs water through osmosis and subsequent factors like evaporation or the loss of small molecules.