Could this be a source of error for the osmosis sac:
The mass of the eggs may be not be accurate as oxygen from the atmosphere is able to diffuse into it, causing an altercation in the masses?
This is quite vague. Just make mentions of inaccurate result collection as human-made errors are inevitable. Moreover other errors can be made due to the incompetency of the naked eye at a molecular level. Using the apparatus incorrectly, misreading the results etc...
i would say something else. It's just like saying " the mass of the eggs may not be accurate as a fly landed on the scale when you were weighing them"- you are over-thinking it.
If you can provide a brief outline of your experiment i could help you
This is okay but the fly situation is quite objective and you would rather want to say "to prevent the error of inaccurate/unreliable results in future experiments the isolation of the eggs is paramount. Furthermore repetition of the practical will minimise the level of inaccuracy.