To find the pressure of the air in the flask when 70 mL of water is added, we first determine the volume of air remaining in the flask. Since the maximum volume of the flask is 250 mL, the volume of air is:

$\text{Volume of air} = \text{Maximum volume} - \text{Volume of water} = 250 \, \text{mL} - 70 \, \text{mL} = 180 \, \text{mL}$

Next, we can substitute this volume into the equation of the best-fit line, which is:

$y = 0.00550x - 0.000645$

Here, $x$ represents the volume of air (in mL), and $y$ represents the pressure (in atm). Substituting $x = 180$:

$y = 0.00550(180) - 0.000645$

Now, calculate:

$y = 0.99 - 0.000645 = 0.989355 \, \text{atm}$

Thus, the pressure of the air in the flask when 70 mL of water is added is approximately 0.989 atm.

Questions for Further Exploration:

1. How does the addition of more water affect the pressure in the flask?
2. What assumptions are made in the relationship between volume and pressure in this scenario?
3. How would the graph change if the relationship were not linear?
4. What would happen to the pressure if the flask had a smaller maximum volume?
5. Can you explain the physical principles behind the relationship observed (e.g., Boyle's Law)?

Tip:

Always ensure that your measurements and calculations maintain consistent units to avoid errors in results.