A student carried out the following procedure to measure the pressure of carbon dioxide in a soft drink bottle. First, she weighed the bottle (853.5 g). Next, she carefully removed the cap to let the CO2 gas escape. She then reweighed the bottle with the cap (851.3 g). Finally, she measured the volume of the soft drink (452.4 mL). Given that Henry's law constant for CO2 in water at 25°C is 3.4 x 10-2 mol/L ? atm, calculate the pressure of CO2 in the original bottle. Why is this pressure only an estimate of the true value?