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# Gas Laws (Chemistry)?

Anna Marie is working a problem that reads “The pressure in a bicycle tire is at 105 psi at 25*C in Fresno. You take the bicycle up to Huntington, where the temperature is -5*C. What is the pressure in the tire? You can assume that volume and the number of particles is constant. Anna Marie’s solution looks like this: 105 psi (-5*C/25*C) = -21 psi. She feels pretty confident she has set up the problem correctly. You disagree with her answer. What don’t you like about her answer? How would you change about her solution? Why do you want to make that change?

### 2 Answers

- pisgahchemistLv 71 year agoFavorite Answer
Time pressure ....

Does Anna Marie understand the difference between the absolute pressure in a tire and gauge pressure? Does the person who wrote the question?

1. Make sure that the pressure of the air in the tire is the absolute pressure, and not the gauge pressure. Gauge pressure is the pressure the tire gauge reads which is in addition to the ambient pressure. Since no mention of the ambient pressures are made, we will assume it is the absolute pressure.

2. Temperature must be in an absolute temperature scale. Kelvin will do nicely.

K = C + 273.15

P1V1 / T1 = P2V2 / T2 ............ combined gas law

P1 / T2 = P2 / T2 ..................... assume V is constant, Amontons's law

P2 = P1T2 / T1 ........................ solve for P2

P2 = 105 psi x 268K / 298K ..... T is in Kelvin

P2 = 94.4 psi

- ChemTeamLv 71 year ago
Change the degrees Celsius to Kelvin, then solve. Also, no such thing as a negative pressure.