What must the pressure be in a balloon that holds 5.1L of gas at 20*C if there are two moles of gas in the balloon?

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 p = nRT / V R = 0.08206 atmosphere-liters /mol-Kelvin or you can use R = 8.3127 kpa-liters / mol-Kelvin or R = 62.37 mmHg-liters / mol-Kelvin.  Lets use the last one p = (2 moles)(62.37mmHg-Liter / mol-Kelvin)(20 deg Celsius +273) / 5.1 L = 7165.9mmHg You could also adjust the standard pressure at STP which is 760 mmHg by multiplying the temperature ratio (293/273), the decreased volume ratio (22.4L / 5.1L) and the number of moles ratio (2 moles / 1 mole).  If you do this, you get the same answer.  This makes sense since increased temperature will increase the pressure, increased number of moles will increase the pressure and decreased volume will increase the pressure.