1. Calculate the density of oxygen at STP using the ideal gas law.

2. A tire is filled with air at `15°C` to a gauge pressure of `220\ kPa`. If the tire reaches a temperature of `38°C`, what fraction of the original air must be removed if the original pressure of `220\ kPa` is to be maintained?

3. Estimate the number of (a) moles, and (b) molecules of water in all the Earth's oceans. Assume water covers `75%` of the Earth to an average depth of `3\ km`.