Partial pressures and a real gas

You will work with a mixture of gases, and then test whether a gas is close to ideal.

The idea. Each gas in a mixture fills the whole volume, so its partial pressure is $p_i = n_iRT/V$. The total is the sum. For a real gas, the ratio $pV/(nRT)$ shows how far it is from ideal.

Your tasks.

1. In partial_pressures, moles is a dictionary such as {"oxygen": 0.2, "helium": 0.8}. Return a dictionary of the partial pressures in Pa.
2. In mole_fraction, return the mole fraction of the gas name.
3. In ratio, return $pV/(nRT)$ for a measured pressure p.