Calculate the concentration of the water in this glass.

This is quite a classic chemistry interview question for oxbridge and one that helped me understand a little bit better about how the interviewers want you to think in the exam.

They will often ask something as if they want an exact answer, where as in actual fact, they don't mind you estimating and guessing a bit as you explain your thought process along the way.

Whilst not actually being that difficult a question, I think this question exemplifies this as people often get to be obsessed with thinking about the exact glass in front of them - rather than seeing the bigger picture and thinking about the chemistry.

Solution:
1. Estimate the amount of water in the glass.
It would be nice if the value is close, but in actual fact it doesn't matter, as this will not effect the concentration of the water.

2. Remember that water has a density of 1g/ml, and so work out the mass of the water.

-Concentration = Moles/Volume (in dm^3) &

-Moles = Mass/R.F.MMoles of water = Mass calculated/~18
e.g 1 litre=1000g

So, 55.56 moles of water in the glass.

Concentration = 55.56/1 = 55.5 mol/dm^3