Question

Calculate the molarity of Pure water (d= 1g/l)

Answer 1

Molarity is no of moles per litre.1g/ml density means that 1L (=1000 ml) weighs 1000g.
Mol wt of water is 18 g/mol
No of moles of water =1000/18 = 55.55 mol
So one litre of water has 55.55 mol water, and so molarity if water is 55.55 molar

Answer 2

55.5 mol is answer of that question