Ok density of water is 1000 g/ 1 L

 

I don't know isn't this 1 L composed completely of water. This is confusing me. Do find the concentration you have to find the moles and divide by 1 L. 1000/18=55 M . I thought concentration of pure water you can't find because the solution is complete water. I hope you understand what I mean. Thanks

the concentration is the number of mols per unit volume right?

 

well, pure water is 55 mol/L

 

simple.

 

its fractional composition is 1 if you care. but there really is no reason why a pure substance can't have a concentration at all, its just equivalent to its molar density.

And what if you vaporize the water, increasing the volume but decreasing the density? Then your pure water will be less concentrated.

Just for the record, to two significant figures 1000/18= 56 not 55.