What is the molarity of a solution that has a concentration of 10.0 g/L of NaCl?

Molar mass of NaCl = (23.0 + 35.5) g/mol = 58.5 g/mol

No. of moles of 10.0 g NaCl = (10.0 g) / (58.5 g/mol) = 0.171 mol

Molarity of the solution = 0.171 mol/L = 0.171 M

Convert 10g to moles.
mol(NaCl) = 10/ (23 + 35.5) = 10 / 58.5 = 0.1709 moles
0.1709 moles is contained in one litre 'L'
Hence molarity is 0.1709 mol L^-1
This can be written as
0.1709 mol dm^-3
or
0.1709 M

(10.0 g/L) / (58.4430 g NaCl/mol) = 0.171 mol/L = 0.171 M NaCl

it's easy you can google all that .. you deal with the atomic numbers and the molar number . then you crunch some numbers .