57.1g * 6.022*10^23 / 22.9 * 2 = 7.51*10^23 molecules
As you mentioned the question in chemistry so it may be a concentration unit I.e mass per volume.
(SOLUTION)
Mass/volume = total mass/ total volume
Now:
Total mass= 42.95 g
Total volume =351.8mL
Put these values in above equation;
Mass/volume = 42.95/351
“”. = 0.122 g/mL
