Question #7b377

There are variety of concentration. First, you must be clear what kind of concentration you want to find.
https://en.wikipedia.org/wiki/Concentration

If you want to calculate mass concentration, divide the mass of the solute #m# by the volume of solution #V#.

#ρ=m/V#

Your answer is wrong because you used the volume of solvent instead of solution.
Dissolving #60# g #NaOH# in #100# ml water, the volume of the solution will be so different from #100# ml.

Instead, you can calculate mass fraction.
#100# ml of water is #100# g and the mass of solution is #60+100=160# g.
Mass fraction of #NaOH# in this solution is #w="mass of solute"/"mass of solution"=60/160=0.375#, or #37.5%#.

[Some evaluation]
Acording to CRC databook, the density of #NaOH# solution with mass fraction #0.360# and #0.380# is #1.3900# and #1.4101# respectively. (g/ml) Data are cited from http://www.edu.utsunomiya-u.ac.jp/chem/v8n1/ashida1/TabC05.htm.

Density of this #NaOH# solution (#w=0.375#) will be about #1.4051#(linear interpolation is applied).

The volume of the solution is about #(160 g)/(1.4051 g/(mL))=113.87# (mL) and
the mass concentration is about #(60(g))/(0.11387(L))=526.91# (g/L).