What is the molar concentration of a solution if 200 ml contains 20 g of sodium hydroxide?

Given:
V solution (NaOH) = 200 ml = 0.2 l
m (NaOH) = 20 g

To find:
See (NaOH) -?

Solution:
1) Calculate the molar mass of NaOH:
M (NaOH) = Mr (NaOH) = Ar (Na) * N (Na) + Ar (O) * N (O) + Ar (H) * N (H) = 23 * 1 + 16 * 1 + 1 * 1 = 40 g / mol;
2) Calculate the amount of NaOH substance:
n (NaOH) = m (NaOH) / M (NaOH) = 20/40 = 0.5 mol;
3) Calculate the molar concentration of the NaOH solution:
Cm (NaOH) = n (NaOH) / V solution (NaOH) = 0.5 / 0.2 = 2.5 M.

Answer: The molar concentration of the NaOH solution is 2.5 M.