50. A 2.0-ml sample of NaOH solution is exactly neutralized by 4.0 mL of 3.0 M HCl solution.
What is the concentration of the NaOH
solution?

Ido not understand. 

The correct answer is "hydrogen".

Electron equations of the given changes are as follow,a)                              cu      →      cu⁺²  + 2e⁻b)                              mg⁺²  +  2 e⁻      →      mgc)                              cl⁻    →      cl  +  1e⁻d)                              s  +  2 e⁻    →      s²⁻e)                                o²⁻      →      o  +  2 e⁻f)                              p  +  3 e⁻    →      p³⁻g)                              fe²⁺    →      fe³⁺  + 1 e⁻h)                              s²⁻    →      s⁴⁺  +  6 e⁻

For lesson: cameras, plant presses, newspaper, paper towels, clip boards, pencils, calculators, field guides, metric measuring tapes, paper, transect squares, colored pencils, ph meters, light meters, computers, etc.for extension: baggies for soil samples collecting, distilled water, hand shovels, beakers, boiled rice, microscopes, well microscope slides or regular microscope slides with cover slips