To calculate the Mr value of sodium hydroxide, I used the periodic table. The Ar of Na is 23, O is 16 and H is 1. The Mr of NaOH is 40. I calculated the mole value of sodium hydroxide by using the Mr.
I then balanced the equation so that I could get the correct mole ratio between the sodium hydroxide and the hydrochloric acid. I found that the ratio is 1:1 so the moles of the sodium hydroxide and hydrochloric acid are the same. I calculated the concentration of NaOH because I knew the volume of NaOh as well as the moles. I used the formula moles/volume.
For instance, NaCl. Na has a charge of +1 where as Cl has a charge of -1. This is because Sodium has 1 electron in the outer shell, and chlorine has 7 electrons in the outer shell. When together, the charges cancel each other out.