If 35.50 cm3 of a naoh solution are required for the complete neutralization of a 25.00cm3 sample of 0.200mol dm-3 h2so4, what is the concentration of the naoh solution?

In this question, you are given the NaOH volume but asked for concentration. 
Don't forget that for every 1 mol of NaOH there will be 1 mol OH- ion, but for every 1 mol of H2SO4 there will be 2 mol of H- ion.
To neutralize you need the same amount of OH- and H+, so the equation should be:

OH-= H+
35.50cm3 * x*1= 25cm3* 0.2mol/dm3 *2
x= 10/35.5 mol/dm3= 0.2816/dm3

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