mHCl = 7,3% : 100% x 200 = 14,6 g
=) nHCl = 14,6 : 36,5=0,4 mol
PTHH NaOH + HCl --> NaCl + H2O 
theo pt 
nNaOH = nHCl = 0,4 mol
=) mNaOH = 0,4 . 40 = 16 g
=) mdd NaOH = 16:(10%:100%) = 160 g
mdd sau = m dd NaOH + mdd HCl = 160 + 200 = 360g 
nNaCl = nHCl = 0,4 mol
=) mNaCl = 0,4.58,5=23,4 g
=) C% = 23,4 : 360 x 100% = 6,5%