答：
1.混合后 w = （35*0.1 + 65*0.20/(35+65)*100% = 16.5%
2.HCl ------ NaOH
36.5g------40g
16.5%*50---20%*m(NaOH)
得 m(NaOH) = 16.5% *50*40 /36.5/20% = 45.21g
3.反應(yīng)后溶液中溶質(zhì)為NaCl,其物質(zhì)的量與HCl的量相同,溶液總質(zhì)量守恒,故為（50+45.21）=95.21g
則 m(NaCl) = 16.5%*50/36.5*58.5 = 13.22g
w(NaCl) = 13.22/95.21 *100% = 13.9%
綜上所述,1混合后鹽酸中溶質(zhì)的質(zhì)量分?jǐn)?shù)為16.5%；2.所用NaOH溶液的質(zhì)量 為45.21g；
3.反應(yīng)后溶液中溶質(zhì)的質(zhì)量分?jǐn)?shù) 13.9%