NaOH+HCl----->NaCl+H2O
0,1mol----1dm3
x----------0,02dm3
x=0,002mol
1mol NaOH----1mol HCl
x------------------0,002mol HCl
x=0,002mol NaOH
0,11mol------1dm3
x-------0,02dm3
x=0,0022mol
nNaOH= 0,0022mol-0,002mol=0,0002mol
V=20cm3+20cm3=40cm3
[OH-]=0,0002mol/0,04dm3=0,005mol/dm3
pOH= -log0,05*10^-1=2,301
pH= 14-2,301=11,699=11,7