Concentration
=
Moles of solute (moles)
Volume of solution (L)
Thus
Concentration
×
volume
=
moles of solute
0.205
⋅
m
o
l
⋅
L
−
1
×
0.250
⋅
L
=
Moles of solute
=
0.0513
⋅
m
o
l
, an answer in moles as required.
And thus
mass of NaOH
=
moles
×
molar mass
=
0.0513
⋅
m
o
l
×
40.0
⋅
g
⋅
m
o
l
−
1
=
2.05
⋅
g