How many moles are there in 20.0 mL of a 0.125 M solution?

0.00285 moles
Explanation:
Molarity is defined as moles of solute, which in your case is sodium hydroxide,
NaOH
, divided by liters of solution.
molarity
=
moles of solute
liters of solution
SImply put, a
1-M
solution will have
1
mole of solute dissolved in
1
liter of solution.
Now, you know that your solution has a molarity of
0.150 M
and a volume of
19.0 mL
.
Since the volume is much smaller than
1 L
, you can expect to have fewer moles of sodium hydroxide in this sample than you would have had in a full liter of solution.
Convert the volume of the sample from mililiters to liters by using the conversion factor
1 L
=
10
3
mL
to get
19.0
mL
⋅
1 L
10
3
mL
=
19.0
⋅
10
−
3
L
This means that the number of moles of solute you get in this sample will be equal to
c
=
n
V
⇒
n
=
c
⋅
V
n
=
0.150
moles
L
⋅
19.0
⋅
10
−
3
L
=
0.00285 moles NaOH