8.04 mL.
How many moles of NaOH?
Note the unit:
[tex]V = 15.8 \; \textbf{mL} = \dfrac{15.8}{1000}\;\textbf{L} = 0.0158 \; \text{L}[/tex].
[tex]n = c\cdot V = 0.0158 \times0.321 = 0.00507\;\text{mol}[/tex].
How many moles of HCl?
As seen in the equation, HCl and NaOH reacts at a 1:1 ratio.
[tex]n(\text{HCl}) = n(\text{NaOH}) = 0.00507\;\text{mol}[/tex].
How many mL of HCl?
[tex]V = \dfrac{n}{c} =\dfrac{0.00507}{0.631} = 0.00804\;\text{L} = 0.00804\times 1000 \;\textbf{mL} = 8.04\;\text{mL}[/tex].