<span>HCl<span>(aq)</span>+NaOH<span>(aq)</span>→NaCl<span>(aq)</span>+<span>H2</span>O<span>(l)</span></span>
As you can see here, one mole of acid neutralizes one mole of base.
We use the concentration equation, which states that,
<span>c=<span>nv</span></span>
<span>
<span>
<span>
n is the number of moles
</span>
<span>
v is the volume of solution
</span>
</span>
</span>
Rearranging for moles, we get,
<span>n=c⋅v</span>
So, we have:
<span><span>n<span>NaOH</span></span>=0.1 M⋅0.05 L</span>
<span>=0.005 mol</span>
Since one mole of acid neutralizes one mole of base, then we must have: <span><span>n<span>HCl</span></span>=<span>n<span>NaOH</span></span></span>.
And so,
<span><span>c<span>HCl</span></span>=<span><span>n<span>HCl</span></span><span>v<span>HCl</span></span></span></span>
<span>=<span><span>0.005 mol</span><span>0.03 L</span></span></span>
<span>≈0.17 <span>M</span></span>