Calculate the amount of substance of an isotope based on the percent composition
If you have given an accurate account then your teacher is mistaken. We've all gotten confused while trying to talk and think at the same time. 
\begin{align}
\pu{42.3 g} \ce{Cl}\cdot 0.24\frac{m(\ce{^37Cl})}{m(\ce{Cl})} 
&amp;= \pu{10.2 g} \ce{^37Cl} \\
\implies
\frac{\pu{10.2 g}~ \ce{^37Cl}}{\pu{36.97 g//mol}\ce{^37Cl}} &amp;= \pu{0.276 mol}
\end{align}