How much grams of a substance will be left after 2 hours if its half life is 15 minutes?

$= \frac{M}{256} g$, where M = initial mass
In 2 hrs or 120 min there will be$\frac{120}{15} = 8$ half life periods.
For each half life the mass is halved, so after 2 hrs i.e. after 8 half life periods the mass will be $= M \times {\left(\frac{1}{2}\right)}^{8} g = \frac{M}{256} g$