#### Answer

The spring constant should be 98 N/m

#### Work Step by Step

The force with which a spring pulls is $F = kx$, where $k$ is the spring constant and $x$ is the stretch distance. When an object hangs on the spring, the spring's force is equal to the weight of the object.
Let's assume that the spring stretches by 1.0 cm when a mass of 100 grams hangs on the spring. We can find the spring constant $k$.
$F = mg$
$kx = mg$
$k = \frac{mg}{x}$
$k = \frac{(0.100~kg)(9.80~m/s^2)}{0.010~m}$
$k = 98~N/m$
The spring constant should be 98 N/m