To find out how much sugar Ms. Johnson used, we need to calculate half of the sugar available in the canister.
The amount of sugar in the canister is \( \frac{7}{8} \) cups. To find half of this amount, we multiply by \( \frac{1}{2} \):
\[ \text{Amount of sugar used} = \frac{1}{2} \times \frac{7}{8} \]
Now, we can multiply the fractions:
\[ \frac{1 \times 7}{2 \times 8} = \frac{7}{16} \]
So, Ms. Johnson used \( \frac{7}{16} \) cups of sugar for the dessert.