Answer
The concentration of $Na_2CO_3$ in that $Na_2CO_3$ solution is equal to 19.4 % (m/m).
Work Step by Step
1. Write the concentration expression:
$Concentration \space (m/m) = \frac{mass \space of \space solute \space }{mass \space of \space solution \space} \times 100 \%$
2. Substitute the values, and calculate the concentration:
$Concentration \space (m/m) = \frac{48.5 \space g }{250.0 \space g} \times 100 \% = 19.4 \%$