Step 1: Use the relation for work done at constant pressure.
For an ideal gas expanding at constant pressure,
\[
W=P\Delta V
\]
Given,
\[
W=200\,\text{J}
\]
For an ideal gas,
\[
P\Delta V=nR\Delta T
\]
Hence,
\[
nR\Delta T=200\,\text{J}
\]
Step 2: Find change in internal energy.
For a monatomic ideal gas,
\[
\Delta U=\frac{3}{2}nR\Delta T
\]
Since
\[
nR\Delta T=200,
\]
we get
\[
\Delta U=\frac{3}{2}\times 200
\]
\[
\Delta U=300\,\text{J}
\]
Step 3: Apply the first law of thermodynamics.
According to the first law,
\[
Q=\Delta U+W
\]
So,
\[
Q=300+200
\]
\[
Q=500\,\text{J}
\]
Step 4: Final conclusion.
Hence, the heat absorbed by the gas is
\[
\boxed{500\,\text{J}}
\]